Human mitochondrial pyrophosphatase: cDNA cloning and analysis of the gene in patients with mtDNA depletion syndromes

Pyrophosphatases (PPases) catalyze the hydrolysis of inorganic pyrophosphate generated in several cellular enzymatic reactions. A novel human pyrophosphatase cDNA encoding a 334-amino-acid protein approximately 60% identical to the previously identified human cytosolic PPase was cloned and characterized. The novel enzyme, named PPase-2, was enzymatically active and catalyzed hydrolysis of pyrophosphate at a rate similar to that of the previously identified PPase-1. A functional mitochondrial import signal sequence was identified in the N-terminus of PPase-2, which targeted the enzyme to the mitochondrial matrix. The human pyrophosphatase 2 gene (PPase-2) was mapped to chromosome 4q25 and the 1.4-kb mRNA was ubiquitously expressed in human tissues, with highest levels in muscle, liver, and kidney. The yeast homologue of the mitochondrial PPase-2 is required for mitochondrial DNA maintenance and yeast cells lacking the enzyme exhibit mitochondrial DNA depletion. We sequenced the PPA2 gene in 13 patients with mitochondrial DNA depletion syndromes (MDS) of unknown cause to determine if mutations in the PPA2 gene of these patients were associated with this disease. No pathogenic mutations were identified in the PPA2 gene of these patients and we found no evidence that PPA2 gene mutations are a common cause of MDS in humans.