Age-resolving Osteopetrosis: A Rat Model Implicating Microphthalmia and the Related Transcription Factor TFE3

Abstract

Microphthalmia (Mi) is a basic helix-loop-helix–leucine zipper (b-HLH-ZIP) transcription factor implicated in pigmentation, mast cells, and bone development. Two dominant-negative mi alleles ( mi/ mi and Mi ^or / Mi ^or ) in mice cause osteopetrosis. In contrast, osteopetrosis has not been observed in a number of recessive mi alleles, suggesting the existence of Mi protein partners important in osteoclast function. An osteopetrotic rat of unknown genetic defect ( mib) has been described whose skeletal sclerosis improves dramatically with age and that is associated with pigmentation defects reminiscent of mouse mi alleles. Here we report that this rat strain harbors a large genomic deletion encompassing the 3′ half of mi including most of the b-HLH-ZIP region. Osteoclasts from these animals lack Mi protein in contrast to wild-type rat, mouse, and human osteoclasts. Mi is not detectable in primary osteoblasts. In addition TFE3, a b-HLH-ZIP transcription factor related to Mi, was found to be expressed in osteoclasts, but not osteoblasts, and to coimmunoprecipitate with Mi. These results demonstrate the existence of members of a family of biochemically related transcription factors that may cooperate to play a central role in osteoclast function and possibly in age-related osteoclast homeostasis.