An Alternative Splicing Variant in Clcn7–/– Mice Prevents Osteopetrosis but Not Neural and Retinal Degeneration

The ubiquitously expressed chloride channel 7 (CLCN7) is present within the ruffled border of osteoclasts. Mutations in the CLCN7 gene in humans (homologous to murine Clcn7) are responsible for several types of osteopetrosis in humans, and deficiencies in CLCN7 can present with retinal degeneration and a neuronal storage disease. A previously reported Clcn7 –/– mouse showed diffuse osteopetrosis accompanied by severe retinal and neuronal degeneration. In contrast, the authors produced a novel Clcn7 –/– mutant where mice did not develop osteopetrosis but still developed lethal neural and retinal degeneration. In these mice, there was a rapid progressive loss of the outer nuclear layer and photoreceptor layers of the retina. Laminar degeneration and necrosis of neurons in layers IV and V of the cerebral cortex and in the CA2/CA3 regions of the hippocampus were associated with intraneuronal accumulations of autofluorescent granules (periodic acid–Schiff positive). The extensive reactive gliosis was always associated with the accumulation of intraneuronal cytoplasmic material. The authors found, through quantitative real time polymerase chain reaction analyses, that an alternate Clcn7 transcript (previously identified only in bone marrow) showed minimal expression in the brain and eye but moderate expression in bone, which correlates with rescue of the osteopetrotic phenotype in the face of continued retinal and neuronal degeneration. Findings in this knockout mouse model prove that osteopetrotic compression of the brain is not responsible for neuronal and retinal degeneration in CLCN7-deficient mice; rather, they suggest that neurotoxicity is most likely due to lysosomal dysfunction as a result of the functional lack of this chloride channel in the central nervous system and eye.