Inhibition of MAFB and PI3K/AKT-signaling reveals a potential therapeutic strategy for multicentric carpotarsal osteolysis-associated nephropathy

Multicentric carpotarsal osteolysis (MCTO) is an ultra-rare skeletal dysplasia characterized by aggressive osteolysis and in many patients, progressive renal failure. Previous work identified autosomal dominant missense mutations in the transactivation domain of the transcription factor MAFB as the cause of MCTO. Because the pathological mechanism resulting in MCTO is not clear, we used mice carrying the MCTO mutation (MafbMCTO/MCTO mice) to determine the role of MAFB. Using a cycloheximide chase assay in HEK293T cells, we found that the MCTO mutation resulted in reduced degradation and thus accumulation of recombinant MAFB protein. MafbMCTO/MCTO mice exhibited focal segmental glomerulosclerosis (FSGS) similar to what is observed in MCTO patients. To analyze the effect of the excessive presence of MAFB, we generated heterozygous MafbMCTO/- mice by crossing MafbMCTO/MCTO and Mafb+/- animals. Such MafbMCTO/- mice did not suffer albuminuria and exhibited histologically normal kidney appearance. The finding that MCTO mutation haploinsufficiency rescues animals from nephropathy is consistent with the mutation resulting in excess MAFB as the cause of disease. Next, we performed RNA-seq of the glomeruli from MafbMCTO/MCTO mice. We found marked up-regulation of the PI3K/AKT signaling pathway. Because receptor tyrosine kinases are PI3K/AKT activators, we administered the specific inhibitor imatinib to MafbMCTO/MCTO mice. This resulted in a significant reduction of urinary albumin relative to untreated animals. Taken together, these results suggest that excess MAFB and the activation of PI3K/AKT signaling induce nephropathy in MCTO. MAFB and/or PI3K/AKT signaling may therefore represent new treatment targets in MCTO patients.