The limb girdle muscular dystrophies (LGMD) are muscle-wasting disorders involving muscles of the pelvis and shoulder girdle. Thirteen genetically defined limb girdle muscular dystrophy mutations have been identified with most of the mutated genes encoding structural proteins. However, two LGMD mutations do not encode for structural proteins rather, they possess enzymatic activity. These proteins named calpain 3 (CAPN3) and Trim32 both function in protein degradative pathways. Until now, it has been unclear how mutations in these degradative enzymes might lead to muscle disease. Previously we showed that CAPN3 is important for muscle remodeling and that it acts upstream of the ubiquitin-proteasome pathway. These prior observations have led us to turn our attention to LGMD2H, which involves a mutation in Trim32. Three allelic, autosomal recessive diseases result in mutations in Trim32 including LGMD2H, Sarcotubular myopathy (SM) and Bardet Biedl (BB) syndrome. LGMD2H is believed to be a milder form of muscular dystrophy than SM. Bardet Biedl is a complex and genetically heterogeneous disorder involving retinal dystrophy, obesity, kidney abnormalities and polydactyly. It is currently not understood why different mutations in one gene can result in such clinically diverse phenotypes. In an effort to better understand the biological function of Trim32, we have generated unique reagents such as antibodies, recombinant proteins and a genetically modified mouse lacking Trim32. To address questions about Trim32's biological function and why mutations result in disease, we will perform the following aims:
Aim 1 : We will characterize the phenotype of mice lacking Trim32 (T32KO) to better understand its biological function.
Aim 2 : We will generate an animal model for LGMD2H (knock in mouse carrying the LGMD2H mutation) and compare this mouse to the Trim32 knock out mouse.
Aim 3 : We will explore the biological role of Trim32 in muscle and begin to explore pathogenic mechanisms in LGMD2H.
Aim 4 : We will identify substrates of Trim32 and determine if the LGMD2H mutant has a reduced ability to ubiquitinate these substrates.
