Compound heterozygosity for a frameshift mutation and an upstream deletion that reduces expression of SERPINH1 in siblings with a moderate form of osteogenesis imperfecta

Abstract

SERPINH1 encodes the collagen chaperone HSP47 that binds to arginine-rich sequences in the type I procollagen trimers and provides the final steps in the folding and stabilization of the triple helical domain. Loss of both alleles in mice results in very early embryonic lethality. SERPINH1 mutations have been associated with one of the rarest forms of recessively inherited osteogenesis imperfecta (OI) with a moderate to severe phenotype. We identified a family with non-consanguineous unaffected parents who had two children with moderate short stature, low bone density, and fractures. Both children were compound heterozygotes for two mutations: a frameshift in the last exon that deleted the RER retention signal, and a 5,274 bp deletion 2.37 kb upstream from the transcription start site. The maternally-inherited frameshift allele was expressed at normal levels, but the protein was unstable. The mRNA encoded by the second allele represented about 50% of that from the frameshift-containing allele.The upstream deletion was inherited from the father, and the mRNA encoded by that allele in his cultured dermal fibroblasts was also expressed at a low level, which confirmed that this domain had a regulatory function for SERPINH1. Regulatory mutations are uncommon causes of human genetic disorders, and the ability to measure expression levels in appropriate cells is key to their identification. K E Y W O R D Scollagen folding, osteogenesis imperfecta, regulatory mutation, RER retention signal, SERPINH1

Aberrant binding of mutant HSP47 affects posttranslational modification of type I collagen and leads to osteogenesis imperfecta

“…variants. [15][16][17]21,25] For p.(Leu78Pro), HSP47 was shown to be unstable and completely degraded via the proteasome. [16] Immunofluorescent staining showed normal HSP47 localization to the ER in p.(Leu326Pro) cells [15,25], while in p.(Met237Thr) cells, a punctate pattern was observed, with accumulation of HSP47 in vacuolar-like compartments at the periphery of the cell.…”

“…fibroblasts. [16,17,21] The discrepancy between these observations remains unexplained but could result from variant-specific (compensatory) effects, or, alternatively, the overmodifications caused by the latter variants could be too mild to be picked up in fibroblast culture.…”

“…[16] Subsequently, biallelic SERPINH1 variants were described in seven additional individuals from five unrelated families, suffering from moderately severe to neonatal lethal OI. [17][18][19][20][21] These variants included homozygous or compound heterozygous missense variants (p.(Met237Thr) [17] and p.[(Leu50Arg)];[(Arg405His)] [20]), a homozygous in-frame deletion (p.(Glu105_His108del)) [18], a homozygous frameshift variant (p.(Glu113Valfs � 8)) [19], and a compound heterozygous nonsense variant (p.(Asp412 � ) combined with a >5 kb genomic deletion located 2.37 kb upstream of the transcription start site. [21] Here, we report the ninth patient suffering from severe OI caused by a novel homozygous SERPINH1 missense variant (p.(R222S)).…”