Compliance with ethical standards: the study was approved by the Ethics Committee of Medical Institute of the Peoples' Friendship University of Russia (protocol № 13 dated December 19, 2019); the study design was consistent with the Declaration of Helsinki guidelines regarding the research involving identifiable human tissue samples. The informed consent was submitted by all patients.

Author contribution: Kudryavtsev GYu, Kudryavtsevа YaYu, Solovyeva NA — literature survey, manuscript writing; Kudryavtseva LV, Osipov VA — statistical data processing; Mikhaleva LM — sample collection and processing; Babichenko II — study concept and design.