In this study we observed significant differences in the neuroanatomical profiles of male children with autistic disorder with FXS relative to those who did not have FXS. Specifically, boys with both FXS and autism had substantially enlarged CN volume and smaller AMY volume compared to boys with FXS without autism. In contrast, boys with idiopathic autism (no FXS) had only modest enlargement in their CN volumes compared to controls, but more robust enlargement of their AMY volumes. Although observing this double dissociation among selected brain volumes, no significant differences in severity of autistic behavior as measured by subdomains of the ADI-R were detected between these two study groups. This study therefore provides evidence of a substantially different pattern of brain structures in two clinical populations with presentations of autistic behavior. The findings in the present study suggest that heterogeneity may be underestimated in studies attempting to identify common biological underpinnings of individuals meeting DSM IV behavioral criteria for the behaviorally defined syndrome of autism (e.g., genetic linkage studies). Clearly the study of biological mechanisms underlying autistic behavior in etiologically-defined subgroups such as those with FXS, is an important and probably under-employed strategy for dealing with the heterogeneity issue.
The finding of enlarged CN in FXS is consistent with other studies finding enlargement of the CN in FXS [12
], and the magnitude of enlargement (~40%) suggests this is a robust finding in children with FXS. The CN has been implicated in the repetitive behaviors seen in both autism and FXS. One possible cause for such an enlargement of the CN in FXS may be linked to the underlying genetics of the disorder. An association has been reported between a measure of FMR1
gene inactivation (activation ratio-AR) with caudate volume, and an association of IQ with both caudate and ventricular volumes [49
]. We also know that FMRP has also been shown to play a direct role in brain development [50
] and decreased FMRI protein has been associated with the cognitive deficits seen in FXS [51
]. The significant CN enlargement observed in FXS, in this study almost 3–4 times greater than in controls and about 3 times greater than cases with autism, may be more related to the FXS mutation versus diagnosis of autism, since the findings in the FXS autistic and non-autistic individuals are the same as those in the overall FXS group, regardless of autism status.
We report decreased AMY volume in our FXS group (with and without autism) and enlarged AMY in our AUT group. Our finding of increased AMY volume in our AUT sample is consistent with other reports of increased AMY in young children with autism, where AMY enlargement has also been associated with deficits in social behavior [21
] and social orienting [26
]. Our finding of decreased AMY volume in boys with FXS is contrary to some reports of AMY enlargement in FXS [28
], but consistent with studies that included very young children [48
]. Dalton and colleagues hypothesized that early AMY enlargement is the result of amygdalar hyperactivity and hypertrophy, in response to the aversive nature of social stimulation in autism, with subsequent decreased AMY volume occurring as the aversive stimulation becomes chronic [56
]. Here we find support to suggest that children with FXS have decreased AMY despite having social deficits characteristic of autism. On the other hand, children with autism have enlargement of the AMY, yet display the same behaviors. It may be that having an AMY either too large or too small are ‘two sides of the same coin’ in the same way that hyper or hypo function of the MECP2 gene both result in the Rett Syndrome phenotype [58
We did not find phenotypic differences in autistic behavior, as measured by the ADI-R and ADOS in our FXS+Aut and AUT groups. The defining features of autism as they appear in the DSM, while perhaps good ways to characterize the most impairing clinical features of autism, are not necessarily the best phenotypic features for separating out the underyling etiologic heterogeneity. Other behavioral features such as hyperarousal, thought to be more characteristic of FXS, may better distinguish groups of autistic individuals with and without FXS and may eventually find a place in studies of the autistic phenotype. Clearly additional studies which seek to identify different behavioral profiles in autistic individuals with and without FXS would provide important clues to meaningfully subsetting the autism phenotype.
In this study, we also found evidence for significantly enlarged CN volume in FXS compared to our control group, and with and TYP and DD subgroups. We found significant enlargement in the PUT and GP in the FXS group compared to the controls. This is the first report of significant enlargement in the PUT and GP structures in FXS, and as part of the fronto-striatal circuit with the caudate, this enlargement provides support for a neuroanatomical abnormality in this pathway in FXS. We also found significant group differences for the HIP volumes between the FXS and DD groups.
There are several limitations in the current study. The size of our subgroups of the controls (DD and TYP) and FXS (with and without autism) was modest and is a limitation. The heterogeneity we observe in idiopathic autism may have also limited the generalizability of the findings in this study. We only examined male children and therefore our findings may not generalize to females. Lastly, the measures we employed for behavioral assessment (ADI-R, ADOS) were developed for categorical diagnosis of autistic disorder and not for use in the way we have done—contrasting items and domains. Tools designed for this that have better dimensional qualities and that examine more varied behaviors (e.g., arousal, face processing using eye gaze, etc.) may have revealed behavioral differences that more closely correlated with the neuroanatomical differences observed between the groups. The study also has a number of strengths, most notable the large sample size of our autism and FXS groups within the narrow age range we examined. There is good evidence of age-dependent variation in neuroanatomical structures in autism (albeit indirectly from cross sectional studies [42
]) and both FXS and ideopathic autism are well known to be developmental conditions where presentations vary with age.
In conclusion, this study offers a unique examination of early brain development in two behavioral overlapping disorders, FXS and autism, and finds two distinct patterns of brain morphology. The present study finds uniquely different neurodevelopmental profiles for these two behaviorally similar disorders and suggests that comparative neuroimaging studies may provide the best window into teasing apart genetically meaningful aspects of the autism phenotype in FXS. These findings also underscore the importance of addressing heterogeneity in studies of autistic behavior. Simply mentioning that it exists and that it may be the cause of a lack of replication, may not be enough as the continued failure to take etiologic heterogeneity into account in autism is likely to continue to handicap our best efforts to find circumscribed genetic and neurobiologic mechanisms underlying this condition. Studies examining the molecular basis for this difference may provide the best approach to getting a foot-hold into the pathogenesis of autistic behavior. For example, mouse studies have revealed that while social deficits qualitatively similar to those seen in autism are present in C57B6 FX (−/−) mice, they are not observed in FX (−/−) on a FVB background. Such findings suggest that it is the effect of interacting genes (and perhaps environment) on FMR1 that may have a role in causing abnormal social behavior, and similarly, studying genes interacting with FMR1 in humans with autism may provide clues to the genetic profiles causing autism in this and perhaps even subgroups of autistic individuals. Clearly working in an iterative fashion—going from genotype to phenotype and back again from phenotype to genotype, may eventually reveal more subtle phenotypic variations, perhaps outside of the traditional defining features of autism, to distinguish underlying etiologically-defined subgroups that could be employed to find new autism susceptibility genes.