Robo receptors interact with ligands of the Slit family. The nematode C. elegans has one Robo receptor (SAX-3) and one Slit protein (SLT-1), which direct ventral axon guidance and guidance at the midline. In larvae, slt-1 expression in dorsal muscles repels axons to promote ventral guidance. SLT-1 acts through the SAX-3 receptor, in parallel with the ventral attractant UNC-6 (Netrin). Removing both UNC-6 and SLT-1 eliminates all ventral guidance information for some axons, revealing an underlying longitudinal guidance pathway. In the embryo, slt-1 is expressed at high levels in anterior epidermis. Embryonic expression of SLT-1 provides anterior-posterior guidance information to migrating CAN neurons. Surprisingly, slt-1 mutants do not exhibit the nerve ring and epithelial defects of sax-3 mutants, suggesting that SAX-3 has both Slit-dependent and Slit-independent functions in development.

Autistic spectrum disorders (ASDs) are characterized by impairments in language, social skills, and repetitive behaviors, often accompanied by intellectual disability. Advances in the genetics of ASDs are providing new glimpses into the underlying neurobiological mechanisms disrupted in these conditions. These glimpses on one hand reinforce the idea that synapse development and plasticity are one of the major pathways disrupted in autism, but beyond that are providing fresh molecular support to the idea of mechanistic parallels between idiopathic ASD and specific syndromic neurodevelopmental disorders like fragile X syndrome (FXS). Fragile X syndrome is already recognized as the most common identifiable genetic cause of intellectual disability and ASDs, with many overlapping phenotypic features. Fragile X syndrome is associated with a variety of cognitive, behavioral, physical, and medical problems, which are managed through supportive treatment. Recent major advances in the understanding of the underlying neurobiology in FXS have led to the discovery of agents that rescue phenotypes in the FXS mouse model, and early clinical trials of targeted treatments in humans with FXS. Thus translational strategies in FXS may be poised to serve as models for ASD and other cognitive disorders.

BACKGROUND: Brain development follows a different trajectory in children with autism spectrum disorders (ASD) than in typically developing children. A proxy for neurodevelopment could be head circumference (HC), but studies assessing HC and its clinical correlates in ASD have been inconsistent. This study investigates HC and clinical correlates in the Simons Simplex Collection cohort. METHODS: We used a mixed linear model to estimate effects of covariates and the deviation from the expected HC given parental HC (genetic deviation). After excluding individuals with incomplete data, 7225 individuals in 1891 families remained for analysis. We examined the relationship between HC/genetic deviation of HC and clinical parameters. RESULTS: Gender, age, height, weight, genetic ancestry, and ASD status were significant predictors of HC (estimate of the ASD effect = .2 cm). HC was approximately normally distributed in probands and unaffected relatives, with only a few outliers. Genetic deviation of HC was also normally distributed, consistent with a random sampling of parental genes. Whereas larger HC than expected was associated with ASD symptom severity and regression, IQ decreased with the absolute value of the genetic deviation of HC. CONCLUSIONS: Measured against expected values derived from covariates of ASD subjects, statistical outliers for HC were uncommon. HC is a strongly heritable trait, and population norms for HC would be far more accurate if covariates including genetic ancestry, height, and age were taken into account. The association of diminishing IQ with absolute deviation from predicted HC values suggests HC could reflect subtle underlying brain development and warrants further investigation.