Moderately severe osteogenesis imperfecta-like osteochondrodysplasia associated with heterozygous variants in both COL1A2 and TRPV4.

Osteogenesis imperfecta (OI) is a genetically heterogeneous connective tissue disorder with major findings of bone fragility and deformity and short stature. Most patients have dominant mutations in COL1A1 or COL1A2. Heterozygous defects in transient receptor protein vanilloid 4 (TRPV4), encoding a Ca2+ permeable channel highly expressed in chondrocytes, osteoblasts, and osteoclasts, have been associated with skeletal dysplasias characterized by short stature, bone deformity, and osteoarthritis. We present the unusual situation of a patient with heterozygous variants in 2 genes known to cause bone dysplasia. We investigated variant function and potential contribution to phenotype. A 20-mo-old female was referred due to in utero genetic testing that revealed heterozygosity for both a novel COL1A2 splice variant c.2565+3_256+6delAAGT and a TRPV4 variant c.1108G>A; p.G370S. She had findings of type III OI as well as rhizomelia, unexpected for OI caused by collagen structural mutation. Polymerase chain reaction analysis of COL1A2 cDNA from patient fibroblasts confirmed in- and out-of-frame alternatively spliced transcripts. Type I collagen expression was reduced. Proband collagen was overmodified, and significant intracellular retention led to reduced secretion. Although patient fibroblasts showed higher TRPV4 mRNA levels, both patient fibroblasts and human embryonic kidney (HEK) cells transfected with WT or mutant TRPV4 constructs showed normal protein levels. However, analysis of patient fibroblasts and coexpression of GFP-tagged TRPV4 p.G370S and WT TRPV4 in HEK cells demonstrated reduced mutant TRPV4 protein at the plasma membrane compared to control fibroblasts and WT TRPV4 alone, respectively. Patient fibroblasts and the transfected HEK cells showed significantly decreased calcium influx in response to TRPV4 agonist. We propose that the TRPV4 variant is potentially causes the patient's rhizomelia in the context of OI. Expanded utilization of broad skeletal genetic testing panels or whole exome sequencing (WES) is expected to reveal additional cases with multi-gene contributions, illuminating cellular and matrix pathway interactions.