Developmental dysplasia of the hip caused by homozygous <i>TRIM33</i> pathogenic variant affecting downstream BMP pathway.

Abstract

Developmental dysplasia of the hip (DDH), formerly termed congenital dislocation of the hip, is the most common congenital disease of the musculoskeletal system in newborns. While familial predilection to DDH has been well documented, the molecular genetics/pathways of this common disorder are poorly understood. Linkage analysis and whole exome sequencing; real-time PCR studies of skin fibroblasts. Consanguineous Bedouin kindred presented with DDH with apparent autosomal recessive heredity. Linkage analysis and whole exome sequencing delineated a single 3.2 Mbp disease-associated chromosome 1 locus (maximal multipoint Logarithm of the Odds score 2.3), containing a single homozygous variant with a relevant expression pattern: addition of threonine in TRIM33 (NM_015906.4); c.1648_1650dup. <i>TRIM33</i> encodes a protein that acts both in the TGF-β and the BMP pathways; however, it has been mostly studied regarding its function in the TGF-β pathway. Since BMPs are known to act in bone formation, we focused on the BMP pathway, in which TRIM33 functions as a transcription factor, both an activator and repressor. Skin fibroblasts of two affected girls and a heterozygous <i>TRIM33</i> variant carrier were assayed through reverse-transcription PCR for expression of genes known to be downstream of TRIM33 in the BMP pathway: fibroblasts of affected individuals showed significantly reduced expression of <i>DLX5</i>, significantly increased expression of <i>BGLAP</i>, increased expression of <i>ALPL</i> and no change in expression of <i>RUNX2</i> or of <i>TRIM33</i> itself. DDH can be caused by a biallelic variant in <i>TRIM33</i>, affecting the BMP pathway.