Clinicopathologic and molecular analysis of embryonal rhabdomyosarcoma of the genitourinary tract: evidence for a distinct DICER1-associated subgroup

Abstract

Embryonal rhabdomyosarcoma (ERMS) of the uterus has recently been shown to frequently harbor DICER1 mutations. Interestingly, only rare cases of extrauterine DICER1-associated ERMS, mostly located in the genitourinary tract, have been reported to date. Our goal was to study clinicopathologic and molecular profiles of DICER1-mutant (DICER1-mut) and DICER1-wild type (DICER1-wt) ERMS in a cohort of genitourinary tumors. We collected a cohort of 17 ERMS including nine uterine (four uterine corpus and five cervix), one vaginal, and seven urinary tract tumors. DNA sequencing revealed mutations of DICER1 in 9/9 uterine ERMS. All other ERMS of our cohort were DICER1-wt. The median age at diagnosis of patients with DICER1-mut and DICER1-wt ERMS was 36 years and 5 years, respectively. Limited follow-up data (available for 15/17 patients) suggested that DICER1-mut ERMS might show a less aggressive clinical course than DICER1-wt ERMS. Histological features only observed in DICER1-mut ERMS were cartilaginous nodules (6/9 DICER1-mut ERMS), in one case accompanied by foci of ossification. Recurrent mutations identified in both DICER1-mut and DICER1-wt ERMS affected KRAS, NRAS, and TP53. Copy number analysis revealed similar structural variations with frequent gains on chromosomes 2, 3, and 8, independent of DICER1 mutation status. Unsupervised hierarchical clustering of array-based whole-genome DNA methylation data of our study cohort together with an extended methylation data set including different RMS subtypes from genitourinary and extra-genitourinary locations (n = 102), revealed a distinct cluster for DICER1-mut ERMS. Such tumors clearly segregated from the clusters of DICER1-wt ERMS, alveolar RMS, and MYOD1-mutant spindle cell and sclerosing RMS. Only one tumor, previously diagnosed as ERMS arising in the maxilla of a 6-year-old boy clustered with DICER1-mut ERMS of the uterus. Subsequent sequencing analysis identified two DICER1 mutations in the latter case. Our results suggest that DICER1-mut ERMS might qualify as a distinct subtype in future classifications of RMS.

Fig. 1. Clinical and molecular features of 17 ERMS of the genitourinary tract.

a Age at diagnosis, gender distribution, sites of origin and DNA sequencing data, (b) variant allele frequency (VAF) of DICER1 alterations suggesting either retained (ROH) or loss (LOH) of heterozygosity and (c) mutational spectrum of DICER1 mutations in 9 uterine ERMS. Cumulative copy-number profiles of (d) 9 DICER1-mut ERMS and (e) 8 DICER1-wt ERMS. Molecular data of ERMS 3 and 4 have previously been reported elsewhere [25].

Fig. 2. Histological features of DICER1-mut ERMS.

a Nodular growth of hypocellular areas resembling the botryoid variant of ERMS, (b) entrapment of epithelium with cuffing of adjacent small blue cells with scant cytoplasm, as well as (c) desmin and (d) myogenin positivity. e Focal rhabdomyoblasts, and (f) anaplasia may be present. g Nodules of chondroid matrix as well as (h) areas of abutting ossification with osteoid matrix and multinucleated osteoclast-like giant cells may be suggestive of DICER1-association.

Fig. 3. Histological features of DICER1-wt ERMS.

a Botryoid variant of ERMS with (b) a distinct cambium layer and (c) proliferation of small blue cells with scant cytoplasm. d Anaplasia and rhabdomyoblasts may be present, however, in contrast to DICER1-mut ERMS no nodules of cartilage or osteoid are present.

Fig. 4. Distinct patterns of DNA methylation in DICER1-mut ERMS.

Unsupervised hierarchical clustering (a) and t-SNE analysis (b) of 9 DICER1-mut and 8 DICER1-wt ERMS, together with a large methylation data set of RMS of genitourinary and extra-genitourinary locations shows distinct cluster formation for ARMS (n = 43), MYOD1-mut SRMS (n = 12), ERMS (n = 46), non-neoplastic striated muscle tissue (control; n = 8) and DICER1-mut ERMS (n = 10).