Cryptic genetic variation in a heat shock protein shapes the expressivity of a mutation affecting stem cell behaviour in C. elegans.

A fundamental question in medical genetics is how the genetic background modifies the phenotypic outcome of key mutations. We address this question by focusing on the epidermal seam cells, which display stem cell properties in Caenorhabditis elegans. We demonstrate that a null mutation in the GATA transcription factor egl-18, which is involved in seam cell fate maintenance, is more tolerated and thus has lower expressivity in the divergent CB4856 isolate from Hawaii than the lab reference strain N2 from Bristol. We identify multiple quantitative trait loci (QTLs) underlying the difference in mutation expressivity between the two isolates. These QTLs reveal cryptic genetic variation, which acts to reinforce seam cell fate through potentiating Wnt signalling. Within one QTL region, a single amino acid deletion in the heat shock protein HSP-110 in CB4856 lowers egl-18 mutation expressivity. Our work underscores that natural variation in conserved heat shock proteins can shape mutation expressivity.