Author contribution: Eliseeva NV — sample design, molecular genetic testing, manuscript writing; Ponomarenko IV — molecular genetic testing, statistical analysis, manuscript writing; Churnosov MI — study concept, manuscript editing; the final version of the manuscript was read and approved by all authors.

Compliance with ethical standards: the study was approved by the Ethics Committee of the Institute of Medicine of the Belgorod State University (protocol № 4 dated May 19, 2015); the informed consent to participation was submitted by all study participants.