Hereditary axonal neuropathy related to MME gene mutation in a family with fetomaternal alloimmune glomerulonephritis.

We report a consanguineous family with a homozygous and heterozygous membrane metallo-endopeptidase (MME) mutation (c.467delC) and two clinical conditions: fetomaternal alloimmune membranous glomerulopathy (FMG) and hereditary motor and sensory axonal neuropathy. The penetrance of both phenotypes was variable. Some individuals experienced unusually fast neurological degradation. Pain and vasomotor signs were frequent complaints, possibly due to a loss of the neutral endopeptidase (NEP, the MME gene product) function and its subsequent inability to degrade substance P and vasomotor peptides. Electrophysiological and nerve biopsy findings were consistent with predominantly axonal neuropathy. This specific clinical phenotype was attributed to a c.467delC MME gene mutation. Diagnosis of such a mutation is important but can be challenging, due to allele dropout. Heterozygous subjects who had already reached the expected age of disease onset had peripheral neuropathy, but also suffered from additional diseases. Neurologists should advise women of childbearing age with MME mutations to seek pre-pregnancy genetic advice and nephrologists should search for neuropathy in patients with FMG.

Mutant p97 exhibits species-specific changes of its ATPase activity and compromises the UBXD9-mediated monomerisation of p97 hexamers.

p97 (VCP) is a homo-hexameric triple-A ATPase that exerts a plethora of cellular processes. Heterozygous missense mutations of p97 cause at least five human neurodegenerative disorders. However, the specific molecular consequences of p97 mutations are hitherto widely unknown. Our in silico structural models of human and Dictyostelium p97 showed that the disease-causing human R93C, R155H, and R155C as well as Dictyostelium R154C, E219K, R154C/E219K p97 mutations constitute variations in surface-exposed locations. In-gel ATPase activity measurements of p97 monomers and hexamers revealed significant mutation- and species-specific differences. While all human p97 mutations led to an increase in ATPase activity, no changes could be detected for the Dictyostelium R154C mutant, which is orthologous to human R155C. The E219K mutation led to an almost complete loss of activity, which was partially recuperated in the R154C/E219K double-mutant indicating p97 inter-domain communication. By means of co-immunoprecipitation experiments we identified an UBX-domain containing Dictyostelium protein as a novel p97 interaction partner. We categorized all UBX-domain containing Dictyostelium proteins and named the interaction partner UBXD9. Pull-down assays and surface plasmon resonance analyses of Dictyostelium UBXD9 or the human orthologue TUG/ASPL/UBXD9 demonstrated direct interactions with p97 as well as species-, mutation- and ATP-dependent differences in the binding affinities. Sucrose density gradient assays revealed that both human and Dictyostelium UBXD9 proteins very efficiently disassembled wild-type, but to a lesser extent mutant p97 hexamers into monomers. Our results are consistent with a scenario in which p97 point mutations lead to differences in enzymatic activities and molecular interactions, which in the long-term result in a late-onset and progressive multisystem disease.