Novel ATXN1/ATXN1L::NUTM2A fusions identified in aggressive infant sarcomas with gene expression and methylation patterns similar to CIC- rearranged sarcoma

CIC -rearranged sarcomas are newly defined undifferentiated soft tissue tumors with CIC- associated fusions, and dismal prognosis. CIC fusions activate PEA3 family genes, ETV1/4/5, leading to tumorigenesis and progression. We report two high-grade CNS sarcomas of unclear histological diagnosis and one disseminated tumor of unknown origin with novel fusions and similar gene-expression/methylation patterns without CIC rearrangement. All three patients were infants with aggressive diseases, and two experienced rapid disease deterioration and death. Whole-transcriptome sequencing identified an ATXN1-NUTM2A fusion in the two CNS tumors and an ATXN1L-NUTM2A fusion in case 3. ETV1/4/5 and WT1 overexpression were observed in all three cases. Methylation analyses predicted CIC -rearranged sarcoma for all cases. Retrospective IHC staining on case 2 demonstrated ETV4 and WT1 overexpression. ATXN1 and ATXN1L interact with CIC forming a transcription repressor complex. We propose that ATXN1/ATXN1L -associated fusions disrupt their interaction with CIC and decrease the transcription repressor complex, leading to downstream PEA3 family gene overexpression. These three cases with novel ATXN1/ATXN1L -associated fusions and features of CIC -rearranged sarcomas may further expand the scope of “ CIC -rearranged” sarcomas to include non- CIC rearrangements. Additional cases are needed to demonstrate if ATXN1/ATXN1L-NUTM2A fusions are associated with younger age and more aggressive diseases.