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Table 1 Relative frequency of mutations associated with the different mitochondrial DNA depletion syndromes (MDS, source: HGMD Professional database: http://www.hgmd.cf.ac.uk/ )

From: Syndromes associated with mitochondrial DNA depletion

Mitochondrial DNA depletion syndromes Period of onset Clinical features Genes (aliases) Chromosomal LOCI Numbers of mutations References*
Hepatocerebral mtDNA depletion syndrome Neonatal, early childhood Hepatic dysfunction; psychomotor delay; hypotonia; lactic acidosis; nystagmus; neurological dysfunction POLG (Polg1/PolgA) 15q25 8 [10]
C10orf2 (Twinkle/PEO1) 10q24 3 [11]
DGUOK (dGK) 2p13 51 [12]
MPV17 (SYM1) 2p23.3 28 [13]
    TK2 16q22-q23.1 1 [14]
Alpers-Huttenlocher syndrome Early childhood Hepatic dysfunction; epilepsia partialis continua; neurological dysfunction POLG (Polg1/PolgA) 15q25 54 [15]
Myopathic mtDNA depletion syndrome Infancy, early childhood Hypotonia; muscle weakness; dysarthria and dysphagia; failure to thrive POLG (Polg1/PolgA) 15q25 1 [5]
TK2 16q22-q23.1 34 [16]
RRM2B (p53R2) 8q23.1 3 [5]
    DGUOK (dGK) 2p13 1 [17]
Encephalomyopathic mtDNA depletion syndrome Infancy Hypotonia; muscle weakness; psychomotor delay; sensorineural hearing impairment; lactic acidosis; neurological dysfunction RRM2B (p53R2) 8q23.1 14 [18]
TK2 16q22-q23.1 1 [19]
SUCLA2 13q12.2 9 [20]
    SUCLG1 2p11.3 13 [21]
Mitochondrial neurogastrointestinal encephalomyopathy Late childhood, adolescence Gastrointestinal dysmotility; weight loss; peripheral neuropathy;ptosis; neurological dysfunction TYMP (ECGF1) 22q13 81 [22]
RRM2B (p53R2) 8q23.1 2 [23]
    POLG (Polg1/PolgA) 15q25 1 [24]
  1. MDS are inherited in an autosomal recessive pattern; mutations in POLG gene outside MDS may be associated with autosomal recessive/dominant pattern of inheritance. *References of the first mutations published associated with the phenotype.