One pedigree with four patients has been recently described with mitochondrial DNA depletion and mutation in SUCLA2 gene leading to succinyl-CoA synthase deficiency. Patients had a Leigh-like encephalomyopathy and deafness but besides the presence of lactic acidosis, the profile of urine organic acid was not reported. We have studied 14 patients with mild ‘unlabelled’ methylmalonic aciduria (MMA) from 11 families. Eight of the families are from the Faroe Islands, having a common ancestor, and three are from southern Italy. Since the reaction catalysed by succinyl-CoA synthase in the tricarboxylic acid (TCA) cycle represents a distal step of the methylmalonic acid pathway, we investigated the SUCLA2 gene as a candidate gene in our patients. Genetic analysis of the gene in the 14 patients confirmed the defect in all patients and led to the identification of three novel mutations (p.Gly118Arg; p.Arg284Cys; c.534 + 1G → A). The defect could be convincingly shown at the protein level and our data also confirm the previously described mitochondrial DNA depletion. Defects in SUCLA2 can be found at the metabolite level and are defined by mildly elevated methylmalonic acid and C4-dicarboxylic carnitine concentrations in body fluids in association with variable lactic acidosis. Clinically the diagnosis should be considered in patients with early/neonatal onset encephalomyopathy, dystonia, deafness and Leigh-like MRI abnormalities mainly affecting the putamen and the caudate nuclei. The frequency of the mutated allele in the Faroese population amounted to 2%, corresponding with an estimated homozygote frequency of 1 : 2500. Our data extend knowledge on the genetic defects causing MMA. Our patients present with an early infantile Leigh-like encephalomyopathy with deafness, and later on a progressive dystonia. Mild MMA, lactic acidosis and specific abnormalities in the carnitine ester profile are the biochemical hallmarks of the disease. In view of the frequency of the mutated allele on the Faroe Islands, measures become feasible to prevent the occurrence of the disease on the islands. We confirm and extend the findings on this inborn error of metabolism in the TCA cycle that must be carefully investigated by accurate metabolite analyses.

Published mutations in deoxyguanosine kinase (DGUOK) cause mitochondrial DNA depletion and a clinical phenotype that consists of neonatal liver failure, nystagmus and hypotonia. In this series, we have identified 15 different mutations in the DGUOK gene from 9 kindreds. Among them, 12 have not previously been reported. Nonsense, splice site, or frame‐shift mutations that produce truncated proteins predominate over missense mutations. All patients who harbor null mutations had early onset liver failure and significant neurological disease. These patients have all died before 2‐years of age. Conversely, two patients carrying missense mutations had isolated liver disease and are alive in their 4th year of life without liver transplant. Five subjects were detected by newborn screening, with elevated tyrosine or phenylalanine. Consequently, this disease should be considered if elevated tyrosine is identified by newborn screening. Mitochondrial DNA content was below 10% of controls in liver in all but one case and modestly reduced in blood cells. With this paper a total of 39 different mutations in DGUOK have been identified. The most frequent mutation, c.763_c.766dupGATT, occurs in 8 unrelated kindreds. 70% of mutations occur in only one kindred, suggesting full sequencing of this gene is required for diagnosis. The presentation of one case with apparent viral hepatitis, without neurological disease, suggests that this disease should be considered in patients with infantile liver failure regardless of the presence of neurological features or apparent infectious etiology.

Sixteen unrelated Southern European patients with the mitochondrial depletion syndrome (MDS) were analyzed for mutations in the TK2 and DGUOK genes. Three novel mutations were identified in TK2 (R183G, R254X, and 142insG). When we analyzed additional genes involved in the dNTPs pool, such as SLC25A19 (DNC) and NT5M (d‐NT2), we did not detect mutations. The current study suggest that scanning the TK2, DGUOK, SLC25A19, and NT5M genes is likely to help about 10% of MDS families in terms of genetic counseling. Also, our findings indicate that genotype‐phenotype correlations are not straightforward in MDS.

Background: Dominant mutations in COL6A1, COL6A2, and COL6A3, the three genes encoding collagen type VI, a ubiquitous extracellular matrix protein, are associated with Bethlem myopathy (BM) and Ullrich scleroatonic muscular dystrophy. Methods: The authors devised a method to screen the entire coding sequence of the three genes by reverse transcriptase–PCR amplification of total RNA from skin fibroblasts and direct sequencing of the resulting 25 overlapping cDNA fragments covering 107 exons. Results: Four splicing and four missense mutations were identified in 16 patients with BM, six of which are novel mutations in COL6A1. Both common and private mutations are localized in the α1 (VI) chain between the regions corresponding to the 3′ end of the NH2-globular domain and the 5′ end of the triple helix, encoded by exons 3 through 14. Conclusions: The clustering of the mutations in a relatively narrow area of the three collagen type VI chains in patients with Bethlem myopathy (BM) suggests that mutations in different regions could result in different phenotypes or in no phenotype at all. Moreover, the detection of mutations in only 60% of the patients suggests the existence of at least another gene associated with BM. The authors propose the direct sequencing of COL6 cDNAs as the first mutation screening analysis in BM, given the high number of exon-skipping events.