Abstract 225: Frequent abnormalities in TP53 and increased genetic instability in myxofibrosarcoma
Authors: Takeuchi Yasuhide , Kunitz Annegret , Halik Adriane , Suzuki Hiromichi , Yoshida Kenichi , Shiraishi Yuichi , Chiba Kenichi , Tanaka Hiroko , Kakiuchi Nobuyuki , Shiozawa Yusuke , Yokoyama Akira , Inoue Yoshikage , Yoshizato Tetsuichi , Aoki Kosuke , Fujii Yoichi , Nannya Yasuhito , Makishima Hideki , Miyano Satoru , Haga Hironori , Damm Frederik , Ogawa Seishi 
Published: September 2020

Abstract
Myxofibrosarcoma (MFS) is a rare subtype of soft tissue sarcomas (STSs) preferentially affecting the elderly. Histologically, MFS is distinct from other STSs, in that it is characterized by the proliferation of pleomorphic spindle cells with varying degrees of myxoid components. However, the molecular pathogenesis of MSF is poorly understood.
We conducted an integrated molecular study involving 70 samples from primary MFS patients, in which samples were analyzed by whole-genome sequencing (WGS, n=5), whole-exome sequencing (WES, n=44), targeted-capture sequencing (TS, n=65), RNA sequencing (n=3), and immunohistochemistry (IHC, n=50). Copy number (CN) alterations were detected by sequencing-based CN analysis. We combined our STS data with those from from the Cancer Genome Atlas and European Genome-phenome Archive cohort, including 116 MSF samples, and compared to the data from other STS samples (n=189). To further investigate the genetic basis of mixed histological components and chronological changes in MFS, we performed WES of multi-regional and/or multi-time-point samples from 8 MSF cases.
A total of 9,246 mutations were identified by WES in 130 primary MFS samples with a median of 55.5 mutations/sample. Mutations were dominated by age-related C to T transitions at CpG sites. When combining the results from TS, among 188 primary MFS samples, most frequently mutated and/or CN-altered genes included TP53 (n=82, 43.6%), RB1 (n=31, 16.5%), ATRX (n=29, 15.4%), and CDKN2A (n=19, 10.2%). A fusion gene involving TRIO was newly identified by RNA sequencing (n=1). A similar mutational profile was observed in undifferentiated pleomorphic sarcoma (UPS), in which TP53 (59.1%), ATRX (34.0%), RB1 (22.7%), and CDKN2A (20.5%) were the major mutational targets, suggesting the common molecular pathogenesis between these two subtypes. Combined with frequent positive staining in IHC (n=22, 44.0%), TP53 was affected in 55.3% of the MFS cases (n=104). Five MFS cases evaluated by WGS showed complex structural abnormalities suggestive of increased genetic instability, where a median of 179 structural variations were detected per sample. WES with multi-regional sampling (n=5) disclosed a high level of intratumor heterogeneity, in which less than 29.0% of mutations were shared by different samples taken from the same tumor. An analysis of longitudinal samples (n=6) revealed significantly higher numbers of mutations in relapse samples (1.6 times on average, p = 0.03). In all cases, TP53 lesions were present from at the time of the initial diagnosis, while most of other lesions were subclonal and acquired during the clinical course.
In summary, the genetic profile of MFS is characterized by frequent abnormalities in TP53, RB1, CDKN2A and ATRX, and closely related to other STSs, particularly to UPS. Clonal TP53 abnormalities resulted in complex chromosomal structure and a high degree of intratumor heterogeneity.
Citation Format: Yasuhide Takeuchi, Annegret Kunitz, Adriane Halik, Hiromichi Suzuki, Kenichi Yoshida, Yuichi Shiraishi, Kenichi Chiba, Hiroko Tanaka, Nobuyuki Kakiuchi, Yusuke Shiozawa, Akira Yokoyama, Yoshikage Inoue, Tetsuichi Yoshizato, Kosuke Aoki, Yoichi Fujii, Yasuhito Nannya, Hideki Makishima, Satoru Miyano, Hironori Haga, Frederik Damm, Seishi Ogawa. Frequent abnormalities in TP53 and increased genetic instability in myxofibrosarcoma [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 225.