The Ash Twins Blog

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.