Recessive osteogenesis imperfecta (OI) is caused by mutations in genes encoding proteins involved in post-translational interactions with type I collagen. A founder mutation in a new gene responsible for recessive OI has recently been reported in Bedouins from Israel and Saudi Arabia, who have a homozygous deletion of TMEM38B exon 4 and surrounding intronic sequence. TMEM38B encodes TRIC-B, an integral ER membrane monovalent cation channel involved in Ca++ release from intracellular stores. However, the molecular mechanisms through which this mutation causes an OI phenotype are unknown. We identified a 20 month-old girl with moderately severe OI, born to consanguineous parents from Saudi Arabia, who is homozygous for the TMEM38B founder mutation. TMEM38B transcripts are 25% of control level, and include six alternatively spliced forms of the transcript, of which one in-frame transcript deletes the central transmembrane domain and putative ion pore. The complete absence of TRIC-B protein was confirmed by Western blot and resulted in decreased intracellular Ca++ concentration and ATP-induced Ca++ flux from the ER. Surprisingly, SDS-Urea PAGE demonstrated increased electrophoretic migration of collagen alpha chains, suggesting altered post-translational modification. Although LH1 transcripts and protein were increased, proband collagen revealed a 30% reduction in helical lysine hydroxylation. Furthermore, the detection of lower stability collagen species (3040% of total collagen) on differential scanning calorimetry, increased intracellular PDI and GRP/BiP, and decreased procollagen pericellular processing, imply that proband procollagen conformation is abnormal. FKBP65, which is stabilized by Ca++ and required for collagen telopeptidyl hydroxylation and crosslinking, was decreased in proband fibroblasts. Matrix deposited in culture by proband fibroblasts has 30% reduction of immaturely and maturely crosslinked collagen. We propose that these data support a role for TRIC-B in intracellular Ca++ mobilization, and that absence of TRIC-B causes OI by dysregulation of multiple Ca++-regulated collagen-specific chaperones and modifying enzymes in the ER.
17 May 2014 - 20 May 2014