The Ash Twins Blog

See previous posts on history, symptoms, neurophysiology, genetics, and molecular biology of Angelman Syndrome.

To briefly review, Angelman Syndrome (AS) is a neurological syndrome which causes patients to be excessively cheerful, giggling, and happy, and mentally retarded with very little or no language. AS occurs when the protein ubiquitin E3A ligase (UbE3A) is lost due to abnormal methylation/imprinting of a region of chromosome 15. UbE3A recognizes proteins and attaches a ubiquitin to them, which causes them to be degraded in the proteasome. Loss of UbE3A in Angelman Syndrome would prevent these proteins from being degraded and lead to an excess of them in cells. In the previous post, I described how Greer et al identified ubiquitination targets of UBE3A: Arc, Ephexin5, HHR23A, Sacsin, Blk, and Mcm7. In this post I consider the function of these proteins.

Arc

Arc/Arg3.1 (activity-regulated cytoskeleton-associated protein/ activity-regulated gene 3.1; henceforth referred to as Arc) has multiple roles in neuronal function (Bramham et al 2008; Bramham et al 2010).  It is an early-immediate gene, meaning that it is transcribed into messenger RNA (mRNA) in association with heightened neural activity (its promoter includes binding sites for cAMP response element-binding protein (CREB), serum response factor (SRF), and myocyte-enhancer factor-2 (MEF2 – a TF which also enhances UbE3A transcription)). Arc mRNA transcripts are shuttled to dendrites and are translated into protein at sites of increased synaptic activity. Arc then associates with endophilin and dynamin to induce the internalization of AMPA receptors by endocytosis, which will function to decrease the synapse’s strength – this process is hypothesized to underlie the candidate neurophysiological mechanism for forgetting known as long term depression (LTD). Arc is also believed to mediate the structural consolidation of long term potentiation (LTP). In the current model (Bramham et al 2008), high synaptic activity induces brain-derived neurotrophic factor (BDNF) release into the synaptic cleft. BDNF binds to the TrkB tyrosine kinase receptor on the postsynaptic membrane. Activated TrkB phosphorylates and activates cytoplasmic proteins which induce Arc transcription (making mRNA with the gene as a blueprint) and local translation (making protein with the mRNA as a blueprint), and Arc indirectly phosphorylates cofilin. Phosphorylated cofilin mediates the expansion of actin filaments (globular proteins which are chained together to form a major component of the cytoskeleton, the cell’s internal scaffold). The enlarged synaptic scaffolding stabilizes the receptors and machinery needed for a stronger synapse. Arc’s key role in LTD and LTP has been confirmed behaviorally and electrophysiologically (Plath et al 2006).

The activity of Arc is exquisitely regulated within neurons (Bramham et al 2008). For example, Arc translation is inhibited by fragile X mental retardation protein (FMRP – the protein which is dysfunctional in Fragile X syndrome [the most common single-gene cause of mental retardation]) at baseline. When synaptic concentrations of the excitatory neurotransmitter glutamate reach a high enough level, metabotropic glutamate receptors (mGluR1) are activated and block FMRP’s inhibition of Arc translation.  mGluR1 activation also induces Arc translation through a kinase: Eukaryotic elongation factor 2 kinase (eEF2K) dissociates from mGluR1 and phosphorylates eukaryotic elongation factor 2 (eEF2). EEF2 then selectively upregulates Arc translation while suppressing translation of other proteins (Park et al 2008). Arc mRNA is also regulated by nonsense-mediated degradation. The 3’ untranslated-region (UTR) of Arc is a target for RNA-binding proteins which speed its degradation. It has been suggested that together these mechanisms could allow for a situation in which each Arc mRNA transcript leads to the production of a single Arc protein.

Arc’s role in neural circuit function is less understood, but there are some interesting clues. McCurry and colleagues, for instance, have identified a key role for Arc in experience-dependent cortical plasticity (2010). In primary visual cortex (V1), Arc is primarily expressed in layer 2-4 pyramidal neurons; interestingly, layer 5 neurons lack Arc, as does the thalamus. In Arc knockout mice, topographic organization in V1 is indistinguishable from wild-type by intrinsic imaging, and visual-evoked responses are normal. Activity-dependent reorganization in V1, on the other hand, is nonexistent in Arc KO mice. Monocular deprivation – the experimental technique in which one eye is sutured shut for a few days during the period of high cortical plasticity (4-5 weeks-old in mice (Gordon 1997), 6 months- to 8 years-old in humans (Vaegan & Taylor, 1979)) – has no effect on V1 organization in KO mice; compare this to wild-type animals, in which this transient deprivation causes a dramatic enlargement of ocular dominance columns representing the open eye at the expense of cortical surface area representing the sutured eye. Similarly, repeated presentation of the same visual stimulus over several days increases the electrophysiological response to that stimulus in WT mice but not in Arc KO mice. The monocular deprivation plasticity is likely mediated by a mGluR-LTD mechanism, and the repeated-stimulus potentiation is likely mediated by LTP, so these experiments strongly implicate Arc in both of these processes. In another study, Wang et al demonstrated that in Arc KO mice, V1 neurons have broader tuning curves relative to WT (2006). Moreover, Arc KO mice have impaired learning and memory in the Morris water maze, contextual fear conditioning, and conditioned taste aversion (Plath et al 2006) Last, induction of LTP and LTD is impaired in the hippocampi of these mice (Plath et al 2006).

What do these experimental findings tell us about the pathophysiology of AS? AS patients probably have a dysregulated, hyperactive Arc which is not degraded after it performs its function (Greer et al 2010). Tonic Arc activity should lead to the functionally-contradictory decrease in AMPA receptor density and increase in synapse size and stability. The overall effect on synaptic strength could vary from synapse to synapse, but a gross dysregulation of individual synapse strengths would be inevitable. The implication for network function would be an inability to fine-tune and optimize synaptic connections. This model could be tested by generating an Arc-overexpressing transgenic mouse.

Ephexin5

Ephexin5 is an uncharacterized Ras homology (rho) guanine nucleotide exchange factor (rho-GEF). Rho-GEFs are a class of enzymes which activate rho GTPases by catalyzing the exchange of GDP for GTP (Attalieb et al 2010). Rho GTPases are a subclass of the Ras superfamily small GTPases, and their major role is regulation of actin cytoskeleton dynamics (Etienne-Manneville & Hall 2002). In addition to their activation by rho-GEFs, Rho GTPases are deactivated by GTPase activating proteins (GAPs), which catalyze the GTP-to-GDP transition, and inhibited by guanine dissociation inhibitors (GDIs), which lock GTPases in the GDP-bound inactive state. Ephexin 5’s role in intracellular cascades is not worked out, but the related rho-GEF ephexin1 has been shown to catalyze the GDP-to-GTP activation in the rho GTPases RhoA, Cdc42, and Rac1, is the downstream target of the EphA receptor, regulates growth cone dynamics in developing neurons, and is required for the morphological “plaque-to-pretzel” maturation of the neuromuscular junction (Shamah et al 2001; Shi et al 2010). Greer et al cite unpublished findings to claim that ephexin5 restricts the number of synapses formed by a neuron (2010).

Working from the unpublished premise that ephexin5 restricts synapse number, overactive Ephexin5 would be expected to decrease the number of synapses on neurons. This hypothesis comports with Dindot et al’s findings in mouse maternal UbE3A knockouts that spine density is significantly decreased (2008). Dindot et al also found highly variable spine morphology compared to WT, which would also be expected given dysregulation of rho GTPase effects on the actin cytoskeleton.

HHR23A

HHR23A (human homologue of RAD23 A) is a unique protein that participates in both DNA repair and the proteasome system. In a complex with other proteins, HHR23a mediates nucleotide-excision repair, an adaptive response to ultraviolet (UV) DNA damage. The first 80 amino acids of HHR23A form a ubiquitin-like domain (UbL), which has high structural homology to ubiquitin and can bind to many of the same active sites as ubiquitin – the 19S proteasomal subunit, for example. It also has a ubiquitin-binding domain, which binds to ubiquitinated protein moieties. The current model is that the protein’s UbL domain binds and inhibits the proteasome, which prevents degradation of NER proteins bound to HHR23a. The significance of this protein’s expression for AS is unresolved.

Sacsin

Sacsin is another protein demonstrated to be a UbE3A ubiquitination target by Greer et al (2010). It is a little-studied protein, the mutation of which has been shown to cause spastic ataxia (Takiyama 2007). This autosomal recessive spastic ataxia was originally identified in the Charlevoix-Saguenay region of Quebec, where the mutant allele is exceptionally common due to a small founder population. As expected by the prominent motor-related symptoms, Sacsin is expressed primarily in large neurons of the motor system – for example, Betz cells in primary motor cortex and Purkinje cells in the cerebellum (Parfitt et al 2009). Like HHR23A, Sacsin has a UbL domain which interacts with the proteasome. Experiments suggest that Sacsin cooperates with Heat shock protein 70 (Hsp70) to stabilize protein folding and mediate proteasomal degradation of ataxia-related genes. Knockdown of Sacsin in heterologous cells increases the toxicity of polyglutamine-expanded ataxin-1 (the molecular lesion underlying spinocerebellar ataxia type 1)

Loss of ubiquitination would be expected to increase Sacsin levels in AS. As Sacsin appears to protect against toxic aggregates, it is not obvious how heightened Sacsin levels could lead to neuronal dysfunction. However, given Sacsin’s expression in large motor and Purkinje neurons, dysregulation of this protein is a prime candidate for the prominent motor and cerebellar dysfunction in AS.

Blk

Blk (B lymphoid tyrosine kinase) is a Src family nonreceptor tyrosine kinase which dimerizes with receptor tyrosine kinases to activate intracellular signaling cascades (Oda et al 1999). Src family kinases are involved in many cell processes; most relevant to AS, they are highly enriched in nerve growth cones (Helmke et al 1995). The active form is selectively ubiquitinated and degraded by UbE3A (Oda et al 1999). Dysregulated tyrosine kinase activity at growth cones could lead to abnormal neural circuit development.

Mcm7

MCM7 is a member of the minichromosome maintenance (MCM) complex (Lei 2005). The MCM complex moves with the DNA replication fork and likely performs a helicase function, separating the double-stranded DNA so it may be copied. The significance of this gene’s overactivity for AS is indeterminate.

It is probable that other UbE3A targets have yet to be identified, but the targets reviewed above are likely to play very important roles in AS’s pathogenesis.

Bramham CR, Worley PF, Moore MJ, Guzowski JF (2008) “The immediate early gene arc/arg3.1: regulation, mechanisms, and function.” J Neurosci 28(46):11760–11767

Bramham CR, Alme MN, Bittins M, Kuipers SD, Nair RR, Pai B, Panja D, Schubert M, Soule J, Tiron A, Wibrand K. (2010) “The Arc of synaptic memory.” Exp Brain Res. 200(2):125-40.

Dindot SV, Antalffy BA, Bhattacharjee MB, Beaudet AL. (2008) “The Angelman syndrome ubiquitin ligase localizes to the synapse and nucleus, and maternal deficiency results in abnormal dendritic spine morphology.” Hum Mol Genet 17:111–118.

Gordon JA (1997) “Cellular mechanisms of visual cortical plasticity: A game of cat and mouse” Learning and Memory, 4 (3):245-261.

Helmke S, Pfenninger KH. “Growth cone enrichment and cytoskeletal association of non-receptor tyrosine kinases” Cell Motil Cytoskeleton. 1995;30(3):194-207.

Lei M (2005) The MCM complex: its role in DNA replication and implications for cancer therapy. Curr Cancer Drug Targets. 5(5):365-80.

McCurry CL, Shepherd JD, Tropea D, Wang KH, Bear MF, Sur M. (2010) “Loss of Arc renders the visual cortex impervious to the effects of sensory experience or deprivation.” Nat Neurosci. 13(4):450-7.

Oda H, Kumar S, Howley PM. (1999). “Regulation of the Src family tyrosine kinase Blk through E6AP-mediated ubiquitination” PNAS 96:9557–62

Parfitt DA, Michael GJ, Vermeulen EG, Prodromou NV, Webb TR, Gallo JM, Cheetham ME, Nicoll WS, Blatch GL, Chapple JP. (2009) The ataxia protein sacsin is a functional co-chaperone that protects against polyglutamine-expanded ataxin-1. Hum Mol Genet. 18(9):1556-65. Epub 2009 Feb 10.

Park S, Park JM, Kim S, Kim JA, Shepherd JD, Smith-Hicks CL, Chowdhury S, Kaufmann W, Kuhl D, Ryazanov AG, Huganir RL, Linden DJ, Worley PF. (2008) “Elongation factor 2 and fragile X mental retardation protein control the dynamic translation of Arc/Arg3.1 essential for mGluR-LTD.” Neuron. 59(1):70-83.

Plath N, Ohana O, Dammermann B, Errington ML, Schmitz D, Gross C, Mao X, Engelsberg A, Mahlke C, Welzl H, Kobalz U, Stawrakakis A, Fernandez E, Waltereit R, Bick-Sander A, Therstappen E, Cooke SF, Blanquet V, Wurst W, Salmen B, Bosl MR, Lipp HP, Grant SG, Bliss TV, Wolfer DP, Kuhl D (2006) “Arc/Arg3.1 is essential for the consolidation of synaptic plasticity and memories.” Neuron 52(3):437–444

Shamah SM, Lin MZ, Goldberg JL, Estrach S, Sahin M, Hu L, Bazalakova M, Neve RL, Corfas G, Debant A, Greenberg ME. (2001) “EphA receptors regulate growth cone dynamics through the novel guanine nucleotide exchange factor ephexin.” Cell. 105(2):233-44.

Shi L, Butt B, Ip F, Dai Y, Jiang L, Yung W, Greenberg ME, Fu A, Ip NY (2010) “Ephexin1 Is Required for Structural Maturation and Neurotransmission at the Neuromuscular Junction” Neuron 65(2):204-216

Takiyama Y. (2007) Sacsinopathies: sacsin-related ataxia. Cerebellum. :1-7.

Vaegan & Taylor (1979). “Critical period for deprivation amblyopia in children.” Transactions of the Ophthalmological Society (UK) 99: 432-439.

See previous posts on history, symptoms, neurophysiology, and genetics of Angelman Syndrome.

Animal cells have evolved two systems to degrade excess, aged, and abnormal proteins: the ubiquitin-proteasome system and the lysosome (Tai & Schuman 2008). The ubiquitin-proteasome system is responsible for the degradation of most intracellular proteins, while the lysosome degrades most membrane and extracellular proteins.

The ubiquitin-proteasome system mediates essential functions in the nervous system (Tai & Schuman 2008). For example, proteasome inhibitors cause retrograde amnesia in rodents. Ubiquitination and proteasome pathways are essential for the candidate molecular mechanism for memory known as long term potentiation (LTP). Neurons, with their exceptionally large cell volumes and long life (most neurons in the brain are as old as the organism), necessitate especially efficient protein turnover and recycling.

Dysfunction of the ubiquitin-proteasome system has been implicated in the neurodegenerative disorders, such as Alzheimer disease, Parkinson disease, Huntington disease, amyothrophic lateral sclerosis (ALS, a.k.a. Lou Gehrig Disease), and the spinocerebellar ataxias (Lehman et al 2009). In these syndromes aggregates of misfolded proteins collect inside neurons and become resistant to proteasomal degradation. These misfolded proteins may actively inhibit the proteasome, or an upstream mechanism may disrupt proteasome function and permit aggregate formation. Less intuitively, the ubiquitin-proteasome pathway has been incriminated in the autism spectrum disorders (Glessner et al 2009; Lehman et al 2009). This situation makes more sense in light of the recent emergence of the hypothesis that autism is primarily a disorder of protein translation and degradation at dendritic spines (Geschwind 2008, Kelleher & Bear, 2008). Most relevant to the current work, the ubiquitin-proteasome pathway is perturbed in Angelman syndrome.

The proteasome is a 2.5 megadalton macromolecular machine that degrades intracellular proteins (Navon & Clechanover 2009). It consists of a 19S regulatory particle, which binds to ubiquitin and ubiquitin-like moieties to regulate which proteins are degraded, and the 20S core particle, which mediates the amino acid-by-amino acid disassembly of peptides. Proteasomes localize to dendritic spines to precisely regulate synaptic protein content after heightened synaptic activity (Tai & Schuman 2008), and it has recently been shown that this process requires CaM kinase IIα (CaMKIIα) functioning as a scaffolding protein (Bingol et al 2010). As genetic manipulation of CaMKII can rescue the neurological phenotype of the mouse model for AS (van Woerden et al 2007), this molecular connection between CaMKII and the proteasome could represent a essential locus for disease (More in a future post).

Ubiquitination refers to the covalent addition of a 76-amino acid peptide called ubiquitin (called this for historical reasons- was identified in the 1970′s as a protein of unknown function present in all cells) to the ε-amino group of lysine residues on target proteins (Pickart 2001). Key to ubiquitination is the fact that ubiquitin has many lysine residues of its own, so it too can be ubiquitinated, leading to long chains of ubiquitin on targeted proteins. Ubiquitination modulates many key cellular processes by selectively targeting master regulatory proteins for proteasomal degradation. Although degradation is the classic and most common fate for a ubiquitinated protein, ubiquitination can influence proteins in other ways, too, including modulation of ribosomal function, DNA repair, the inflammatory response, and transcription factor binding. Relevant to neuroscientists, ubiquitination can mark a receptor or ion channel for endocytosis (Hicke 1999). Whether ubiquitination signals for degradation or something else probably depends on the length of the polyubiquitin chain, the subcellular localization of the substrate, and the topology of the moiety formed by the ubiquitin-substrate complex (Pickart 2001). For example, polyubiquitin chains linked by K48-G76 bonds are usually destined for proteasomal degradation, whereas chains linked by K63-G76 bonds are implicated in intracellular signaling. Deubiquitinating enzymes (DUBs) depolymerize ubiquitin chains, and can thus reverse a protein’s degradation-fate if there are no proteasomes in close proximity and/or the ubiquitin chain is not long enough (Tai & Schuman 2009).

A dozen ubiquitin-related covalent protein modifiers have been identified in addition to ubiquitin (Jentsch & Pyrowolakis 2000). Most have high homology to ubiquitin but instead of mediating degradation are involved in a wealth of other cellular processes. Sumoylation with SUMO-1 (small ubiquitin-related modifier), for example, can direct proteins to translocate into the nucleus at specific points during the cell cycle. Rubylation with RUB (related to ubiquitin) stabilizes scaffolding proteins for the formation of ubiquitination complexes.

Ubiquitination occurs in a three-step process catalyzed by three ligases: E1, E2, and E3 (Pickart 2001). First, E1 ligase forms a thiol ester with ubiquitin’s G76 carboxyl group, activating the ubiquitin C-terminus. Next, E2 ligase transfers the activated ubiquitin from E1 to E3. Last, E3 ligase catalyzes the covalent linkage of ubiquitin to a protein substrate. Polyubiquitin chain formation sometimes requires an additional E4 ligase. The organization of ligases is hierarchical: There is one E1; several E2s, each of which can load several E3s; and hundreds of E3s, each of which target a few substrate proteins.

E3 ligases therefore determine which proteins are ubiquitinated by recognizing highly specific structural motifs on their target proteins.  The ubiquitination signal recognized by a given E3 is frequently a short primary amino acid sequence. Unfortunately, high-throughput methods for identifying E3 substrates are unavailable, and the specific targets for most E3s are undefined (Tai & Schuman 2008).

E3 ligases subdivide into two classes based on their catalytic domain, the HECT ligases and RING ligases. The HECT (Homologous to E6-AP Carboxy Terminus) domain is a highly conserved E2-binding site. RING ( Really Interesting New Gene) refers to a double-zinc cross-brace structure that also specifically binds E2. The domains, although structurally quite distinct, do not appear to differ significantly in function (Pickart 2001).

By incredible coincidence, ubiquitin E3A ligase (UbE3A), the protein implicated in Angelman Syndrome, was the first HECT ligase to be studied in detail. It was discovered as the E3 ligase which is co-opted by human papillomavirus (HPV) to ubiquitinate and degrade the antineoplastic protein p53. The HPV protein which binds UbE3A to degrade p53 is called E6, and thus UbE3A is also called E6-associated protein (E6-AP).

Until very recently, the normal physiological protein substrates of UbE3A (i.e. when it is not in a complex with E6) were for the most part unknown. The Howley lab showed that HHR23A and the Src kinase Blk are ubiquitinated by UbE3A (Kumar et al 1999; Oda et al 1999), and Banks and Kuhne demonstrates that Mcm7 is ubiquitinated by UbE3A as well (1998).  In the March 2010 issue of Cell, Greer et al. identify the ubiquitination signal of UbE3A and report its protein substrates. In the key experiment, a hemagglutinin-tagged ubiquitin knock-in mouse was crossed with the UbE3A knockout and WT mice. Brain lysate was incubated with anti-hemagglutinin antibody to pull down ubiquitin-bound proteins, which were then identified by mass spectrometry. Any proteins that were pulled down in the WT prep but not the UbE3A prep were likely ubiquitinated specifically by UbE3A. Three novel protein substrates were identified: Arc, Ephexin5, and Sacsin.

UbE3A recognizes these proteins and attaches a ubiquitin to them, which causes them to be degraded. Loss of UbE3A, as in Angelman Syndrome, would prevent these proteins from being degraded and lead to an excess of them in cells. In the next post, I describe the function of these proteins in greater detail to help us think about what the effects of their faulty degradation would be.

Bingol B, Wang CF, Arnott D, Cheng D, Peng J, Sheng M. (2010) “Autophosphorylated CaMKIIalpha acts as a scaffold to recruit proteasomes to dendritic spines.” Cell. 140(4):567-78.

Geschwind DH (2008)“Autism: many genes, common pathways?” Cell. 135(3):391-5.

Glessner JT, Wang K, Cai G (2009) “Autism genome-wide copy number variation reveals ubiquitin and neuronal genes.” Nature 459:569–573

Greer PL, Hanayama R, Bloodgood BL, Mardinly AR, Lipton DM, Flavell SW, Kim TK, Griffith EC, Waldon Z, Maehr R, Ploegh HL, Chowdhury S, Worley PF, Steen J, Greenberg ME. (2010) “The Angelman Syndrome protein Ube3A regulates synapse development by ubiquitinating arc.” Cell. 5;140(5):704-16.

Hicke L. (1999) “Gettin’ down with ubiquitin: turning off cell-surface receptors, transporters and channels.” Trends Cell Biol.9(3):107-12. Review.

Jentsch S & Pyrowolakis G (2000) “Ubiquitin and its kin: how close are the family ties?” Trends Cell Bio 10.

Kelleher RJ, Bear MF. (2008) “The autistic neuron: troubled translation?”  Cell.  135(3):401-6.

Kuhne C, Banks L. (1998) “E3-ubiquitin ligase/E6-AP links multicopy maintenance protein 7 to the ubiquitination pathway by a novel motif, the L2G box.” J. Biol. Chem. 273:34203–9

Kumar S, Talis A, Howley PM. (1999) “Identification of HHR23A as a Substrate for E6-associated Protein-mediated

Ubiquitination” J. Biol. Chem. 274:18785–92

Lehman NL (2009) “The ubiquitin proteasome system in neuropathology.” Acta Neuropathol. 118(3):329-47.

Navon A Ciechanover A (2009) “The 26 S Proteasome: From Basic Mechanisms to Drug Targeting” J. Biol. Chem. 2009 284: 33713-33718.

Oda H, Kumar S, Howley PM. (1999). “Regulation of the Src family tyrosine kinase Blk through E6AP-mediated ubiquitination” PNAS 96:9557–62

Pickart CM (2001) “Mechanisms underlying ubiquitination.” Annu Rev Biochem. 70:503-33. Review.

Tai & Schuman (2008) “Ubiquitin, the proteasome and protein degradation in neuronal function and dysfunction” Nat Rev Neuro.

van Woerden, G.M., Harris, K.D., Hojjati, M.R., Gustin, R.M., Qiu, S., de Avila Freire, R., Jiang, Y.H., Elgersma, Y. and Weeber, E.J. (2007) “Rescue of neurological deficits in a mouse model for Angelman syndrome by reduction of alphaCaMKII inhibitory phosphorylation.” Nat. Neurosci., 10, 280–282.

One of the leading aphorisms in the nomination of justices for the Supreme Court is “Younger is Better.” More years on the court, so goes this view, means that a president can exert a deeper and more durable political influence on the This view has persisted, and will persist, despite these decade-old results in Kosma (1998):

This empirical study measures the influence of 99 retired Supreme Court justices, analyzing over 1.2 million citations to over 24,000 opinions of the Court written between 1793 and 1991. It models the appointment process as the selection of a capital investment, treating a justice’s output as the precedents generated each term and using citations as a proxy for an opinion’s value. This model is applied to the retired justices and their opinions, and its consistency is tested by independently analyzing citations by subsequent Supreme Court and circuit court opinions. . . . Older appointees have been no less influential than young appointees, and, on an annual basis, older appointees have actually been more influential.

Huck, Kirksteiger, & Oechssler (2005) present an evolutionary model of the emergence of an endowment effect. Their model assumes two agent types. One starts with one divisible unit of good x and another starts with one divisible unit of good y. Then they can trade. F(x,y), evolutionary fitness, is strictly concave. U(x,y) is subjective utility; it can differ from F(x,y) because x-agents can put extra subjective value on x-goods and y-agents can do so on y-goods. A positive endowment effect improves the threat point in an agent’s behavior but distorts the consumption mix away from optimality. Because of concavity of F, however, the threat point effect outweighs the distortion. The model therefore predicts a positive endowment effect to emerge and become evolutionarily stable under these conditions.

In the conclusion, the authors comment that

This is quite different from the observation that individuals may have strategic incentives to lie about their true preferences. As convincingly argued by Frank (1988) it is not always possible to credibly signal preferences which one does not hold. In our setting individuals behave sincerely according to their preferences. Neither do they lie nor do they commit themselves to non–credible threats. They simply develop an endowment effect because individuals with an endowment effect end up with more resources and therefore higher fitness. Note, however, that overall the endowment effect causes an inefficiency since there is a suboptimal amount of trade. Feasible allocations which would be mutually beneficial in terms of fitness are not implemented due to the bias in preferences.

It is important to notice that once evolution has brought forth preferences with endowment effects, individuals will reveal their endowment effects not only in bilateral trade but also in incentive compatible market situations. . . .

While the experimental evidence for the existence of an endowment effect is overwhelming, it is not straightforward to test implications of our theory that go beyond this empirically. A possible solution is to test the predictions for animals. There is a large literature documenting that animals fight much harder for objects in their possession, like territories, food or females; see e.g. Maynard Smith (1982 ch. 8). For example, territorial behaviour has been observed among bees, butterflies, baboons, fish, lions, and others. This is often interpreted as a form of an endowment effect (see e.g. Friedman, 2003). Of course, animals do not engage in bargaining as we know it from humans but their behaviour has the flavour of bargaining. A fight among lions is very costly in expectation for both sides. Thus most of the time, a fight for the size of the one’s territory is settled before it really starts, which seems to be efficient (just like the Nash bargaining solution). Thus, a testable implication should be that animals that do not have conflicts about territories etc. should not have an endowment effect.

I clicked to the Balfour web site yesterday to order graduation announcements. Check out this home page:

http://www.balfour.com/

Balfour sells class rings and other paraphernalia associated with high school, college, sports championships, and the military. Put another way, Balfour’s business model is to exploit human tribalistic instincts by selling expensive ritualistic symbols of the only tribal commitments that Americans really buy into.

See previous posts on history, symptoms, and neurophysiology of Angelman Syndrome.

The genetic and epigenetic lesions which lead to Angelman Syndrome (AS) have been studied in great detail and provide a window into the extraordinary complexity of genetic systems.

First, a review of the genetics (Butler 2009). Every cell in the body has two complete copies of the genome (with the exception of the sex chromosomes), 23 chromosomes from the father and 23 chromosomes from the mother. When chromosomes are passed on from parent to offspring, they are encoded with a distinct methylation pattern depending on if they are packaged in the egg (from the mother) or sperm (from the father). Methylation patterns arise from the action of DNA methyltransferases, which attach methyl groups (a carbon bound to three hydrogens) to cytosine nucleotides at specific places in the genome. Consequently, cells in the offspring ‘know’ which of each chromosome pair is from the mother or father. For most proteins, this methylation pattern does not matter – genes from both parents are expressed equally (this is called biallelic expression). For a subset of genes (~1%), however, only the maternally- or paternally-inherited allele is expressed. Gene imprinting represents a mechanism of gene regulation which does not originate in the animal’s genetic code and is thus an example of epigenetics.

The evolutionary significance of genomic imprinting is unknown. As parent-specific imprinting evolved in mammals about 150 million years ago, it has been hypothesized that imprinting arose from parental conflict: “Paternally expressed genes that increase the growth of the offspring, even at the expense of the mother, will increase the genetic fitness of the father. Conversely, maternally expressed genes that restrict resources provided to any one young, allowing her to carry successive pregnancies, will increase her genetic fitness” (Renfree et al 2009). In line with this theory, almost all imprinted genes identified thus far affect embryonic growth and nutrition. Further, imprinting is present in placental mammals and marsupials, but not the egg-laying monotremes. Imprinting also could have evolved as a mechanism to inhibit asexual reproduction and the subsequent deleterious effect of homozygous recessive mutations (Das et al 2009). It may allow the maternal and paternal chromosomes to specialize on different developmental transcription programs within cells, or it may have encouraged the mutation of new traits by enabling a single recessive mutation (on the active allele) to mediate a dominant phenotype.  Last, genomic imprinting could have evolved as gene dosage became more important in cell physiology. This last theory comports with the recent acceleration of segmental genomic duplications during primate evolution (Marques-Bonet et al 2009).

Selective imprinted gene expression is especially common in the brain, and imprinted genes have been disproportionately linked to neuropsychiatric disorders (Butler 2009). Over 100 conditions have been associated with imprinting abnormalities, including Angelman syndrome, Prader-Willi syndrome, Silver-Russell syndrome, Beckwith-Wiedemann syndrome, Albright hereditary osteodystrophy, and uniparental disomy 14.

The q11-13 region of chromosome 15 contains a cluster of imprinted genes. Two of the genes, UbE3A and ATP10C, are maternally-expressed, meaning that only the maternally-inherited allele is active.  70% of Angelman syndrome cases are caused by a deletion of the maternally-inherited version of this chromosomal region, which leads to a lack of functional UbE3A and the symptoms of AS (Dan 2009). Another 5-10% of cases stem from an isolated deletion in the maternally-inherited UbE3A gene, and 3-5% of cases have an imprinting defect resulting in lack of the maternal pattern of DNA methylation required for UbE3A expression. 2-3% of cases are caused by uniparental disomy, a rare condition in which both copies of chromosome 15 are inherited from the father and none from the mother. 1-2% are due to complex genomic abnormalities which lead to the inactivation of UbE3A. In the remaining 10% of cases, etiology remains undetermined.

Although UbE3A disruption appears to be necessary for the symptoms of AS, it may not be sufficient. It is possible that isolated mutation of UbE3A only causes AS in concert with other mutations; this would explain why most cases are due to regional deletion, as many of the non-imprinted genes in the region, including three GABA receptor genes, have important roles in neural function and have been linked to neuropsychiatric disorders. Until a greater number of healthy control genomes are mapped, the prevalence of inactivating UbE3A mutations in the normal population will not be known.

Interestingly, inheritance of the same chromosome 15q11-13 deletion that causes AS, but from the father instead of the mother, causes a completely different disorder known as Prader-Willi syndrome (PWS) (Butler 2009). The most conspicuous phenotype of this disorder is an appetite so voracious that parents must put locks on the pantry and refrigerator. Perhaps as expected, PWS patients are almost unanimously obese. Other signs include mental retardation, hyperactivity, self-injury, short stature, small hands and feet, and characteristic facial features. At least a dozen genes in this region are paternally-expressed imprinted genes, including SNURF-SNPRN, NDN, MKRN3, MAGEL2, and several small nucleolar RNAs (snoRNAs – non-coding RNAs which guide the assembly of ribosomal RNA and transfer RNA (Bachellerie et al 2002)) . None of these genes by itself has yet been shown to be pathogenic for PWS.

A genome-wide microarray study of 859 autism spectrum syndrome (ASD) cases linked copy-number variations in UbE3A, along with three other ubiquitin-pathway genes (PARK2, RFWD2, and FBXO40),  to autism (Glessner et al 2009). Eight ASD samples exhibited duplications in the 15q11-13 regions, compared to none of the control samples. In line with these findings, autistic features are actually disproportionately common in AS (1.9%) and PWS (25.3%) patients compared to the general population (~0.67%) (Veltman et al 2005).

The 15q11-13 region and UbE3A in particular are highly relevant to neuropsychiatric disorders. In the next section, I delve into UbE3A’s molecular biology to gain insight into AS pathogenesis.

Bachellerie, JP; Cavaille J, Huttenhofer A (2002). “The expanding snoRNA world”. Biochimie 84: 775–790.

Butler M (2009) “Genomic imprinting disorders in humans: a mini-review” J Assist Reprod Genet 26:477–486.

Das R, Hampton DD, Jirtle RL. (2009) “Imprinting evolution and human health.” Mamm Genome. 20(9-10):563-72.

Glessner JT, Wang K, Cai G (2009) “Autism genome-wide copy number variation reveals ubiquitin and neuronal genes.” Nature 459:569–573

Marques-Bonet T, Girrajan S, Eichler EE (2009) “The origins and impact of primate segmental duplications” Trends Genetics 25(10).

Renfree, M. B., Papenfuss, A. T., Shaw, G., and Pask, A. J. (2009). “Eggs, embryos and the evolution of imprinting: insights from the platypus genome.” Reprod. Fertil. Dev. 21, 935–942.

Veltman, M.W., Craig, E.E. and Bolton, P.F. (2005) “Autism spectrum disorders in Prader–Willi and Angelman syndromes: a systematic review.” Psychiatr. Genet. 15: 243–254.

See previous posts on an intro to and symptoms of Angelman Syndrome.

One of the more notable features of Angelman Syndrome (AS) is the syndrome’s pathognomonic neurophysiological findings. The electroencephalogram (EEG) in AS is usually very abnormal, and more abnormal than clinically expected (Williams 2005). Three distinct interictal patterns are seen in these patients (Fig. 1; normal EEG in Fig. 2 for comparison) (Dan 2009). The most common pattern is a very large amplitude 2-3 Hz rhythm most prominent in prefrontal leads (Fig. 1 A). Next most common is a symmetrical 4-6 Hz high voltage rhythm (Fig. 1 B). The third pattern, 3-6 Hz activity punctuated by spikes and sharp waves in occipital leads, is associated with eye closure (Fig. 1 C). Paroxysms of laughter have no relation to the EEG, ruling out this feature as a gelastic phenomenon (Williams 2005).

Fig. 1 The three distinct EEG patterns seen in AS. Courtesy Dan 2009

Fig. 2 EEG from a normal subject. Courtesy MGH website

Epileptic seizures occur in 80% of patients with AS (Clay-Smith & Laan 2003). Many different types of seizures have been reported, including absence, myoclonic, atonic, and tonic-clonic seizures (Pelc et al 2008). Seizures commonly progress to nonconvulsive status epilepticus.

Pelc K, Boyd SG, Cheron G, Dan B. (2008b) “Epilepsy in Angelman syndrome.” Seizure 17:211–217.

Other citations: see previous posts

An article in the New York Times Magazine about the moral psychology of babies has the following excerpt:

In the journal Science a couple of months ago, the psychologist Joseph Henrich and several of his colleagues reported a cross-cultural study of 15 diverse populations and found that people’s propensities to behave kindly to strangers and to punish unfairness are strongest in large-scale communities with market economies, where such norms are essential to the smooth functioning of trade. Henrich and his colleagues concluded that much of the morality that humans possess is a consequence of the culture in which they are raised, not their innate capacities.

At the same time, though, people everywhere have some sense of right and wrong. You won’t find a society where people don’t have some notion of fairness, don’t put some value on loyalty and kindness, don’t distinguish between acts of cruelty and innocent mistakes, don’t categorize people as nasty or nice. These universals make evolutionary sense. Since natural selection works, at least in part, at a genetic level, there is a logic to being instinctively kind to our kin, whose survival and well-being promote the spread of our genes. More than that, it is often beneficial for humans to work together with other humans, which means that it would have been adaptive to evaluate the niceness and nastiness of other individuals. All this is reason to consider the innateness of at least basic moral concepts.

In addition, scientists know that certain compassionate feelings and impulses emerge early and apparently universally in human development. These are not moral concepts, exactly, but they seem closely related. . . .

There seems to be something evolutionarily ancient to this empathetic response. If you want to cause a rat distress, you can expose it to the screams of other rats. Human babies, notably, cry more to the cries of other babies than to tape recordings of their own crying, suggesting that they are responding to their awareness of someone else’s pain, not merely to a certain pitch of sound. Babies also seem to want to assuage the pain of others: once they have enough physical competence (starting at about 1 year old), they soothe others in distress by stroking and touching or by handing over a bottle or toy. There are individual differences, to be sure, in the intensity of response: some babies are great soothers; others don’t care as much. But the basic impulse seems common to all. . . .

Some recent studies have explored the existence of behavior in toddlers that is “altruistic” in an even stronger sense — like when they give up their time and energy to help a stranger accomplish a difficult task. The psychologists Felix Warneken and Michael Tomasello have put toddlers in situations in which an adult is struggling to get something done, like opening a cabinet door with his hands full or trying to get to an object out of reach. The toddlers tend to spontaneously help, even without any prompting, encouragement or reward. . . .

What do these findings about babies’ moral notions tell us about adult morality? Some scholars think that the very existence of an innate moral sense has profound implications. In 1869, Alfred Russel Wallace, who along with Darwin discovered natural selection, wrote that certain human capacities — including “the higher moral faculties” — are richer than what you could expect from a product of biological evolution. He concluded that some sort of godly force must intervene to create these capacities. (Darwin was horrified at this suggestion, writing to Wallace, “I hope you have not murdered too completely your own and my child.”)

A few years ago, in his book “What’s So Great About Christianity,” the social and cultural critic Dinesh D’Souza revived this argument. He conceded that evolution can explain our niceness in instances like kindness to kin, where the niceness has a clear genetic payoff, but he drew the line at “high altruism,” acts of entirely disinterested kindness. For D’Souza, “there is no Darwinian rationale” for why you would give up your seat for an old lady on a bus, an act of nice-guyness that does nothing for your genes. And what about those who donate blood to strangers or sacrifice their lives for a worthy cause? D’Souza reasoned that these stirrings of conscience are best explained not by evolution or psychology but by “the voice of God within our souls.”

The evolutionary psychologist has a quick response to this: To say that a biological trait evolves for a purpose doesn’t mean that it always functions, in the here and now, for that purpose. Sexual arousal, for instance, presumably evolved because of its connection to making babies; but of course we can get aroused in all sorts of situations in which baby-making just isn’t an option — for instance, while looking at pornography. Similarly, our impulse to help others has likely evolved because of the reproductive benefit that it gives us in certain contexts — and it’s not a problem for this argument that some acts of niceness that people perform don’t provide this sort of benefit. (And for what it’s worth, giving up a bus seat for an old lady, although the motives might be psychologically pure, turns out to be a coldbloodedly smart move from a Darwinian standpoint, an easy way to show off yourself as an attractively good person.)

The general argument that critics like Wallace and D’Souza put forward, however, still needs to be taken seriously. The morality of contemporary humans really does outstrip what evolution could possibly have endowed us with; moral actions are often of a sort that have no plausible relation to our reproductive success and don’t appear to be accidental byproducts of evolved adaptations. Many of us care about strangers in faraway lands, sometimes to the extent that we give up resources that could be used for our friends and family; many of us care about the fates of nonhuman animals, so much so that we deprive ourselves of pleasures like rib-eye steak and veal scaloppine. We possess abstract moral notions of equality and freedom for all; we see racism and sexism as evil; we reject slavery and genocide; we try to love our enemies. Of course, our actions typically fall short, often far short, of our moral principles, but these principles do shape, in a substantial way, the world that we live in. It makes sense then to marvel at the extent of our moral insight and to reject the notion that it can be explained in the language of natural selection. If this higher morality or higher altruism were found in babies, the case for divine creation would get just a bit stronger.

But it is not present in babies. In fact, our initial moral sense appears to be biased toward our own kind. There’s plenty of research showing that babies have within-group preferences: 3-month-olds prefer the faces of the race that is most familiar to them to those of other races; 11-month-olds prefer individuals who share their own taste in food and expect these individuals to be nicer than those with different tastes; 12-month-olds prefer to learn from someone who speaks their own language over someone who speaks a foreign language. And studies with young children have found that once they are segregated into different groups — even under the most arbitrary of schemes, like wearing different colored T-shirts — they eagerly favor their own groups in their attitudes and their actions. . . .

The aspect of morality that we truly marvel at — its generality and universality — is the product of culture, not of biology. There is no need to posit divine intervention. A fully developed morality is the product of cultural development, of the accumulation of rational insight and hard-earned innovations. The morality we start off with is primitive, not merely in the obvious sense that it’s incomplete, but in the deeper sense that when individuals and societies aspire toward an enlightened morality — one in which all beings capable of reason and suffering are on an equal footing, where all people are equal — they are fighting with what children have from the get-go.

In his original report “’Puppet Children’: A report on three cases” (1965), Angelman identified many of the key features of Angelman syndrome (AS):

Their flat heads, jerky movements, protruding tongues and bouts of laughter give them a superficial resemblance to puppets, an unscientific name but one which may provide for easy identification. It will be seen that all these children possess a number of characteristic features in common and may be summarised as follows:

1. A horizontal depression in the occipital region of the skull, present at birth. Also brachycephaly associated with microcephaly, becoming more obvious as growth proceeds, but not due to premature fusion of the coronal sutures.
2. A varying degree of primary optic atrophy, associated with incomplete development of the choroid.
3. Abnormal air encephalograms indicating some degree of cerebral atrophy associated with ventricular dilatation.
4. Very frequent fits resembling a hypsarrhythmic state and a profound degree of mental retardation.
5. Easily provoked and prolonged paroxysms of laughter.
6. Ataxia, with weakness of the limbs and trunk resembling that seen in cerebellar deficiency.
7. Ability to protrude the tongue to an unusual degree.

Add to this a profound developmental delay, speech impairment, and abnormal sleep patterns, and a near-complete picture of Angelman syndrome is formed (Figure 1) (Dan 2009).

Figure 1 – Children with Angelman Syndrome. Images courtesy (Clay-Smith & Laan 2003)

As AS patients reach adulthood, their behavior becomes quieter and they are less hyperactive, but their sociability and easy laughter persists (Clay-Smith & Laan 2003). Some adult AS patients become aggressive, especially if frustrated by communication difficulties. Puberty occurs normally in AS patients, and some have intact fertility. The distinct AS facial features become even more pronounced. Most patients develop muscular rigidity and scoliosis, and many patients become obese.

Inspection of the gross neuroanatomy reveals a small brain with some atrophy most prominent in the cerebellum (Jay et al 1991). Golgi staining of AS cortex shows a prominent decrease in dendritic arborization and spine number in layer 3 and 5 pyramidal neurons. Decreased GABA levels are present in cerebellum, and increased glutamate levels are present in cerebral cortex.

Angelman syndrome is an uncommon neurodevelopmental syndrome, with an estimated prevalence of 1-in-10,000 to 1-in-12,000 individuals (Petersen et al 1995; Steffenberg et al 1996). The only potential risk factor identified for Angelman Syndrome is assisted reproductive technology i.e. in vitro fertilization, but given the rarity of AS and imprinting disorders in general there is currently insufficient evidence to establish the association (Owen & Segars 2009).

There are as yet no good treatments for AS. Seizures can be treated with antiepileptic medication, but they are conspicuously difficult to manage (Williams 2005). Physical therapy may help with the motor deficits. Melatonin can be prescribed to minimize night-time sleeplessness.

Clayton-Smith J, Laan L. (2003) “Angelman syndrome: a review of the clinical and genetic aspects.” J Med Genet. 40(2):87-95. Review.

Dan B. (2009) “Angelman syndrome: current understanding and research prospects.” Epilepsia. 50(11):2331-9.

Jay V, Becker LE, Chan FW, Perry TL Sr.(1991) “Puppet-like syndrome of Angelman: a pathologic and neurochemical study. Neurology. 41(3):416-22.

Owen CM, Segars JH (2009) “Imprinting disorders and assisted reproductive technology.” Semin Reprod Med. 27(5):417-28

Petersen MB, Brondum-Nielsen K, Hansen LK, Wulff K. (1995) “Clinical, cytogenetic, and molecular diagnosis of Angelman syndrome: estimated prevalence rate in a Danish county.” Am J Med Genet 60:261–2.

Steffenburg S, Gillberg CL, Steffenburg U, Kyllerman M. (1996) “Autism in Angelman syndrome: a population-based study.” Pediatr Neurol 14:131–6.

Williams C (2005) “Neurological aspects of the Angelman syndrome” Brain & Development 27: 88–94

The human genome contains over 25000 genes encoding a dizzying number of proteins. These proteins together produce all of the biological processes necessary to give us life. The great majority of these genes are of unknown function, and finding a handhold to begin to grasp a gene’s role is surprisingly difficult, especially in the case of genes controlling nervous system development and operation. Spontaneous mutations in these genes provide just such a handhold.

It is a general property of DNA that it mutates in a somewhat random fashion. Any given person probably harbors about 175 new mutations in his/her genome, most or all of which are phenotypically insignificant. In some cases a mutation can alter or disrupt the function of a protein and cause a visible change in the individual’s anatomy or physiology. By identifying individuals which express common patterns of symptoms and screening their DNA, physician-scientists have been able to identify many genes which are involved in nervous system function. Dr. Harry Angelman’s recognition of his eponymous syndrome provides just such an example. Angelman reflected on his discovery of the Happy Puppet syndrome in a personal letter to his friend Dr. Charles Williams in 1991:

“The history of medicine is full of interesting stories about the discovery of illnesses. The saga of Angelman’s syndrome is one such story. It was purely by chance that nearly thirty years ago (e.g., circa 1964) three handicapped children were admitted at various times to my children’s ward in England. They had a variety of disabilities and although at first sight they seemed to be suffering from different conditions I felt that there was a common cause for their illness. The diagnosis was purely a clinical one because in spite of technical investigations which today are more refined I was unable to establish scientific proof that the three children all had the same handicap. In view of this I hesitated to write about them in the medical journals. However, when on holiday in Italy I happened to see an oil painting in the Castelvecchio museum in Verona called . . . a Boy with a Puppet [Fig. 1]. The boy’s laughing face and the fact that my patients exhibited jerky movements gave me the idea of writing an article about the three children with a title of Puppet Children. It was not a name that pleased all parents but it served as a means of combining the three little patients into a single group. Later the name was changed to Angelman syndrome. This article was published in 1965 and after some initial interest lay almost forgotten until the early eighties.”

It was not until the 1990’s that Angelman Syndrome was convincingly linked to genetic abnormalities in a specific region of chromosome 15 (Nichols 1993), and it was not until 1997 that mutations in the gene encoding ubiquitin E3A ligase (UbE3A, also known as E6-associated protein, E6AP) were shown to be sufficient to cause the disorder (Kishino et al 1997). Since these seminal discoveries, our understanding of the cellular and molecular changes that lead to the symptoms of Angelman Syndrome has advanced considerably.

Angelman Syndrome’s distinct symptom profile, neurophysiology, epigenetics, molecular biology, and systems pathophysiology make it an especially fascinating disorder from the perspective of the neurobiology of disease. In the following several posts, I will review these aspects of the disorder.

Kishino T, Lalande M, Wagstaff J. (1997) “UBE3A/E6-AP mutations cause Angelman syndrome.” Nat Genet 15:70–73.

Nicholls, R.D. (1993) “Genomic imprinting and candidate genes in the Prader-Willi and Angelman syndromes.” Curr. Opin. Genet. Dev. 3: 445−456.