Monoallelic expression can govern penetrance of inborn errors of immunity.

Inborn errors of immunity (IEIs) are genetic disorders that underlie susceptibility to infection, autoimmunity, autoinflammation, allergy and/or malignancy1. Incomplete penetrance is common among IEIs despite their monogenic basis2. Here we investigate the contribution of autosomal random monoallelic expression (aRMAE), a somatic commitment to the expression of one allele3,4, to phenotypic variability observed in families with IEIs. Using a clonal primary T cell system to assess aRMAE status of genes in healthy individuals, we find that 4.30% of IEI genes and 5.20% of all genes undergo aRMAE. Perturbing H3K27me3 and DNA methylation alters allele expression commitment, in support of two proposed mechanisms5,6 for the regulation of aRMAE. We tested peripheral blood mononuclear cells from individuals with IEIs with shared genetic lesions but discordant clinical phenotypes for aRMAE. Among two relatives who were heterozygous for a mutation in PLCG2 (delEx19), an antibody deficiency phenotype corresponds to selective mutant allele expression in B cells. By contrast, among relatives who were heterozygous for a mutation in JAK1 (c.2099G>A; p.S700N), the unaffected carrier T cells predominantly expressed the wild-type JAK1 allele, whereas the affected carrier T cells exhibited biallelic expression. Allelic expression bias was also documented in phenotypically discordant family members with mutations in STAT1 and CARD11. This study highlights the importance of considering both the genotype and the 'transcriptotype' in analyses of the penetrance and expressivity of monogenic disorders.