The dominant LIM Domain Binding protein 3 (LDB3) mutation p.A165V is the most common cause of MFM4. Disease mechanisms underlying MFM4 are not yet defined, partly due to lack of mouse models.

LDB3-A165V mutant mice were generated by homologous recombination. Mutant mice were compared to wildtype littermates in a longitudinal follow up over 10 months. Protein expression changes were examined using Reverse Phase Protein Array (RPPA). Mice were subjected to chronic exercise (6 weeks downhill running). Autophagy flux was measured in mice following colchicine administration.

Beginning at 3 months of age, four paw grip strength was significantly reduced in mutant mice (p < 0.01, n = 35). Maximal isometric contractile force was 147.8+/-5.2 mN/mm² in mutant and 187.4+/-3.2 mN/mm² in wildtype mice (p < 0.01, n = 3-4) in EDL ex-vivo without evidence of fatigability. Muscle mass was reduced in mutant mice (p < 0.05, n = 5). By 6 months, hallmark pathology changes of MFM were observed by immunostaining and electron microscopy, followed at 8 months by vacuolar degenerative myopathy on GT staining. RPPA analysis at 4 months identified altered expression of PKC alpha, a known LDB3 interactor, and indicated altered activity of Akt and GSK3. Exercise capacity in 3-4 month old mutant mice was unchanged. Effects of exercise on muscle structure and function in mutant mice are underway.

LDB3-A165V mice develop progressive myofibrillar myopathy reminiscent of human patients.

Our studies in heterozygous and homozygous mice indicate a dominant gain of function by mutant LDB3. Changes in signaling pathways precede MFM pathology in LDB3-A165V mutant mice and suggest a role in pathogenesis and, potentially, will lead to identification of therapeutic targets in MFM.