Advances in Autism Genetics: on the Threshold of a New Neurobiology
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REVIEWS Advances in autism genetics: on the threshold of a new neurobiology Brett S. Abrahams and Daniel H. Geschwind Abstract | Autism is a heterogeneous syndrome defined by impairments in three core domains: social interaction, language and range of interests. Recent work has led to the identification of several autism susceptibility genes and an increased appreciation of the contribution of de novo and inherited copy number variation. Promising strategies are also being applied to identify common genetic risk variants. Systems biology approaches, including array-based expression profiling, are poised to provide additional insights into this group of disorders, in which heterogeneity, both genetic and phenotypic, is emerging as a dominant theme. Gene association studies Autistic disorder is the most severe end of a group of into the ASDs. This work, in concert with important A set of methods that is used neurodevelopmental disorders referred to as autism technical advances, made it possible to carry out the to determine the correlation spectrum disorders (ASDs), all of which share the com- first candidate gene association studies and resequenc- (positive or negative) between mon feature of dysfunctional reciprocal social interac- ing efforts in the late 1990s. Whole-genome linkage a defined genetic variant and a studies phenotype of interest. tion. A meta-analysis of ASD prevalence rates suggests followed, and were used to identify additional that approximately 37 in 10,000 individuals are affected1. loci of potential interest. Although the application of Whole-genome linkage study ASDs encompass several clinically defined conditions genome-wide techniques to assess copy number variation A statistical evaluation of (see BOX 1 and the Diagnostic and Statistical Manual of (CNV) has only just begun3–5, these studies have already genetic variation throughout Mental Disorders), pervasive developmental disorder identified a large number of potentially important novel the genome that is used to identify polymorphic loci that — not otherwise specified and autistic disorder are the candidate loci. segregate with a phenotype of most common, whereas Asperger syndrome appears less Thus, in contrast to the complete absence of any interest. frequently. Boys are at increased risk for the ASDs, an biological understanding of the ASDs as recently as 30 effect that becomes even more pronounced in so-called years ago, we now know that defined mutations, genetic Copy number variation (CNV). The insertion or deletion high-functioning cases. syndromes and de novo CNV account for about 10–20% of a relatively large DNA A chronological overview of research in the ASDs of ASD cases (TABLE 1; Supplementary information S1 fragment (>50 kb). underscores the short history of genetic work in this (table)). However, the striking finding that none of these area as well as the diversity of the methods used. Before known causes accounts for more than 1–2% of cases is the 1970s, autism was not widely appreciated to have a reminiscent of mental retardation (MR), an overlapping strong biological basis. Instead, various psychodynamic but distinct neurodevelopmental syndrome for which interpretations, including the role of a cold or aloof there is no single major genetic cause, but rather many style of mothering, were invoked as potential causes. relatively rare mutations. Despite this heterogeneity in Programs in Neurogenetics and Neurobehavioural The importance of genetic contributions became clear the ASDs, several biological themes, including defective 6 7 Genetics, Neurology in the 1980s, when the co-occurrence of chromosomal synaptic function and abnormal brain connectivity Department, and Semel disorders and rare syndromes with the ASDs were (BOX 3), have been hypothesized to link rare and com- Institute for Neuroscience noted2. Subsequent twin and family studies provided mon variants at the level of biological function. Still, the and Behaviour, David Geffen School of Medicine, University additional support for a complex genetic aetiology, but relative proportion of ASDs that are explained by either of California at Los Angeles, these were limited by the lack of uniform diagnostic rare or common genetic variation (or both) remains to Los Angeles, California criteria. The development of validated diagnostic and be determined. 90095-1769 USA. assessment tools in the early 1990s, most notably the In the face of this uncertainty, multiple parallel Correspondence to B.S.A or Autism Diagnostic Interview — Revised (ADI-R) and approaches are necessary to advance our understand- D.H.G e-mails: [email protected]; the Autism Diagnostic Observation Schedule (ADOS), ing of the genetic factors underlying the ASDs. These [email protected] addressed these concerns and these tools have proven approaches include whole-genome and pathway-based doi:10.1038/nrg2346 crucial to the advancement of international research association studies, dense resequencing to identify natURE REviEWS | GENETICS voLUME 9 | MAY 2008 | 1 REVIEWS 8 Relative risk mutations, and the continued collection of large well- affected . Third, siblings and parents of an affected child The ratio of disease incidence characterized patient cohorts and their relatives for are more likely than controls to show subtle cognitive in two groups of individuals genotype–phenotype studies. Here we review this excit- or behavioural features that are qualitatively similar that differ with regards to any ing and rapidly evolving field in which diverse genetic to those observed in probands9,10 (the broader autism associated factor (such as genetic polymorphism, findings have begun to define potential biological phenotype); this is consistent with the segregation of environmental exposure or mechanisms of disease. quantitative sub-threshold traits within these families. perinatal insult). Fourth, independent twin studies, although small, ASD as a complex genetic trait indicate that concordance rates for monozygotic twins Community-based cohort Autism has a strong genetic basis. Several lines of evi- (70–90%) are several-fold higher than the correspond- A group of individuals that 11,12 have been selected randomly dence support genetic factors as a predominant cause ing values for dizygotic twins (0–10%) . An important from a population (as opposed of the ASDs. First is the growing body of literature question for future work will be to clarify how environ- to those showing a specific demonstrating that mutations or structural variation mental and genetic factors interact to influence risk and phenotype of interest). in any of several genes can dramatically increase dis- presentation (BOX 1). ease risk. Second, the relative risk of a child being diag- nosed with autism is increased at least 25-fold over the Genetic models of ASDs. Central to this question of how population prevalence in families in which a sibling is genetic variation comes to influence phenotypic pres- entation is whether distinct aspects of ASDs are subject to independent genetic modulation or, alternatively, Box 1 | Classification and prevalence of ASDs they are sensitive to largely overlapping risk factors. community-based cohort Specific impairments in each of three core domains before age three are required Recent work in a revealed that, for a diagnosis of autistic disorder (13 per 10,000). Within the social domain, despite high heritability values (>0.64) for social, com- 13 impaired use of nonverbal communication (facial expressions or body language) or a munication and repetitive and/or restrictive domains , reduction in spontaneous attempts to share interests with others are common. only modest co-variation was observed between them. Features in the language domain manifest as delayed or absent speech or Similarly, individuals with extreme scores in one difficulties initiating or sustaining a conversation. Abnormalities in the restricted domain did not necessarily have extreme scores in and/or repetitive domain can present as abnormal preoccupations, inflexible others. Additional support for this oligogenic model, adherence to routines or rituals, or repetitive motor behaviours. Males are over- in which disease results from the combined action of represented compared with females (approximately 4:1), an effect that is seemingly multiple interacting genes, comes from linkage studies unrelated to X-linked genetic variation. Interestingly, this male-to-female ratio that have identified distinct loci for endophenotypes that approaches 1:1 when only severe cases of autistic disorder are considered. 14–16 Additional terms are useful for describing affected children on the basis of are related to different core domains . Such results phenotypic presentation, although one should note that diagnosis in the autism are also in keeping with the view that several risk alleles spectrum disorders (ASDs) is in many cases complicated by the presence of severe act together to modulate risk in families with multiple cognitive delay. Individuals with Asperger syndrome (2.6 per 10,000) show affected siblings8. impairments in the social and restricted and/or repetitive domains, but most use At the same time, it should be recognized that the language in an age-appropriate manner and are not mentally retarded. Males are relationship between core domains in the ASDs might also over-represented among these cases (approximately 8:1). Individuals with not be properly reflected in community-based samples. pervasive developmental disorder — not otherwise specified (PDD-NOS;