Ataxia–Telangiectasia Mutated (ATM) Protein Expression in Uterine Smooth Muscle Tumors

Abstract

Background: Ataxia-telangiectasia-mutated (ATM) is a serine-threonine protein kinase essential in cellular response to double-strand DNA damage. It undergoes auto- phosphorylation after DNA damage and subsequently initiates a signalling cascade that involves the phosphorylation of several other substrates, many of which play an important tumor-suppressing role. When ATM is functional, cells can respond to stress using this DNA damage response pathway. Dysregulation of this pathway in uterine leiomyosarcoma contributes to genomic instability. Analysis of ATM protein expression in uterine smooth muscle tumors has not been investigated. Design: Expression of ATM and phosphorylated ATM (p-ATM) was evaluated on 69 uterine leiomyosarcomas, 18 leiomyomas and 28 myometrium. Immunohistochemistry was performed with monoclonal antibodies for ATM (Clone Y170, Abcam) and p-ATM (Clone EP1890Y, Abcam) on formalin-fixed paraffin embedded tissue. Staining of the nucleus was considered positive. Lymphocytes served as internal positive controls. Immunohistochemical expression was analysed by using a histoscore, generated by multiplying the intensity (0, 1, 2, 3) with the extent (0, 10%, 25%, 50%, 75% and 100%). The final score ranged from 0 to 300, with 0 indicated complete loss of expression. Results: The median age of leiomyosarcoma patients was 50 years (range 33–80). All patients underwent hysterectomy with bilateral salpingo-oophorectomy and without preoperative chemo or radiation therapy. All tumors showed spindle cells differentiation, with diffuse nuclear atypia and >10 mitoses per 10 high power fields (0.55 mm field diameter). Tumor cell necrosis was present in 46 (70%). Abnormal expression (reduced or loss of expression) of either marker was observed in 65/69 (94%) leiomyosarcomas. In 42 (60.9%), there was a complete loss of staining. The remaining cases had a range of histoscores which ranged from 0 – 300 (Table 1). Retained expression of both markers were present in both the leiomyoma and myometrium groups. There was a significant difference in ATM and p-ATM expression between myometrium and leiomyosarcoma (p<0.05), and between leiomyoma and leiomyosarcoma (p<0.05). Follow-up information was obtained for 65 cases, which ranged from 1 to 276 months (median, 26 months). Forty patients (61.2%) died of disease and two (3%) were alive with disease. Among the group of leiomyosarcoma patients who had tumors which showed complete loss of expression of either ATM, p-ATM, or both markers, 28 (71.8%) either died of disease or alive with disease at last follow-up (p<0.05). Conclusions: • Abnormal expression of ATM or p-ATM was present in 94% of leiomyosarcoma, with complete loss of expression of either one or both markers in 60.9%, reflecting the frequent dysregulation of DNA damage response. • Retained expression of ATM and p-ATM was observed in leiomyoma and myometrium. • Within the leiomyosarcoma group, although the majority had complete loss of expression in either or both markers, the remaining tumors expressed ATM and p-ATM at various intensities and extents. Therefore, they were not found be robust diagnostic markers for distinguishing benign and malignant tumors. • Loss of expression of either ATM, p-ATM, or both markers, was a poor prognostic factor in our cohort.