Mary C. O'Driscoll

Background: Germline pathogenic variants in NSD1 cause Sotos syndrome, a developmental disorder characterised by overgrowth, intellectual disability, macrocephaly, developmental anomalies, and, in some cases, tumour development. Familial cases of Sotos syndrome are rare and genotype–phenotype correlations are not well described. NSD1, a lysine-specific histone methyltransferase, is an important epigenetic regulator and pathogenic variants in NSD1 are associated with a distinctive blood DNA methylation pattern (episignature). We described a family with an NSD1 exon 3 deletion and an atypical clinical phenotype. Methods: DNA episignature profiling was undertaken with a next generation sequencing-based approach. Results: Within the family, the three affected individuals showed clinical variability with the proband being most severely affected, although none showed unequivocal macrocephaly or the characteristic facial features of Sotos syndrome. DNA methylation profiling was performed in the three affected family members, eight individuals with Sotos syndrome, and compared to control samples. The eight individuals with Sotos syndrome displayed genome-wide hypomethylation as previously described. DNA hypomethylation was also apparent in the three family members with the NSD1 exon 3 deletion with the proband being most similar to the episignature observed in confirmed Sotos syndrome patients. The two more mildly affected relatives had less pronounced DNA hypomethylation. Conclusions: A familial germline exon 3 NSD1 deletion was associated with mild Sotos syndrome phenotype with variable expressivity and a DNA methylation episignature that was less marked in milder cases than in individuals with classical Sotos syndrome. These findings support the use of methylation episignature analysis to explore intrafamilial variability in chromatin disorders.

Agenesis of the corpus callosum (ACC) is a common brain malformation of variable clinical expression that is seen in many syndromes of various etiologies. Although ACC is predominantly genetic, few genes have as yet been identified. We have constructed and analyzed a comprehensive map of ACC loci across the human genome using data generated from 374 patients with ACC and structural chromosome rearrangements, most having heterozygous loss or gain of genomic sequence and a few carrying apparently balanced rearrangements hypothesized to disrupt key functional genes. This cohort includes more than 100 previously unpublished patients. The subjects were ascertained from several large research databases as well as the published literature over the last 35 years. We identified 12 genomic loci that are consistently associated with ACC, and at least 30 other recurrent loci that may also contain genes that cause or contribute to ACC. Our data also support the hypothesis that many ACC loci confer susceptibility to other brain malformations as well as ACC, such as cerebellar hypoplasia, microcephaly, and polymicrogyria. The database presented here provides a valuable resource for diagnosis and management of individuals with ACC and individuals with chromosome rearrangements in whom ACC should be suspected, and of course for identifying ACC causal and contributory genes. Well‐defined diagnostic criteria, improved scanning techniques, and increased recognition of associated abnormalities will further facilitate gene mapping and allow definition of distinct syndromes within this heterogeneous group of patients.