Uterine leiomyosarcomas (ULMSs) are aggressive smooth muscle tumors associated with poor clinical outcome. Despite previous cytogenetic and molecular studies, their molecular background has remained elusive. To examine somatic variation in ULMS, we performed exome sequencing on 19 tumors. Altogether, 43 genes were mutated in at least two ULMSs. Most frequently mutated genes included tumor protein P53 (TP53; 6/19; 33%), alpha thalassemia/mental retardation syndrome X-linked (ATRX; 5/19; 26%), and mediator complex subunit 12 (MED12; 4/19; 21%). Unlike ATRX mutations, both TP53 and MED12 alterations have repeatedly been associated with ULMSs. All the observed ATRX alterations were either nonsense or frameshift mutations. ATRX protein levels were reliably analyzed by immunohistochemistry in altogether 44 ULMSs, and the majority of tumors (23/44; 52%) showed clearly reduced expression. Loss of ATRX expression has been associated with alternative lengthening of telomeres (ALT), and thus the telomere length was analyzed with telomere-specific fluorescence in situ hybridization. The ALT phenotype was confirmed in all ULMSs showing diminished ATRX expression. Exome data also revealed one nonsense mutation in death-domain associated protein (DAXX), another gene previously associated with ALT, and the tumor showed ALT positivity. In conclusion, exome sequencing revealed that TP53, ATRX, and MED12 are frequently mutated in ULMSs. ALT phenotype was commonly seen in tumors, indicating that ATR inhibitors, which were recently suggested as possible new drugs for ATRX-deficient tumors, could provide a potential novel therapeutic option for ULMS. Uterine leiomyosarcomas are rare, malignant smooth muscle tumors with a poor 5-year survival and high recurrence rate. They account for 1–2% of all uterine malignancies with an estimated incidence of 0.4/100,000 women per year. The symptoms and signs of this tumor type widely overlap with those of common benign uterine leiomyomas, making early diagnosis of uterine leiomyosarcomas difficult. Currently, the diagnosis of these tumors is often incidental and postoperative. Despite previous cytogenetic and molecular studies, their molecular background has remained elusive. Identification of novel molecular genetic characteristics in uterine leiomyosarcomas is clinically relevant to further improve the diagnosis and prognosis of the patients. Here, we performed exome sequencing on 19 tumors, revealing frequent mutations in TP53, ATRX, and MED12. The discovery of frequent inactivating ATRX mutations provides a potential novel therapeutic target for uterine leiomyosarcomas.

ARID1A has been identified as a novel tumor suppressor gene in ovarian cancer and subsequently in various other tumor types. ARID1A belongs to the ARID domain containing gene family, which comprises of 15 genes involved, for example, in transcriptional regulation, proliferation and chromatin remodeling. In this study, we used exome sequencing data to analyze the mutation frequency of all the ARID domain containing genes in 25 microsatellite unstable (MSI) colorectal cancers (CRCs) as a first systematic effort to characterize the mutation pattern of the whole ARID gene family. Genes which fulfilled the selection criteria in this discovery set (mutations in at least 4/25 [16%] samples, including at least one nonsense or splice site mutation) were chosen for further analysis in an independent validation set of 21 MSI CRCs. We found that in addition to ARID1A, which was mutated in 39% of the tumors (18/46), also ARID1B (13%, 6/46), ARID2 (13%, 6/46) and ARID4A (20%, 9/46) were frequently mutated. In all these genes, the mutations were distributed along the entire length of the gene, thus distinguishing them from typical MSI target genes previously described. Our results indicate that in addition to ARID1A, other members of the ARID gene family may play a role in MSI CRC.

Uterine smooth muscle tumors range from benign leiomyomas to malignant leiomyosarcomas. Based on numerous molecular studies, leiomyomas and leiomyosarcomas mostly lack shared mutations and the majority of tumors are believed to develop through distinct mechanisms. To further characterize the molecular variability among uterine smooth muscle tumors, and simultaneously insinuate their potential malignant progression, we examined the frequency of known genetic leiomyoma driver alterations (MED12 mutations, HMGA2 overexpression, biallelic FH inactivation) in 65 conventional leiomyomas, 94 histopathological leiomyoma variants (18 leiomyomas with bizarre nuclei, 22 cellular, 29 highly cellular, and 25 mitotically active leiomyomas), and 51 leiomyosarcomas. Of the 210 tumors analyzed, 107 had mutations in one of the three driver genes. No tumor had more than one mutation confirming that all alterations are mutually exclusive. MED12 mutations were the most common alterations in conventional and mitotically active leiomyomas and leiomyosarcomas, while leiomyomas with bizarre nuclei were most often FH deficient and cellular tumors showed frequent HMGA2 overexpression. Highly cellular leiomyomas displayed the least amount of alterations leaving the majority of tumors with no known driver aberration. Our results indicate that based on the molecular background, histopathological leiomyoma subtypes do not only differ from conventional leiomyomas, but also from each other. The presence of leiomyoma driver alterations in nearly one third of leiomyosarcomas suggests that some tumors arise through leiomyoma precursor lesion or that these mutations provide growth advantage also to highly aggressive cancers. It is clinically relevant to understand the molecular background of various smooth muscle tumor subtypes, as it may lead to improved diagnosis and personalized treatments in the future.