[Values of ATRX protein expression and ALT activation in differential diagnosis of uterine leiomyosarcoma]

Objective: To investigate the correlation between ATRX expression and alternative lengthening of telomeres (ALT) activation status in uterine smooth muscle tumors, and to evaluate the values of the two indicators in diagnosing uterine leiomyosarcoma (uLMS). Methods: Cases of uLMS and its mimickers diagnosed at the Department of Pathology, Peking University Third Hospital, Beijing, China from January 2024 to October 2025 were retrospectively identified and analyzed. Immunohistochemistry was used to detect ATRX protein expression, while telomere-specific fluorescence in situ hybridization was applied to determine ALT activation status. Next-generation sequencing (NGS) was conducted to detect ATRX gene variations in some cases. Fisher's exact test was used to analyze the correlation between ATRX expression and ALT status, as well as the differences in ATRX expression and ALT activation among different tumor types. Kappa test was applied to evaluate the agreement between ATRX expression and ALT status. Results: There were 60 cases of uLMS, 36 cases of smooth muscle tumors of uncertain malignant potential (STUMP), 77 cases of uterine leiomyoma (uLM), and 6 cases of high-grade endometrial stromal sarcoma. NGS revealed that ATRX gene mutations were mostly accompanied by complete loss of protein expression, while copy number reduction was mostly associated with decreased protein expression. The concordant rate between ATRX expression and ALT activation status was 86.4%, with a significant correlation (P<0.001) and substantial consistency (Kappa=0.68). The rates of complete ATRX loss (35/59, 59.3%) and ALT activation (49/60, 81.7%) in uLMS were significantly higher than those in the other groups: 14.3% (5/35) and 17.1% (6/35) in STUMP, 0 (0/72) and 2.6% (2/77) in uLM, and 0 (0/6) for both markers in high-grade endometrial stromal sarcoma. All intergroup differences were statistically significant (all P<0.001). Conclusions: ATRX loss is highly correlated with ALT activation status. Combined detection of ATRX expression and ALT activation status can effectively assist in the differential diagnosis of uLMS from its mimickers. In addition, the presence of ATRX loss and/or ALT activation in STUMP and uLM suggests a malignant potential and seems to warrant close clinical follow-up and individualized diagnosis and treatment decisions.