[Study on the association between CYP2D6*10 polymorphism and adverse reactions to Tamoxifen].

<b>Objective:</b> We aimed to investigate the relationship between CYP2D6*10 gene polymorphisms and adverse reactions associated with tamoxifen treatment in breast cancer patients, and assess the value of CYP2D6*10 gene polymorphism testing in guiding the use of medications in endocrine therapy for breast cancer. <b>Methods:</b> 177 breast cancer patients with HR-positive and postoperative tamoxifen were admitted to the First Affiliated Hospital of Nanjing Medical University from November 2012 to December 2021. Their clinicopathologic data were collected for follow-up observation of adverse reactions related to tamoxifen treatment. After two years of tamoxifen treatment, finger blood of these patients was taken for CYP2D6 gene polymorphism detection. Moreover, databases including RNAfold, QTLbase, 3DSNP v2.0, RegulomeDB 2.2, and HaploReg v4.2 were used to predict the annotation of proximal and distal interactions of CYP2D6 polymorphic sites between genes and regulatory elements. <b>Results:</b> Genotyping analysis revealed 40 patients (22.6%) with the CC genotype, 79 (44.6%) with the CT genotype, and 58 (32.8%) with the TT genotype. Common adverse reactions to tamoxifen included abnormal liver function (58), fatty liver (81), uterine fibroids (12), and endometrial surgery for endometrial thickening (17). Univariate and multivariate logistic regression analyses showed that there were no significant statistical differences between the CC and CT+TT genotypes in terms of liver damage, new-onset fatty liver, uterine fibroids, or tumor recurrence and metastasis (<i>P</i>＞0.05). Notably, endometrial thickening was more significant in patients with the CT+TT genotype (4.37±3.82 mm) than in those with the CC genotype (2.43±2.96 mm), with a statistically significant difference between them (<i>P</i>＜0.01). Bioinformatic analysis suggested that in breast tissues, the CYP2D6*10 polymorphic locus had a significant expression quantitative trait locus (eQTL) effect with CYP2D6, and its genetic variations could affect the binding of CYP2D6 to transcription factors, which might modulate the expression of CYP2D6 through changes in secondary structure and chromatin modifications, etc., and thus affect the tamoxifen drug sensitivity. Further eQTL analysis showed significant correlation between CYP2D6 expression levels with different genotypes of the CYP2D6 rs1065852 polymorphism in breast tissues (<i>P</i>＜0.01). <b>Conclusion:</b> Tamoxifen remains a primary therapeutic agent for premenopausal HR-positive breast cancer patients, and its efficacy is influenced by polymorphisms in the CYP2D6*10. It is recommended that for breast cancer patients carrying the CYP2D6 CT and TT genotypes, endometrial monitoring should be strengthened during treatment with tamoxifen, and the medication should be adjusted in a timely manner.