Mpv17 / PMP22 familyThe 22-kDa peroxisomal membrane protein (PMP22) is a major component of peroxisomal membranes. PMP22 seems to be involved in pore forming activity and may contribute to the unspecific permeability of the organelle membrane. PMP22 is synthesized on free cytosolic ribosomes and then directed to the peroxisome membrane by specific targeting information. Mpv17 is a closely related peroxisomal protein. In mouse, the Mpv17 protein is involved in the development of early-onset glomerulosclerosis. More recently a homolog of Mpv17 in S. cerevisiae has been been found to be an integral membrane protein of the inner mitochondrial membrane where it has been proposed to have a role in ethanol metabolism and tolerance during heat-shock. Defects in MPV17 is associated with mitochondrial DNA depletion syndrome (MDDS) and Navajo neurohepatopathy (NNH). MDDS is a clinically heterogeneous group of disorders characterized by a reduction in mitochondrial DNA (mtDNA) copy number. Primary mtDNA depletion is inherited as an autosomal recessive trait and may affect single organs, typically muscle or liver, or multiple tissues. Individuals with the hepatocerebral form of mitochondrial DNA depletion syndrome have early progressive liver failure and neurologic abnormalities, hypoglycemia, and increased lactate in body fluids. NNH is an autosomal recessive disease that is prevalent among Navajo children in the South Western states of America. The major clinical features are hepatopathy, peripheral neuropathy, corneal anesthesia and scarring, acral mutilation, cerebral leukoencephalopathy, failure to thrive, and recurrent metabolic acidosis with intercurrent infections. Infantile, childhood, and classic forms of NNH have been described. Mitochondrial DNA depletion was detected in the livers of patients, suggesting a primary defect in mtDNA maintenance.