A frame-shift mutation in the type I parathyroid hormone (PTH)/PTH-relatedpeptide receptor causing blomstrand lethal osteochondrodysplasia

Blomstrand osteochondrodysplasia (BOCD) is a rare lethal skeletal dysplasia characterized by accelerated endochondral and intramembranous ossification . Comparison of the characteristics of BOCD with type I PTH/PTH-related pep tide (PTHrP) receptor-ablated mice reveals striking similarities that are m ost prominent in the growth plate. In both eases, the growth plate is reduc ed in size due to a strongly diminished zone of resting cartilage and the n ear absence of columnar arrangement of proliferating chondrocytes. This ove rall similarity suggested that an inactivating mutation of the PTH/PTHrP re ceptor might be the underlying genetic defect causing BOCD. Indeed, inactiv ating mutations of the PTH/PTHrP receptor have been recently identified in two cases of BOCD. We describe here a novel inactivating mutation in the PTH/PTHrP receptor. S equence analysis of all coding exons of the type I PTH/PTHrP receptor gene and complementary DNA of a case with BOCD identified a homozygous point mut ation in exon EL2 in which one nucleotide (G at position 1122) was absent. The mutation was inherited from both parents, supporting the autosomal rece ssive nature of the disease. The missense mutation resulted in a shift in t he open reading frame, leading to a truncated protein that completely diver ged from the wild-type sequence after amino acid 364. The mutant receptor, therefore, lacked transmembrane domains 5, 6, and 7; the connecting intra- and extracellular loops; and the cytoplasmic tail. Functional analysis of t he mutant receptor in COS-7 cells and of dermal fibroblasts obtained from t he case proved that the mutation was indeed inactivating. Neither the trans iently transfected COS-7 cells nor the dermal fibroblasts responded to a ch allenge with PTH or PTHrP with a rise in intracellular cAMP levels, in shar p contrast to control cells. Our results provide further evidence that BOCD is caused by inactivating mutations of the type I PTH/PTHrP receptor and u nderscore the importance of this receptor in mammalian skeletal development .