Prevalence of CNVs on the X chromosome in patients with neurodevelopmental disorders.

Background: The X chromosome is enriched with genes related to brain development, and the hemizygous state of these genes in men causes some difficulties in the clinical interpretation of copy number variations (CNVs). In this study, we present data on the frequency and spectrum of CNVs on the X chromosome in a cohort of patients with neurodevelopmental disorders (NDDs).

Methods: Chromosomal microarray analysis was performed for 1175 patients with NDDs. CNVs were confirmed by real-time quantitative PCR. X chromosome inactivation was analysed by methyl-sensitive PCR. To determine the pathogenic significance of the CNVs, several criteria, including the origin (inherited or de novo), variant type (microdeletion or microduplication), and X chromosome inactivation pattern in asymptomatic and symptomatic carriers, were considered. Additionally, the spectrum, size and molecular bases of copy number changes in genes or gene regions involved in the development of the pathological phenotype in each patient were considered.

Results: CNVs on the X chromosome were identified in 33 patients (2.8%). Duplications and triplications (27 cases) were four times more common than deletions (6 cases). In 74% of patients, CNVs were of maternal origin; in 10% they were of paternal origin; and in 16% they arose de novo. The frequency of skewed X inactivation among family members who were healthy carriers of pathogenic and likely pathogenic CNVs and variants of uncertain significance (VUSs) on the X chromosome was 23%. For the first time, we reported several CNVs, including a pathogenic microdeletion at Xq26.1q26.2 involving the ARHGAP36 gene and a microduplication at Xp22.2 involving the OFD1 gene, CONCLUSIONS: This study expands on the frequency and spectrum of CNVs in patients with NDDs. Pathogenic variants on the X chromosome were present in 15% of cases, LP in 12%, VUS in 57%, and LB in 16% of cases. Previously unreported CNVs aid in the identification of new structural variants and genes associated with X-linked intellectual disability. We propose to consider the X-chromosome inactivation status when assessing the pathogenetic significance of CNVs using the ACMG algorithm (American College of Medical Genetics).