C‐Myc Overexpression in Adolescent and Young Adult Breast Cancer: Distinct From Older Adults With Relevantly Expressed Cholecystokinin B Receptor

ABSTRACT Breast cancer in adolescents and young adults has poorer clinical outcomes, but the role of MYC in this group remains unclear. We aimed to elucidate the characteristics of MYC expression in breast cancer among adolescents and young adults. MYC expression in 42 adolescents and young adults and 110 older adults were analyzed using immunohistochemistry, fluorescence in situ hybridization, quantitative polymerase chain reaction, and RNA sequencing. Immunohistochemical c‐myc expression was higher in adolescents and young adults group compared to older adults, without MYC gene amplification. In older adults, c‐myc expression was associated with more aggressive features. Adolescents and young adults group showed higher c‐myc expression even in tumors with less aggressive features, such as estrogen receptor positive, low Ki‐67 labeling index, and early clinical stage, than older adults. RNA sequencing revealed higher expression of cholecystokinin B receptor and lower expression of uridine diphosphate glucuronosyltransferase 2 family member B4 in c‐myc positive tumors of adolescents and young adults group. The preference of positive cases for both c‐myc and cholecystokinin B receptor was significantly higher in adolescents and young adults group. In conclusion, c‐myc overexpression makes adolescents and young adults breast cancer more aggressive through multifaceted roles including relevantly expressed cholecystokinin B receptor. Clinical trial registration: This study is not a clinical trial.