The Role of E3-Ubiquitin Ligase Complex in Genitourinary Tract Development and Function

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Summary: Despite their high prevalence, the molecular basis for common genitourinary (GU) congenital defects is poorly understood. We have identified and validated gene lesions at the KCTD13/16p11.2 locus as responsible for lower GU tract abnormalities, particularly cryptorchidism and hypospadias. KCTD13 encodes a substrate-specific adapter of a BCR (BTB-CUL3-RBX1) E3 ubiquitin-protein ligase complex, which regulates the actin cytoskeleton and cell migration via ubiquitination and degradation of RHOA. Notably, RHOA has been implicated in the maintenance of Sertoli-germ cell junctions to promote gametogenesis and gonads development. E3 ubiquitin ligases also regulate the androgen receptor (AR), which is key to male genitalia development and differentiation. We found that KCTD13 is robustly expressed in the GU tract and have recently shown that in comparison to normal controls, gene copy number variants are remarkably common in patients with GU birth defects. Moreover, we obtained in vitro and in vivo evidence that KCTD13 loss affects AR levels in testis and penis. We also observed that haploinsufficient and Kctd13 deficient mice had significantly higher incidence of cryptorchidism and decreased size of testes, seminal vesicles and penis size in conjunction with spermatogenic defects, causing severe subfertility. Further, Kctd13 null mice revealed a significant decrease in masculinization factor SOX9 levels and concomitant upregulation of the feminization factor RHOA. While our collective data suggest a key role of KCTD13 in male GU development, the mechanisms of how this molecule impinges upon the AR and/or SOX9 axes remain a major gap of knowledge. We hypothesize that gene dosage changes in KCTD13 alter the signaling of the masculinization axes, leading to abnormal GU tract development, defective gonad formation, undervirilization, and subfertility. In line with our reasoning, we will test two plausible independent mechanisms by which KCTD13 mediates lower GU tract development and differentiation. First, we hypothesize and test that gene dosage changes in KCTD13 affect GU tract development by affecting AR degradation, subcellular localization and downstream gene targets (AIM 1). Second, we hypothesize and test that KCTD13 affects the expression of masculinization factor SOX9 directly by modulating SOX9 ubiquitination or indirectly by regulating RHOA degradation, which in turn leads to defects in testis and penile development and differentiation (AIM 2). Finally, we identify and characterize the mutations in KCTD13- CUL3 pathway that are critical in GU development such that they could be used to generate a diagnostic kit for patients with disorders of sexual development (AIM3). Completion of the studies in this proposal will advance our understanding of the molecular mechanisms that underlie common GU birth defects.