A model incorporating potential skewed X-inactivation in MZ girls suggests that X-linked QTLs exist for several social behaviours including autism spectrum disorder

The Differential Heritability of Social Adjustment by Sex

Sex differences in social adjustment are frequently observed; however, there has been very little research on adaptability in the individual and social domains. The aim of this study was to investigate the sex difference in social abilities, such as high self-appeal, sociability, school adaptation, and home adaptation between school-age males and females. The sample for this study included both same-sex and opposite-sex twin pairs: a total of 467 twin pairs. We classified them into three groups: a group of those in lower classes of elementary school, a group of those in higher classes of elementary school, and a group of those in junior high school. The heritability of school adaptation was estimated to be 95% in males and 54% in females in the junior high school group. The full sex-limitation model showed a better fit in this group, and this means that a qualitative genetic difference exists. For school adaptation, there was no sex difference in lower elementary school classes; however, a quantitative difference appeared in higher classes of elementary school. Moreover, a qualitative difference appeared in junior high school. From this research, it became clear that sex differences in heritability exist for school adaptation, and there was a marked increase from the elementary school children to the junior high school children.

Commentary on "A Role for the X Chromosome in Sex Differences in Variability in General Intelligence?" (Johnson et al., 2009)

Johnson et al.'s (2009) article highlights the role of X-chromosomal genes in general intelligence and draws attention to their potential role in explaining the observed greater variance for this trait in males and their excess at both extremes of the distribution. We note that this would result from a simple additive effect of X-linked intelligence genes and also discuss the potentially important contribution of recessive deleterious loci. The buffering effect of heterozygosity in females will be partly constrained by the skewing of X-inactivation patterns increasing the variance of females beyond what is expected. Furthermore, escape of some X-linked genes from in-activation may also be relevant to male-female variance comparisons. We also comment on the difficulty of establishing the extent to which the X chromosome is enriched for intelligence genes and point out that their estimates of the proportion of genes influencing general intelligence that might be located on the X chromosome rely on some doubtful premises, especially concerning the likely equivalence of X-linked gene action in males and females. Finally, we discuss the increasingly compelling evidence for the accumulation of genes on the X chromosome that have selective benefit to males, including those implicated infertility and some manifestations of intelligence.

ASD in females: are we overstating the gender difference in diagnosis?

Autism spectrum disorder (ASD) is predominantly diagnosed in males. In explaining the differential rate of occurrence in males and females, biogenetic models have been proposed and studied, often to the exclusion of potential social or cultural factors that may influence different rates of diagnosis across the genders. In this theoretical paper, we consider sociocultural influences that may contribute to the differential expression of ASD in females and partially explain potential underidentification of ASD in females. Based on our synthesis of the extant literature, we propose that ASD may be underidentified in affected females without co-occurring intellectual impairment, owing to a pattern of subtle yet potentially meaningful gender differences in symptom manifestation (e.g., less unusual stereotyped and repetitive behaviors in females, increased prevalence of internalizing problems in females) and gender inequities in research on the ASD phenotype that potentially contributes to biases in assessment tools and diagnostic practices. We offer recommendations for future research directions on gender differences in ASD, and we suggest implications to inform best practice and policy for the assessment of females with ASD.

A review of the role of female gender in autism spectrum disorders

This paper reviews the literature exploring gender differences associated with the clinical presentation of autism spectrum disorders (ASD). The potentially mediating effect of comorbid psychopathology, biological and neurodevelopmental implications on these gender differences is also discussed. A vastly heterogeneous condition, while females on the lower-functioning end of the spectrum appear to be more severely affected, an altered clinical manifestation of the disorder among high-functioning females may consequently result in many being un- or mis-diagnosed. To date, there is strong bias in the literature towards the clinical presentation of ASD in males. It is imperative that future research explores gender differences across the autism spectrum, in order to improve researchers', clinicians' and the public's understanding of this debilitating disorder.

Twins Early Development Study (TEDS): a genetically sensitive investigation of cognitive and behavioral development from childhood to young adulthood

The Twins Early Development Study (TEDS) is a large longitudinal sample of twins born in England and Wales between 1994 and 1996. The focus of TEDS has been on cognitive and behavioral development, including difficulties in the context of normal development. TEDS began when multiple births were identified from birth records and the families were invited to take part in the study; 16,810 pairs of twins were originally enrolled in TEDS. More than 10,000 of these twin pairs remain enrolled in the study to date. DNA has been collected for more than 7,000 pairs, and genome-wide genotyping data for two million DNA markers are available for 3,500 individuals. The TEDS families have taken part in studies when the twins were aged 2, 3, 4, 7, 8, 9, 10, 12, 14, and 16 years of age. Data collection is currently underway to assess the adult destinations of the twins as they move from school to university and the workplace. Between January 2012 and December 2014, all of the TEDS twins will turn 18, and the study will transition to an adult sample. TEDS represents an outstanding resource for investigating the developmental effects of genes and environments on complex quantitative traits from childhood to young adulthood and beyond.

Systematic resequencing of X-chromosome synaptic genes in autism spectrum disorder and schizophrenia

Autism spectrum disorder (ASD) and schizophrenia (SCZ) are two common neurodevelopmental syndromes that result from the combined effects of environmental and genetic factors. We set out to test the hypothesis that rare variants in many different genes, including de novo variants, could predispose to these conditions in a fraction of cases. In addition, for both disorders, males are either more significantly or more severely affected than females, which may be explained in part by X-linked genetic factors. Therefore, we directly sequenced 111 X-linked synaptic genes in individuals with ASD (n = 142; 122 males and 20 females) or SCZ (n = 143; 95 males and 48 females). We identified >200 non-synonymous variants, with an excess of rare damaging variants, which suggest the presence of disease-causing mutations. Truncating mutations in genes encoding the calcium-related protein IL1RAPL1 (already described in Piton et al. Hum Mol Genet 2008) and the monoamine degradation enzyme monoamine oxidase B were found in ASD and SCZ, respectively. Moreover, several promising non-synonymous rare variants were identified in genes encoding proteins involved in regulation of neurite outgrowth and other various synaptic functions (MECP2, TM4SF2/TSPAN7, PPP1R3F, PSMD10, MCF2, SLITRK2, GPRASP2, and OPHN1).

Sex differences in cognitive domains and their clinical correlates in higher-functioning autism spectrum disorders

Despite the skewed sex ratio, few studies have addressed possible cognitive sex differences in autism spectrum disorders (ASDs). This study compared visual attention to detail (ATTD) and selected executive functions (EF) in 35 males and 21 females with higher-functioning ASD and unaffected sibling controls. Females with ASD outperformed males on EF as assessed by the Trail Making Test B-A. Males with ASD showed superior performance for ATTD as measured by the Block Design Test (BD) when compared with females. EF difficulties in males were correlated with more stereotypic behaviours and interests on the Autism Diagnostic Interview-Revised or the Autism Diagnostic Observation Schedule. The results indicated clinically meaningful cognitive sex differences in ASD, particularly an association between EF and stereotypic behaviours and interests. ATTD as a potential basis for specific cognitive strengths (e.g. scientific/savant skills) might be more pronounced in males with ASD.

The origins and evolution of genetic disease risk in modern humans

Patterns and risks of human disease have evolved. In this article, I review evidence regarding the importance of recent adaptive evolution, positive selection, and genomic conflicts in shaping the genetic and phenotypic architectures of polygenic human diseases. Strong recent selection in human populations can create and maintain genetically based disease risk primarily through three processes: increased scope for dysregulation from recent human adaptations, divergent optima generated by intraspecific genomic conflicts, and transient or stable deleterious by-products of positive selection caused by antagonistic pleiotropy, ultimately due to trade-offs at the levels of molecular genetics, development, and physiology. Human disease due to these processes appears to be concentrated in three sets of phenotypes: cognition and emotion, reproductive traits, and life-history traits related to long life-span. Diverse, convergent lines of evidence suggest that a small set of tissues whose pleiotropic patterns of gene function and expression are under especially strong selection-brain, placenta, testis, prostate, breast, and ovary-has mediated a considerable proportion of disease risk in modern humans.

No major role for X-inactivation in variations of intelligence and behavioral problems at middle childhood

Although members of monozygotic (MZ) twin pairs are identical in genomic sequence, epigenetic mechanisms may occasion difference in gene expression and, consequently, twin discordance in complex traits. Recent work suggests that the epigenetic process of X-inactivation in female individuals may impact on intelligence and child behavioral problems. The timing of X-inactivation has been linked to chorionic splitting in MZ twins. Dichorionic monozygotic (DC-MZ) twinning, unlike monochorionic monozygotic (MC-MZ) twinning, occurs prior to the time of X-inactivation in female organisms. Therefore, the hypothesis of a causal role of X-inactivation in intelligence and behavioral problems can be analyzed by modeling the statistical interaction between sex and chorion type for within-pair differences in these traits in MZ twins. In this study, the effect of X-inactivation on childhood behavioral problems, measured with the CBCL, was studied in a sample of 324 MZ twin pairs from the EFPTS and the effect of X-inactivation on IQ was studied in a sample of 272 twin pairs from the same twin survey. Information on chorion type, gestational age, and birth weight was additionally collated. No significant statistical interaction was found between sex and chorion type, indicating that X-inactivation is not likely involved in variations in intelligence or behavioral problems in middle childhood. Further studies are required to replicate these findings and may explore the role of X-inactivation at different ages or at the extreme scores in the spectrum of intelligence and behavioral problems or may focus on other epigenetic mechanisms.