The Role of KBTBD2 in Bone Development and SHORT Syndrome

PROJECT SUMMARY Childhood growth disorders have significant consequences in adult life, including body size, work and reproductive performance, and the risk of chronic diseases. Our long-term goal is to understand the molecular mechanisms underlying childhood growth disorders and translate this knowledge into novel therapeutic

strategies. Given the physiological similarities between humans and mice, studying mouse mutants has provided us with fundamental insights. However, there are critical gaps in our understanding of growth disorders in mice, including the redundancy of genes involved in normal growth and development, as well as the unknown critical

genes. To address these gaps, we utilized a mouse forward genetic screen platform with automated meiotic mapping to identify mutations causing growth disorders rapidly and unbiasedly. In previous studies, we identified a null allele of Kbtbd2 called “teeny”, which exhibited severe insulin resistance, lipodystrophy, fatty liver, and

growth retardation. Our research uncovered the crucial role of KBTBD2 in regulating insulin signaling in adipose tissues, contributing to most of the metabolic phenotypes observed in teeny mice. However, the cause of growth retardation in teeny mice is still unknown. In this proposal, through knock-out of Kbtbd2 and osteogenic

differentiation of bone marrow-derived mesenchymal stromal cells (BMSCs), we discovered an intrinsic role of KBTBD2 in regulating osteogenesis by targeting p85α, the regulatory subunit of PI3K and a key downstream node of IGF-1 signaling. Interestingly, the teeny phenotype closely resembles SHORT syndrome, a genetic

disorder caused by mutations in the PIK3R1 gene, encoding p85α and related isoforms. We found that the recurrent R649W mutation in p85α disrupted its interaction with KBTBD2, resulting in decreased ubiquitination and degradation of p85α by KBTBD2. Based on these results, our central hypothesis is that KBTBD2 regulates

the abundance of p85α to enable IGF-1 signaling activation during osteogenesis, and the p85α R649W mutation leads to SHORT syndrome due to reduced association with the KBTBD2 protein. To test this hypothesis, we propose two specific aims. Aim 1 will elucidate the molecular mechanism of KBTBD2 in bone development. Aim

2 will investigate the role of KBTBD2 in the pathogenesis of SHORT syndrome. Completion of the proposed work will provide a comprehensive understanding of KBTBD2's role in skeletal development. As the KBTBD2 protein is highly conserved between mice and humans and the p85α mutant disrupts the normal function of KBTBD2 in

SHORT syndrome, our study may yield new therapeutic strategies for treating childhood growth disorders in the future.