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Background. By 2020, breast cancer (BC) has become the most frequent malignancy in the world. The most common type of BC is HR+/HER2-negative cancer,2540% of which harbors PIK3CA mutations that affect the catalytic subunit of the PI3K protein. PIK3CA alterations are actionable, as such neoplasms can be treated with a combination of fulvestrant and the PI3K inhibitor alpelisib. As PIK3CA mutations have an extremely versatile effect on the characteristics of a tumor cell, numerous associations of PIK3CA mutations and various clinico-pathological characteristics of BC can be traced. Aim. Our aim was to clarify the information on the frequency and spectrum of PIK3CA mutations in Russian patients with HR+/HER2- advanced BC, and to study the association of PIK3CA mutations with clinical and pathological parameters of BC. Materials and methods. Tissue samples from 694 patients with HR+/HER2- advanced BC (mixed population of primary metastatic and relapsed tumors) who received any line of anti-cancer treatment in Dec 2020 to June 2021 in Russian Federation were analyzed by high-resolution melting, allele-specific PCR, digital droplet PCR and Sanger sequencing (exons 7,9, and 20 of the PIK3CA gene). Mutation rates in different BC subgroups were compared using the Fishers exact test. The age at diagnosis in patients with different PIK3CA status was compared using the MannWhitney U-test. The relationship between the PIK3CA status and the degree of tumor differentiation was compared using the CochraneArmitage test for trends. Luminal A and B BC expression subtypes were distinguished with surrogate IHC markers according to St.-Gallen recommendations (2013). Results. Mutations were identified in 220/694 (32%) BC patients. The three most frequent missense substitutions in the PIK3CA gene (p.E542K, p.E545K, and p.H1047R) accounted for 190/220 (86%) mutations. Associations of PIK3CA mutations with luminal A subtype of BC, low proliferation index, small size of the primary tumor, and absence of signs of hereditary cancer were revealed. Associations of mutations in the kinase domain of PIK3CA (p.H1047R) with late recurrence of locally advanced BC and with non-Slavic ethnic origin of patients were found. Conclusion. PIK3CA mutation rate of 32% confirms high prevalence of mutation in Russian population, with some differences reflecting the ethnic origin of patients.

Relevance. PIK3CA belongs to the top three most frequently mutated genes in breast cancer (BC), especially in estrogen receptor (ER) positive, HER2 negative BC subtype. With an approval of selective PI3K-alpha inhibitor, alpelisib, this alteration has become actionable in ER+HER2- tumors. The frequency and spectrum of PIK3CA alterations in various cohorts is affected by a number of factors, including the distribution of BC expression subtypes, histological types, patient age, and even ethnicity. Aim. Aim of the current study was to characterize the frequency and spectrum of PIK3CA alterations in Russian BC patients. Materials and methods. The analysis of PIK3CA exon 7, 9 and 20 mutations was performed in a cohort of Russian ER+HER2- BC patients by a combination of high-resolution melting analysis, allele-specific PCR, and digital droplet PCR. Results. PIK3CA lesions were identified in 62/206 (30%) patients. Noteworthy, 59/62 (95%) of the identified variants were represented by the three most common p.E542K, p.E545K, and p.H1047R substitutions. The analysis of clinical and morphological characteristics revealed the trends towards association of PIK3CA mutations with older age and more frequent metastatic lung involvement. Conclusion. The obtained data on the frequency and spectrum of PIK3CA somatic aberrations can be helpful when organizing molecular genetic testing of breast cancer patients and using PI3K inhibitors in Russian population.