Compliance with ethical standards: the study was approved by the Ethics Committee of National Research Mordovia State University (Protocol No. 12 dated December 14, 2008). Written informed consent was obtained from all study participants. Blood samples were collected in compliance with the Declaration of Helsinki (2008), the protocol of European Convention on Human Rights and Biomedicine (1999) and the additional protocol to the Convention on Human Rights and Biomedicine concerning Biomedical Research (2005).

Author contribution: Radaeva OA designed the study, analyzed the results, formulated the conclusions and wrote the manuscript; Simbirtsev AS formulated the objective of the study, revised its conclusions and the manuscript itself; Gromova EV designed the study, carried out laboratory tests, and contributed to writing the manuscript; Iskandiarova MS analyzed the literature, supervised blood collection, followed up with the patients, contributed to writing the manuscript; Belyaeva SV analyzed the literature, supervised blood collection, followed up with the patients.