Molecular and clinical genetics of mitochondrial diseases due to POLG mutations

Abstract

Mutations in the POLG gene have emerged as one of the most common causes of inherited mitochondrial disease in children and adults. They are responsible for a heterogeneous group of at least 6 major phenotypes of neurodegenerative disease that include: 1) childhood Myocerebrohepatopathy Spectrum disorders (MCHS), 2) Alpers syndrome, 3) Ataxia Neuropathy Spectrum (ANS) disorders, 4) Myoclonus Epilepsy Myopathy Sensory Ataxia (MEMSA), 5) autosomal recessive Progressive External Ophthalmoplegia (arPEO), and 6) autosomal dominant Progressive External Ophthalmoplegia (adPEO). Due to the clinical heterogeneity, time-dependent evolution of symptoms, overlapping phenotypes, and inconsistencies in muscle pathology findings, definitive diagnosis relies on the molecular finding of deleterious mutations. We sequenced the exons and flanking intron region from approximately 350 patients displaying a phenotype consistent with POLG related mitochondrial disease and found informative mutations in 61 (17%). Two mutant alleles were identified in 31 unrelated index patients with autosomal recessive POLG-related disorders. Among them, 20 (67%) had Alpers syndrome, 4 (13%) had arPEO, and 3 (10%) had ANS. In addition, 30 patients carrying one altered POLG allele were found. A total of 25 novel alterations were identified, including 6 null mutations. We describe the predicted structural/functional and clinical importance of the previously unreported missense variants and discuss their likelihood of being pathogenic. In conclusion, sequence analysis allows the identification of mutations responsible for POLG-related disorders and, in most of the autosomal recessive cases where two mutant alleles are found in trans, finding deleterious mutations can provide an unequivocal diagnosis of the disease.

Figure 1

A. Amino acid alignment of the human POLG amino acid reference sequence versus POLG sequences of various species, with the locations of novel missense variants superimposed. B. New POLG Mutations. The position of the 25 novel mutations is indicated with reference to the exon-intron structure of the POLG gene locus, and the corresponding domains of the POLG protein.

Figure 2

Molecular modeling of the human DNA polymerase gamma active site with amino acids of interest indicated. Critical amino acids important for catalysis are colored in. Those residues modified by POLG mutations are decorated: magenta for those carboxylic residues involved in chelating the active site Mg²⁺ ions (grey spheres); blue for those amino acids critical in dNTP recognition. Amino acid side chains colored in orange represent the sites of substitutions caused by novel disease mutations identified in POLG in this study. An incoming dNTP molecule is shown in turquoise. A. The polymerase active site viewed with the palm domain labeled. B. The polymerase active site rotated to view the fingers domain.