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5-Methyl-2'-deoxycytidine (mC) and the product of its controlled oxidation, 5-hydroxymethyl-2'-cytidine (hmC), play a key role in the epigenetic regulation of gene expression, the cell differentiation, and the carcinogenesis. Due to spontaneious deamination, genomic CpG sites containing mC and hmC serve as mutagenesis hotspots. In addition, error-prone translesion and reparative DNA polymerases may serve as additional source of mutations in the lesion-containing regions with CpG sites. In the present work, we performed in vitro analysis of the accuracy of nucleotide incorporation opposite to mC and hmC by human DNA polymerases Polβ, Polλ, Polη, Polι, PoIκ and primase polymerase PrimPol. The results of the study show a high accuracy of copying mC and hmC by the reparative DNA polymerases polymerases Polβ and Polλ, while Polη, Polι, PoIκ, and PrimPol copied mC and hmC with less accuracy evident by incorporation of dAMP and dTMP. The same spectrum of error-prone dNMP incorporation was also noted at sites with unmodified cytosines.

AOA are autosomal recessive ataxias with a common feature of oculomotor apraxia (OA) - inability to coordinate eye movements. The group includes AOA1 (APTX gene), relatively common AOA2 (SETX gene) and AOA3 (PIK 3R5 gene) described in 2012 in a Saudi family. OA is typical also for Louis-Bar ataxia-telangiectasia and its variants. А first Russian AOA2 case confirmed by DNA test is presented. The disease in a 25-year-old male started in 18 years, in 23 years he lost independent walking due to incoordination and weakness. OA produced few symptoms and was not recorded previously. Sensorimotor axonal polyneuropathy was confirmed by EMG. MRI showed cerebellar atrophy. Alpha-fetoprotein level was tenfold raised. A hereditary ataxia was considered from the disease onset, and a number of genetic tests were performed, but AOA2 was recognized only seven years later. On direct sequencing of SETX exons 6-8 a novel frame-shift mutation с.2623-2626 del 4 in heterozygous state was detected which is sufficient for AOA2 confirmation; the allelic mutation is in search. Recently a first Russian AOA1 case in a 15-year-old girl was also confirmed in our laboratory: compound-heterozygosity for two novel APTX mutations was detected. Evidently AOA are underestimated in clinical diagnostics while DNA testing permits genetic prophylaxis in families. OA should be purposefully searched for in children and young adults suspicious of autosomal recessive ataxias.