Abstract 738: Myxofibrosarcoma is characterized by frequent abnormalities in TP53 and increased genetic instability
Authors: Takeuchi Yasuhide , Kunitz Annegret , Suzuki Hiromichi , Yoshida Kenichi , Shiraishi Yuichi , Chiba Kenichi , Tanaka Hiroko , Shimamura Teppei , Kakiuchi Nobuyuki , Shiozawa Yusuke , Yokoyama Akira , Yoshizato Tetsuichi , Aoki Kosuke , Fujii Yoichi , Nannya Yasuhito , Makishima Hideki , Miyano Satoru , Haga Hironori , Damm Frederik , Ogawa Seishi 
Published: August 2019

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 (n=65), RNA sequencing (n=3), and immunohistochemistry (IHC, n=50). Copy number (CN) alterations were detected by sequence-based CN analysis. The obtained genomic data were combined with those from STS cases from the Cancer Genome Atlas (TCGA) cohort, including 17 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 4,613 mutations were identified by WES in 61 primary MFS samples with a median of 44.0 mutations/sample. Mutations were dominated by age-related C to T transitions at CpG sites. Among 84 primary MFS samples, most frequently mutated and/or CN-altered genes included TP53 (n=64, 76.2%), RB1 (n=29, 34.5%), CDKN2A (n=25, 30.0%), and ATRX (n=18, 21.4%). A fusion gene involving TRIO was newly identified by RNA sequencing in a single case. 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 as many as 83.3% of the MFS cases (n=70). 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. Finally, 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 initial diagnosis, while most of the 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, Hiromichi Suzuki, Kenichi Yoshida, Yuichi Shiraishi, Kenichi Chiba, Hiroko Tanaka, Teppei Shimamura, Nobuyuki Kakiuchi, Yusuke Shiozawa, Akira Yokoyama, Tetsuichi Yoshizato, Kosuke Aoki, Yoichi Fujii, Yasuhito Nannya, Hideki Makishima, Satoru Miyano, Hironori Haga, Frederik Damm, Seishi Ogawa. Myxofibrosarcoma is characterized by frequent abnormalities in TP53 and increased genetic instability [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 738.