Girls with autism spectrum disorder (ASD) appear to have more genetic mutations than boys with ASD; differences in brain structure and function; advocate caution in drawing conclusions regarding female ASD based on male-predominant cohorts

A neurogenetic analysis of female autism. Allison Jack et al. Brain, awab064, April 16 2021. https://doi.org/10.1093/brain/awab064

Abstract: Females versus males are less frequently diagnosed with autism spectrum disorder (ASD), and while understanding sex differences is critical to delineating the systems biology of the condition, female ASD is understudied. We integrated functional MRI and genetic data in a sex-balanced sample of ASD and typically developing youth (8–17 years old) to characterize female-specific pathways of ASD risk. Our primary objectives were to: (i) characterize female ASD (n = 45) brain response to human motion, relative to matched typically developing female youth (n = 45); and (ii) evaluate whether genetic data could provide further insight into the potential relevance of these brain functional differences. For our first objective we found that ASD females showed markedly reduced response versus typically developing females, particularly in sensorimotor, striatal, and frontal regions. This difference between ASD and typically developing females does not resemble differences between ASD (n = 47) and typically developing males (n = 47), even though neural response did not significantly differ between female and male ASD. For our second objective, we found that ASD females (n = 61), versus males (n = 66), showed larger median size of rare copy number variants containing gene(s) expressed in early life (10 postconceptual weeks to 2 years) in regions implicated by the typically developing female > female functional MRI contrast. Post hoc analyses suggested this difference was primarily driven by copy number variants containing gene(s) expressed in striatum. This striatal finding was reproducible among n = 2075 probands (291 female) from an independent cohort. Together, our findings suggest that striatal impacts may contribute to pathways of risk in female ASD and advocate caution in drawing conclusions regarding female ASD based on male-predominant cohorts.

Keywords: autism spectrum disorder, functional MRI, genetics, striatum, social perception

Discussion

This ASDf-enriched sample has yielded a number of novel insights into female neuro-endophenotypes of social motion perception and potential contributors to female risk for ASD. While functional MRI highlights widespread functional differences between ASDf and TDf viewing human motion, analysis of the size of rare CNVs containing genes expressed in these functional MRI-identified brain regions suggests that potential impacts to striatum may be related to a sex-differential process of risk in early development. These larger ASDf CNVs support the FPE model prediction of greater genetic load in ASDf versus ASDm. Below, we discuss findings related to our major research objectives: (i) characterization of a functional MRI-based profile of ASDf (versus TDf) response to socially meaningful motion; and (ii) integration of functional MRI and genetics data.

First, we observed that the ASDf brain response during human action observation is characterized by less recruitment of parietal and posterior frontal cortex relative to TDf, particularly right somatosensory cortex, motor/premotor areas, and the putaminal region of striatum. This is distinct both from the ASD neural response associated with this paradigm in previous ASDm-predominant literature,13,14 and from trend-level TDm > ASDm results in this sample, which exhibit minimal overlap with TDf > ASDf. One prominent peak of TDf > ASDf occurred in right PMv, a region putatively associated with ‘mirroring’ properties,47,48 and which some suggest may help observers ‘fill in’ information missing from point-light human motion displays.49 Somatosensory regions detected in TDf > ASDf also display putative mirroring properties.50 Thus, greater recruitment of these regions by TDf might imply stronger engagement of such processes. PMv was not represented in BrainSpan, and was thus excluded from our Objective 2 analyses.

To contextualize our TDf > ASDf results, we also analysed differences in response between TDf and TDm, TDm and ASDm, and between ASDf and ASDm. TDf showed increased response to BIO > SCRAM relative to TDm in a variety of frontal and parietal regions. As in the sample of typically developing adults from the study by Anderson and colleagues20, TDf versus TDm demonstrated greater BIO > SCRAM activation within right DFC, although other regions demonstrating typically developing child (e.g. ventromedial prefrontal cortex) or typically developing adult (e.g. amygdala) sex differences in their cohort did not replicate in our sample, possibly due to differences in the age ranges of our samples. Many of the regions that emerged from our TDf > TDm contrast overlapped with those represented in the TDf > ASDf map, including right-lateralized anterior insula, IFG, DFC, MFG, and bilateral aIPS and paracingulate. Together, these regions resemble the salience and central executive brain networks. The salience network contains bilateral fronto-insular cortex and dorsal anterior cingulate, and contributes to monitoring and detection of salient stimuli.51 The central executive network is correlated with right fronto-insular activity and includes DFC, supplementary motor area, and lateral parietal cortices; these systems together play a role in attention, working memory, and cognitive control.52 The executive and salience sites recruited more strongly by TDf could play a number of roles potentially contributory to resilience in social perception. Right anterior insula contributes to detection of novel salient stimuli51 and switching between the task-negative (default) and task-positive central executive network53; activity in right anterior insula, IFG, and MFG/DFC can indicate renewed attention to a stimulus.54 These functions suggest more robust attentional reorienting among TDf to the human stimulus after a scrambled block, and/or greater attribution of salience to BIO displays by TDf than either TDm or ASDf.

In previous work examining resting state functional connectivity in our GENDAAR cohort, we found that typically developing youth demonstrated sex differences in functional connectivity of the salience but not the central executive network, while ASD youth showed the opposite pattern, with sex differences in the central executive, but not the salience network.55 Given our previous results, and the role of the salience network in managing switching to the central executive network,53 the TDf > ASDf differences we observed in response to social stimuli within nodes of these two networks could be driven by intrinsic neurotypical sex differences in the salience network that are not evident in ASD. Unfortunately, while our present results, and those of our previous resting state work, suggest that anterior insula and aIPS might have relevance to TDf resilience in social perception, these regions were not characterized in BrainSpan, and thus could not be assessed in our Objective 2 analyses.

We did not detect significant differences between ASDf and ASDm in their functional MRI neural response to biological motion. Moreover, contrary to extant literature, ASDm did not differ from TDm on this task. In exploratory follow-up analyses, we considered whether the TDm > ASDm pattern might be similar to that of TDf > ASDf, but below our threshold for statistical detection. Under a more lenient method for statistical inference, ASDm versus TDm displayed right pSTS hypoactivation similar to that found in previous work,13,14 suggesting that modern methods of functional MRI statistical inference may reduce our power to detect this effect in exchange for greater type I error control. TDf > ASDf did not overlap with TDm > ASDm under this more lenient method. Thus, while ASDf and ASDm response to human motion did not significantly differ, at the same time what distinguishes ASDf from TDf does not appear similar to what distinguishes ASDm from TDm.

While ASDf and ASDm functional brain response did not differ, genetic analyses demonstrated significant differences between these groups. Specifically, ASDf (versus ASDm) exhibited larger size of rare CNVs containing genes expressed during early development of striatum. This finding, accompanied by ASDf (versus TDf) hypoactivation of putamen (a component of the striatum) during social perception, suggests that potential impacts to striatum may be an element of developmental risk for ASD trajectories in girls. Previously, putaminal disruptions in ASD versus typically developing individuals have been documented,56–61 albeit largely in ASDm-exclusive or ASDm-predominant samples. We interpret our findings as suggesting that striatal involvement, while not unique to ASDf, may have a particularly important role in ASDf aetiologies. The putamen, historically attributed a primarily motoric role, also appears involved in social and language functions.62 Among typically developing individuals, the putamen receives projections from motor/premotor (primarily terminating in dorsolateral/central putamen), and prefrontal cortex (primarily terminating in anterior putamen), and appears to serve as an interface between information about motivational value and voluntary behaviour.63,64 Recent work using resting state functional MRI data suggests that while TDm (females not assessed) demonstrate distinct functional segregation of putamen into anterior and posterior segments, putamen in ASDm appears as one functional unit.56 In the present investigation, we observed the peak coordinate of TDf > ASDf striatal response in a region of right anterior putamen characterized as having structural connectivity primarily to executive prefrontal regions (including MFG and DFC65) It also may be notable that in addition to reduced ASDf response in M1C, we observed larger size of CNVs containing genes expressed in M1C in many (though not all) of our control tests. Taken together, this pattern of results could indicate disturbances to the striatomotor-cortical system more broadly and, thus, processes of linking information about motivational value to action. Differential putaminal recruitment during social perception might reflect differing organization of functional connectivity, in which the region is linked to the central executive network and, perhaps, associated protective functions for TDf but not ASDf. Genetic disruptions specifically impacting striatal cortex during development may underlie such functional atypicalities, and have greater impact via disruption of female protective mechanisms. The general lack of female characterization in the literature on ASD putaminal disruptions, however, makes it difficult to draw strong conclusions along these lines. Future work should analyse ASDf and TDf patterns of functional connectivity and gene co-expression among these regions to clarify this possibility.

When considering together our findings of robust TDf > ASDf and TDf > TDm differences in brain function, lack of ASD sex differences in brain response, and greater ASDf versus ASDm size of CNVs containing genes expressed in early striatal development, the overall picture presented is complex but not inconsistent with an FPE model. While the FPE predicts that ASDf should have greater genetic load than ASDm—a prediction supported by our findings—this does not necessarily equate to greater symptomaticity or disruption of brain function. While some ASDf may lack resilience factors typically found in TDf, other ASDf may retain aspects of female protection that make their phenotype less severe than it might otherwise have been given their greater aetiological load. Moreover, female resilience factors may also have sociocultural aspects (e.g. more emotion-oriented talk to daughters versus sons66); the different socialization experiences that an ASDf might encounter could lead, by adolescence, to a brain profile that does not significantly differ from ASDm despite greater genetic load.

In sum, our findings provide new insights into ASDf brain response during social perception, reveal a potential substrate of female risk for ASD trajectories, and illuminate unique qualities of TDf response to human motion relative to TDm. In addition to the basic systems for processing social motion engaged by both sexes, TDf (unique from TDm or ASDf) recruit additional salience and central executive systems. Further, relative to TDf, ASDf show reduced recruitment of striatum during this perceptual task. Compared to ASDm, ASDf (both in our cohort and an independent sample) demonstrate larger size of rare CNVs containing genes expressed in early striatal development, suggesting that, for ASDf, potential impacts to striatum may be particularly relevant. Our results demonstrate the risk of drawing conclusions regarding ASDf based on work comprised of ASDm-predominant samples, and argue for continued attention to the unique characteristics of ASDf.