COL1A2 and COL2A1 expression in temporal bone of lethal osteogenesis imperfecta

COL1A2 and COL2A1 genes are expressed at high levels in many cochlear cells of 16- to 23-week-old human fetuses. Given these prior observations and the rare opportunity to obtain temporal bones from a deceased neonate with osteogenesis imperfecta (OI) type II, we determined the cellular distribution and level of expression of COL1A2 mRNA in OI type II inner ear compared with the expression in second-trimester human fetal cochlea. Expression of COL2A1 mRNA was assessed for its normal role in OI type II neonatal cochlea and to address potential spatial and temporal changes along with our observations in fetal cochlea. We describe our tissue in situ hybridization protocol and document its usefulness in assessing gene expression in human temporal bone obtained at autopsy.

RNA-RNA in situ hybridization was performed in formaldehyde-fixed, decalcified, paraffin-embedded temporal bone sections from a neonate with OI type II. Semi-quantitative assessment of gene expression was performed by visual inspection of grain densities.

COL1A2 and COL2A1 were expressed at moderate-to-high levels in many membranous cochlear cells, and no dramatic alterations in pattern or level of expression of these genes was noted compared with human fetal cochlea. Consistent with in vitro studies, expression of COL1A2 in osteoblasts lining enchondral and endosteal layers is less than that in identical cells of the fetal otic capsule undergoing osteoid deposition and mineralization. Expression of COL1A2 mRNA in osteoblasts lining the outer periosteum of otic capsule is markedly higher than osteoblasts lining enchondral and endosteal layers, suggesting that differential expression may exist between osteoblasts lining endosteal, enchondral, and periosteal surfaces of bone in OI type II.