Autism spectrum disorders are a group of developmental syndromes characterized by fundamental deficits in social communication and language development and accompanied by highly restricted interests, stereotyped repetitive behaviors, or both. The canonical presentation is defined by deficits in all three domains, whereas disorders along the spectrum involve one or more core impairments (Volkmar et al., 2009
The evidence for a genetic contribution to ASD is very strong, based on family and twin studies (Bailey et al., 1995
; Folstein and Rutter, 1977
), the overlap of ASD with known genetic disorders (Fombonne et al., 1997
), and recent molecular data (addressed below). There is a strong male predominance (Volkmar et al., 2004
). Despite ambitious efforts aimed at identifying common and rare alleles, the number of genes or loci that are accepted as carrying definitive risk remains small, and not immune from debate. In contrast, the list of intriguing and plausible candidates (see http://www.mindspec.org/auTSb.html
) is proliferating exponentially; a phenomenon that can make deciphering the current literature quite difficult.
The absence of specific and sensitive biological markers for ASD, and the consequent reliance on syndromic categorization and subjective assessment presents predictable challenges, as it does in all areas of psychiatric diagnosis (Volkmar et al., 2009
). These issues extend well beyond the scope of the current discussion; however a key phenomenological question that bears directly on the interpretation of the autism genetics literature involves the overlap of ASD and intellectual disability (ID), which is present in approximately 70% of individuals meeting full diagnostic criteria for autism (Chakrabarti and Fombonne, 2001
), and approximately 45%–50% for the entire range of ASD diagnoses.
It is important to note here that there is little debate that social impairment and cognitive delay are readily distinguishable in individuals with mild to moderate ID. While this differentiation becomes less reliable in cases of severe to profound delay, debate over this co-occurrence runs deeper and reflects a historical interest in identifying specific risks for autism as opposed to “general disruptions of brain development.” This issue is similarly present with regard to the study of individuals with ASD and seizure, present in 10–25% of individuals with autism (Volkmar and Pauls, 2003
) and is particularly relevant to the study of syndromic autism i.e. ASD in the context of a known genetic syndrome or observed in individuals presenting with marked dysmorphology, structural brain abnormalities or other evidence of a genetic syndrome, but absent a known cause.
In part these questions reflect an ongoing debate regarding whether social disability, characterized in the context of known genetic disorders, is identical to that observed in idiopathic ASD (Moss and Howlin, 2009
), an issue that may be difficult to resolve given the challenges inherent in designing blinded studies of syndromes characterized by distinctive physical features. However, there is a second line of reasoning, based on twin and epidemiological data, that contends that the high rate of ASD in individuals with MR among clinically ascertained populations is an epiphenomenon resulting from a normal distribution of autism traits in the population which is made manifest by the relatively poor “compensatory mechanisms” in individuals with ID (). This view proposes that discovery efforts in cases of overlapping ID an ASD will identify genes for the former; that a different set of genes underlie social ability and disability; and that the optimal strategy for gene discovery with regard to autism would focus on the population of ASD without intellectual disability (Skuse, 2007
Figure 2 Two models of the overlap of ASD and ID: 2a represents a paradigm well articulated by Skuse (2007) which holds that the study of syndromic ASD is likely to identify genes resulting in ID and that co-occuring ASD is a consequence of a reduced compensatory (more ...)
While these debates are ongoing, several considerations deserve mention: First in addition to the increasing application of standardized instruments and blinding methods to enhance the diagnostic reliability of ASD in the context of known syndromes, the not infrequent detection of syndromic mutations in individuals with idiopathic ASD is evidence that these variations may lead to behavioral phenomenon that are, for all practical purposes, indistinguishable from idiopathic ASD. Moreover, given that autism and related conditions are syndromic diagnoses and manifestly not unitary biological entities, there would seem to be little question that individuals that are rigorously characterized with ASD have ASD. The more difficult question of whether it is a worthwhile strategy to search for variants contributing to ASD in the context of ID and to pursue the biology of rare syndromes in an effort to understand autism remains contested. As noted above, a central rationale for outlier strategies is that a genetic variation that is definitively identified as causing or contributing to ASD with ID or syndromic ASD offers potential traction with regard to cellular and molecular mechanisms of pathogenesis in those cases. The question of whether this biology is generalizeable remains to be demonstrated. However, there is already solid evidence supporting the use of identical strategies in other common complex and heterogeneous conditions (Ji et al., 2008
; Romeo et al., 2009
). Moreover, as discussed in more depth below, current evidence already points to some convergence in the molecular mechanisms implicated through the study of syndromic forms of ASD and those suggested by genes that have the shown the strongest evidence so far for contribution to idiopathic ASD (Bourgeron, 2009
; Toro et al., 2010
). Finally, the observation that identical mutations may lead not only to ID and ASD, but to Schizophrenia and possibly other neuropsychiatric disorders (addressed below) would seem to argue for a model based on the plieotropy of genes underlying fundamental neuronal processes (), a phenomena that is highlighted by recent findings from our group with regard to the genetics of structural brain disorders (Bulgivar et al, 2010).
Early and Rare Variant Findings
These debates bear substantially on the interpretation of the results of gene discovery efforts. An observer inclined to compartmentalize syndromic ASD and ASD in the context of ID would likely contend that the field awaits definitive identification of a risk locus for idiopathic ASD. Someone holding the alternative perspective would likely mark the cloning of the Fragile X Mental Retardation Protein (FMRP) as the first autism gene, given a long standing and well-documented excess of the ASD phenotype observed in boys carrying canonical mutations and the repeated observation of FMRP mutations identified in cohorts of individuals with idiopathic ASD (Brown et al., 1986
; Fombonne et al., 1997
; Harris et al., 2008
; Hernandez et al., 2009
; Levitas et al., 1983
A number of other well-defined syndromes have either been found to have a greater than expected frequency of individuals with ASD or to have core features that overlap with ASD or both (Hoffman and State, 2010
). Among the substantial list, in addition to fragile X, the data for an appreciably increased prevalence of mutation carriers among cases of idiopathic ASD is so far best established with regard to tuberous sclerosis (Smalley, 1998
; Smalley et al., 1992
). Similarly, the overlap among defining features of syndromes such as Angelman or Rett and ASD provides a complementary approach to studying the biology of autism. The contribution of these and other rare syndromes to the development of neurobiological models of ASD pathogenesis is discussed in ensuing sections.
The first rare coding mutation in individuals with putative idiopathic ASD (eg the absence of a known genetic syndrome or clear evidence of a monogenic disorder) was identified in the neuronal adhesion molecule, Neuroligin 4X
(Jamain et al., 2003
). Based on the prior identification of recurrent deletions on the X chromosome in affected individuals, Thomas Bourgeron’s lab sequenced genes within the interval in 36 sibling pairs and 122 trios and found a truncating frame-shift mutation in NGLN4X
arising de novo
in an unaffected mother and transmitted to two brothers, one with Asperger syndrome and the second with “typical” autism. Shortly thereafter, mapping of a multi-generational pedigree affected with both ID and ASD by an independent group led to the discovery of a segregating truncating mutation, nearly identical to the initially reported NLGN4X
mutation (Laumonnier et al., 2004
Over the ensuing half-decade, this finding has been further supported by convincing evidence for recurrent de novo
mutations in individuals with ASD in SHANK3
a post synaptic scaffolding molecule that falls within the 22q13.3 microdeletion syndrome region (Durand et al., 2007
; Gauthier et al., 2009
; Lim et al., 1999
; Moessner et al., 2007
). Moreover recurrent de novo
mutations in other interacting molecules, including NRXN1
(Kim et al., 2008
; Szatmari et al., 2007
) and SHANK2
(Berkel et al., 2010
; Pinto et al., 2010b), functional data showing a role for neuroligins in the establishment of both excitatory and inhibitory synapses (Chubykin et al., 2005
; Graf et al., 2004
; Scheiffele et al., 2000
), and interesting, but less conclusive genetic (Jamain et al., 2003
) and model systems data (Chadman et al., 2008
; Tabuchi et al., 2007
) with regard to the closely-related molecule NGNL3,
have placed NLGN4X
among the most widely accepted genetic findings in idiopathic ASD.
Over the last several years a rapidly expanding list of rare mutations have been described in individuals with ASD, representing so many genes in fact that an exhaustive assessment is not feasible here. However only a small number yet have the property of being observed in rigorous studies by multiple independent investigators. Among these, in addition to NGLN4X, SHANK3
, the neuronal adhesion molecules CNTN4
has been identified by our lab and others via molecular cytogenetic mapping of de novo
rearrangements and CNV analyses (Fernandez et al., 2004
; Glessner et al., 2009
; Roohi et al., 2009
); and, quite recently, SHANK2
, was identified both through the identification of de novo
disruptive mutations in one patient with MR and another with ASD (Berkel et al., 2010
) and a large-scale CNV analysis (Pinto et al., 2010a
A review of the data regarding another neuronal adhesion protein, encoded by the gene Contactin Associated Protein 2
, is illustrative of the general state of the field, with intriguing findings falling just short of definitive evidence: The molecule was first identified as having a role in developmental delay through homozygosity mapping of a rare recessive frame-shift mutation in the Old Order Amish population (Strauss et al., 2006
). The report involved a syndrome of intractable epilepsy accompanied by clinically diagnosed autism. Shortly thereafter, independent reports provided evidence for variations in CNTNAP2
in idiopathic ASD. The mapping of a rare de novo
chromosomal rearrangement by our lab disrupting this locus in a simplex ASD pedigree and the subsequent identification of multiple rare transmitted, missense substitutions at highly conserved positions in affected multiplex families (Bakkaloglu et al., 2008
), the simultaneous findings by two independent groups of common variant association (Alarcon et al., 2008
; Arking et al., 2008
), and a subsequent report of interaction of CNTNAP2
along with a association of a common allele in language impairment phenotype (Vernes et al., 2008
) generated strong interest in this gene and led to its characterization by some authors as a confirmed idiopathic ASD locus.
However, while the initial mapping of a rare recessive mutation was quite compelling, the finding left open the question of whether and how either common or rare variation in this gene contributes to non-syndromic ASD. For example, the mutation burden analysis conducted by our lab was suggestive, showing an approximately 2-fold increase in very rare missense variants in cases versus controls. However, as noted in the initial publication the results did not reach statistical significance apart from a single recurrent transmitted allele; population stratification could not be ruled out as a confound and we lacked the ability to reliably differentiate functional from incidental mutations in cases and controls. Moreover, the two simultaneous common variant association studies identified different alleles that carried risk for differing phenotypes: one involving the diagnosis of ASD and the other mapping a language quantitative trait locus identified in individuals with ASD. Subsequently, only one of three published GWAS studies in ASD provides any support for association of CNTNAP2
with the diagnosis of ASD and this was modest at best (Anney et al., 2010
). Finally, the link to specific language impairment though statistically significant and based on a strong a priori
hypothesis was nonetheless identified through a candidate gene association study and
replication has yet to be attempted in a genome-wide analysis.
This characterization is not meant to call into question the rigor of the aforementioned studies or to detract from interest in CNTNAP2. The presence of multiple lines of evidence emerging nearly simultaneously from independent groups conducting state-of-the-art studies is unusual in the field and many of the outstanding questions have simply not yet been tested adequately to address the question of independent replication. Nonetheless, this summary highlights issues that are reflective of the current state of the science with regard to many of the most intriguing findings in ASD genetics both with regard to common and rare variants: underscoring the question of the relationship of syndromic findings to common forms of the disorder and highlighting the challenges posed by sample size, power, ancestral matching and distinguishing functional from neutral alleles.
Copy Number Variation
Prior to the advent of high-density microarrays and the ability to detect sub-microscopic variations in chromosomal structure, multiple recurrent chromosomal abnormalities were identified in individuals with ASD. The most common and highly penetrant are maternally inherited duplications of chromosome 15q11–13 (Baker et al., 1994
; Hogart et al., 2010
). Rare recurrent microscopic deletions at 2q37, 1q21,22q11, 22q13 among others, have been identified in affected individuals (Bucan et al., 2009
The importance of submicroscopic
copy number variation for ASD was first reported by Michael Wigler’s lab through the identification of a marked excess of de novo
variations in affected singleton probands (10%) compared to probands from multiplex families (3%) or unaffected controls (1%) (Sebat et al., 2007
). A subsequent genome-wide CNV study (Marshall et al., 2008
) confirmed a several fold increase in these events, and suggested a cumulative frequency of approximately 5–10 percent in the simplex ASD population. However, another recent large-scale study, while identifying de novo
variations in 5.6% of simplex ASD probands, did not find a difference compared to probands from multiplex families (5.5%) (Pinto et al., 2010a
). The reasons for the variability observed in this third study have not yet been clarified, although even prior to these results, a hypothesis emerged that the increased frequency identified by Sebat et al (2007)
might apply particularly to large events, and consequently, that the increasing resolution of array platforms might tend to obscure some differences between groups.
The study by the Wigler lab also included a description of a 16p11.2 de novo
deletion and, subsequently, two groups nearly simultaneously found a significant association of recurrent de novo CNVs (Kumar et al., 2008
; Weiss et al., 2008
) at this locus in idiopathic ASD, reporting on partially overlapping samples. The finding was confirmed in a subsequent genome wide investigation of structural variation (Marshall et al., 2008
). Among the three other recent large-scale CNV studies: the 16p11.2 finding was not replicated in one due, in part, to the finding of a higher than reported rate of 16p11.2 CNVs in the control group (Glessner et al., 2009
) and in two others, an independent assessment was precluded by extensive sample overlap with the previously reported cases (Bucan et al., 2009
; Pinto et al., 2010b). Interestingly, a recent study of 4284 individuals with MR and multiple congenital anomalies found 16p11.2 CNVs in 0.3%, consistent with that seen previously among ASD cohorts (Bijlsma et al., 2009
), and providing further evidence for wide ranging phenotypic manifestations resulting 16p11.2 variations (discussed below).
The 16p11.2 data serve as an important example of the manner in which current findings are challenging notions regarding rare variant contributions to ASD. All four deletions families reported by Kumar et al (and described again in Weiss et al) included two affected children, and in three of these pedigrees, only one of the affected siblings carried the de novo
CNV. As noted, based on Mendelian expectations, the observation that within multiple small pedigrees the large de novo
deletion was not necessary for the phenotype would raises eyebrows. Indeed, commenting on the lack of 16p11.2 replication in their CNV analysis, Glessner and colleagues write that their results “…indicate that CNVs at the 16p11.2.2 locus may not be sufficient to be causal variants in ASD.” (Glessner et al., 2009
) In fact, none of the reports of 16p11.2 have provided data suggesting that the variant is uniformly necessary or sufficient to lead to ASD. The issue addressed by these studies is whether this CNV confers risk. The aforementioned studies and our own preliminary data from a CNV analysis of simplex families, provides mounting evidence that it does.
Recent CNV studies have also provided interesting data regarding other previously identified and novel loci. Glessner et al. (2009)
replicated findings at 15q11- 13 and 22q11.21, as well as NRXN1
. As noted, they did not identify a significant difference in cases versus controls for 16q11.2 nor did they observe an association of CNVs at the SHANK3 locus
. Their analyses further highlighted multiple novel loci that were found to cluster via gene ontology analyses in the ubiquitin pathway and among molecules categorized as being involved in neuronal development. Pinto et al, in addition to reporting evidence for SHANK2
almost simultaneously with a report by Berkel et al, identified several additional novel loci and provided strong evidence for X linked inherited deletions of DDX53-PTCHD1
, with 7 reported males among the cases and none among 4964 controls. Their pathway analyses highlighted genes involved in cellular proliferation, projection and motility, and GTPase/Ras signaling (Pinto et al., 2010b
Similar to other child psychiatric disorders, the ASD genetics effort was initially characterized by a focus on idiopathic forms of the syndrome, an early inability to identify evidence for single gene inheritance and, subsequently, a widespread preoccupation with the CVCD hypothesis, investigated via non-parametric linkage and candidate gene association studies (Veenstra-Vanderweele et al., 2004
With regard to non-parametric linkage, the largest study to date included 1,181 multiplex families (Szatmari et al., 2007
) and, along with a dozen others using similar approaches, did not identify highly significant evidence for linkage or result in the mapping of a common variation that accounted for the linkage signals identified. It should be noted that these studies are theoretically capable of identifying either common or rare disease alleles. They typically evaluate sib pairs for regions of the genome shared among affected family members more often than would be expected by chance and have the advantages of not requiring a priori
specification of a mode of inheritance and of being robust to allelic heterogeneity.
A likely explanation for the lack of definitive results emerging from these investigations to date is the greater than anticipated degree of locus heterogeneity so far observed in ASD. Particularly in light of the modest sample sizes employed, a great diversity of genes all contributing to ASD would tend to obscure evidence for excess sharing at any given locus. However, as noted previously, the reemergence of linkage analyses of all types as important tools in the context of next generation sequencing technologies is a distinct possibility.
With regard to common variants, despite considerable challenges, several studies conducted just prior to the GWAS era resulted in notable findings. These included the identification of EN2
(Benayed et al., 2009
; Benayed et al., 2005
), the MET
oncogene (Campbell et al., 2006
; Jackson et al., 2009
), and CNTNAP2
(Alarcon et al., 2008
; Arking et al., 2008
; Vernes et al., 2008
). All three have shown some evidence for replication but continue to be the subject of debate due, in part, to their absence from the most promising results emerging from the recent genome wide association studies.
As with other areas of medicine, GWAS have emerged as the gold standard for the identification of common alleles carrying small effects and three relatively large studies in autism have recently been completed. The first studied 780 families and an additional 1204 probands and identified significant association of ASD to an intergenic region of chromosome 5p14.1 mapping between the neuronal adhesion molecules Cadherin 9
and Cadherin 10
(Wang et al., 2009
). The second involved on a cohort of 1031 families and found association to an intragenic SNP near the gene Semaphorin 5A
(Weiss et al., 2009
) and a third used a discovery cohort of 1558 individual and found genome wide evidence for association of MACROD2
(Anney et al., 2010
). As a likely reflection of both the heterogeneity of ASD and the challenge of identifying alleles of very modest effect, none of these studies confirmed the others’ findings.
While the field certainly hoped for replication of identical SNPs across these samples, the findings are nonetheless consistent with similar studies of complex conditions, including very modest odds ratios. Despite cohort sizes that would be considered large for child psychiatry, none of these studies were well powered to replicate findings from the other studies (Anney et al., 2010
). In addition, a comparison of these results with the candidate gene results mentioned above underscores an ongoing debate regarding both the impact of phenotypic heterogeneity and the best approach to addressing this problem. Both the MET
data provide evidence that association is robust either within a specific subgroup of affected individuals or with an endophenotype as opposed to the categorical diagnosis.
There has been a long-standing interest in using phenotypes that fall along the path from genetic variation to the syndromic clinical outcome across all of psychiatry. This is understandable given the clear limitations to our current categorical diagnostic approaches; the identification of more homogenous and biologically relevant entities could very well transform the search for common variants. The challenges in ASD, and for other neuropsychiatric phenotypes, include the difficulty in identifying relevant phenomenon a priori and the related confound of multiple comparisons if a variety of possibilities are examined through the course of a study. Moreover, it is worth noting in the instances where loci contributing to ASD have been most convincingly demonstrated, similar or identical rare mutations confer a very broad range of phenotypes. These results suggest that the effective “distance” between variations in the sequencing or structure of the DNA and resulting brain phenotypes may be quite large. It is difficult to imagine in retrospect what endophenotype would have been useful to detect these mutations, though admittedly the situation may be different for common versus rare variants. However, in other areas of medicine, common variant discovery has been very successful even in highly heterogeneous disorders in leveraging a combination of clinical diagnoses and large cohorts. Given the magnitude of effects identified in those studies and those so far suggested by initial GWAS in ASD, it is clear that the field is just now approaching the sample sizes necessary to begin to answer the question of common variant contributions. Finally, it is worth mentioning that as definitive replicated common variants are identified, the ability to then clarify relevant endophenotypes and a range of genotype-phenotype correlations will be dramatically enhanced.
Molecular findings and diagnostic boundaries
As suggested above, one of the most interesting and thought provoking recent observation in the field has been the wide range of neuropsychiatric manifestations that now appear to emerge from identical rare variants. Indeed the conceptual challenges of integrating the co-occurrence of MR and ASD pales in comparison to the possibility that functionally identical mutations may lead to ASD, MR, seizure disorder, Schizophrenia, ADHD, Tourette syndrome, OCD or some combination of the above. Nonetheless data both from structural and sequencing studies suggests this may be the case: For example 22q11.2 deletions, have long been implicated in the risk for psychosis in addition to the evidence with regard to ASD (Guilmatre et al., 2009
; Vassos et al., 2010
); CNVs at 16p11.2 were observed in individuals with Schizophrenia (Weiss et al., 2008
), a finding that has recently been supported by a large case control association analysis suggesting a particular role for duplications in this interval (McCarthy et al., 2009
) and de novo
mutations in SHANK3 have been identified in individuals with Schizophrenia as well as those with ASD (Gauthier et al., 2010
). Structural variations at CNTNAP2
have been reported not only in ASD and Schizophrenia but Tourette syndrome as well (Friedman et al., 2008
; Verkerk et al., 2003
), and this overlap has also been observed with regard to NLGN4X
(Lawson-Yuen et al., 2008
At present the data pointing to overlapping risks for Schizophrenia and ASD is particularly compelling (McCarthy et al., 2009
), with the prevailing hypothesis that duplications predispose to schizophrenia and a variety of other developmental outcomes including ASD, while deletions do not seem to play a role in the risk for psychosis. The prospect of any shared molecular mechanisms is somewhat ironic. Autism was initially conceptualized as a form of childhood psychosis, but with advances in standardized diagnostic approaches, this idea was rejected (Volkmar and Pauls, 2003
). While there is some overlap between ASD and the social withdrawal seen in individuals with schizophrenia, the natural history of the latter is quite distinctive versus the early onset that defines ASD, as are the positive symptoms of schizophrenia: auditory hallucinations and delusions have not been described as more common among individuals with idiopathic ASD.
Not surprisingly, the suggestion of overlap among so many developmental neuropsychiatric disorders opens the door on a variety of interesting debates. As suggested above, the notion of diagnostic substitution has been raised, but this would seems unlikely to explain all of the recent data given the range of signs and symptoms involved and the strikingly distinct developmental profiles. Given the clear demonstration of incomplete penetrance for implicated variants, it is also likely that some of the many observations are incidental findings. Again, the type of large- scale association analysis of 16p11.2 variations reported by McCarthy et al is precisely the methodology that will be required to provide clear answers to this question. The apparently broad range of phenotypic outcomes also poses some interesting challenges to the model that suggests the overlap of MR and ASD is a reflection of genes for the former uncovering the normal distribution of ASD traits in the population (). in contrast, the emergence of a wide range of phenotypes from, for instance, 16p11.2 duplications would seem most consistent with a model of highly pleotrophic effects of mutations that influence fundamental neurobiological processes ().
Emerging Neurobiological Models
At the molecular level, evidence in favor of this latter model has begun to converge at the synapse. The identification of NLGN4X mutations and the independent findings of rare variants in NRXN1, SHANK3 and SHANK2, have further focused attention on the function of neuroligin-neurexin complex and related molecules in the post synaptic density (PSD), a specialized region of the excitatory synapse ().
A highly selected view of the excitatory synapse highlighting genes strongly implicted via rare variant studies of idiopathic ASD (blue boxes) and those identified through the study of syndromic ASD (red boxes)
Perhaps most interestingly, this data converges with the evidence pointing to a key role for the Fragile X Mental Retardation Protein at glutamatergic synapses (Bear et al., 2004
; Huber et al., 2002
; Nakamoto et al., 2007
). The elaboration of this biology has led to an extremely intriguing hypothesis: that the cognitive and social phenotypes may be mediated through deficits in plasticity relating to long-term depression and that this process is potentially reversible via targeting of metabotropic glutamate receptors or related signaling cascades (Bear et al., 2004
; Dolen et al., 2010
). The notion, supported by recent model systems data (Chang et al., 2008
; Dolen et al., 2007
), that fragile X syndrome may be amenable to intervention throughout the lifespan is now being translated into clinical trials in individuals with Fragile X as well as with idiopathic ASD.
The intersection of studies of syndromic and idiopathic autism has also focused attention on the PI3K-AKT-mTOR pathway. As noted, there is strong evidence that rare mutations in TSC-1/TSC-2 increase the risk for ASD; the data in this regard for NF-1 and PTEN are also convincing (Butler et al., 2005
; Buxbaum et al., 2007
). These molecules point to the rapamycin-sensitive mTOR–raptor complex, a key regulator of protein synthesis and cell growth. Further, binding of hepatocyte growth factor to the MET
oncogene results in activation of a variety of signaling cascades, a process that is regulated in part by PTEN. These findings point to two intriguing possibilities: first, that targeting of this pathway, as is feasible with rapamycin and other compounds, may provide a novel avenue for treatment (Ehninger et al., 2009
; Ehninger et al., 2008
); and second that the data may converge with the biology implicated by fragile X to further refine the understanding of the molecular mechanisms leading to human developmental disorders (Narayanan et al., 2007
; Sharma et al., 2010
Of course, the question of whether the pathways implicated by syndromic ASD, which anchor both the mGluR and mTOR hypotheses, may have broader relevance for idiopathic ASD is highlighted by these findings. Ongoing studies, both to further elaborate basic neurobiology and to address the question in the clinic will help provide the answers. What is indisputable is that the conceptual transition reflected in these efforts is remarkable: the notion that mental retardation and ASD associated with FMRP or TSC-1 mutations may not set in stone early in development represents a seismic shift in thinking regarding the opportunities to treat these conditions, and underscores the more general transformative potential of the interplay of human genetic findings and basic neurobiology.
A third area of possible traction in the neurobiology of ASD relates to CNTNAP2
. As noted, the initial evidence for the phenotypic consequences of homozygous truncating mutations was quite strong. Moreover, the investigators who mapped the locus in Old Order Amish had the unusual opportunity to examine CNS pathology, due to surgical intervention for the severe epilepsy phenotype. The gross morphological abnormalities included temporal lobe dysplasia, evidence of abnormal cortical migration, and dysmorphic pyramidal neurons. Given many remaining uncertainties regarding the function of CNTNAP2
in the CNS and some evidence for the presence of the protein at the synapse (Bakkaloglu et al., 2008
), a convergence with the previously described pathways has not been ruled out. Equally interesting however is the suggestion, based on the expression of CNTNAP2 in post mitotic neurons in the developing human cortex and the pathological data suggests that subtle abnormalities in cortical migration or organization might be an independent avenue to ASD.
When viewed in a more global sense, the recent large scale studies of ASD have begun to suggest other intriguing possibilities; the use of pathway analysis to integrate the very large amount of rare variation data emerging from CNV studies has pointed to the ubiquitin pathway, neuronal adhesion molecules and those involved in cellular proliferation, projection and motility, and GTPase/Ras signalling. A review of the data on 16p11.2 and other specific recurrent CNVs is also focusing attention on dosage sensitivity in conferring risk and shaping developmental outcomes (Toro et al., 2010
). The further pursuit of these general leads will be helped tremendously by additional definitive genetic findings that allow for prioritization among the various possibilities, lend greater specificity to testable hypotheses, and provide a clear link to the human phenotype.