# Familial NSD1 Exon 3 Deletion Associated with Phenotypic and Epigenetic Variability

## TL;DR

A family with a genetic mutation in NSD1 shows varied symptoms and DNA methylation patterns, suggesting methylation analysis can help understand genetic disorder variability.

## Contribution

This study identifies a familial NSD1 exon 3 deletion with variable expressivity and epigenetic differences, expanding understanding of Sotos syndrome.

## Key findings

- Three family members with NSD1 exon 3 deletion showed variable clinical severity and DNA methylation patterns.
- DNA hypomethylation was less pronounced in milder cases compared to classical Sotos syndrome patients.
- Methylation episignature analysis can reveal intrafamilial variability in chromatin-related disorders.

## Abstract

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.