Fibroblastic spindle cell tumors are a heterogeneous group of rare soft tissue tumors that are increasingly recognized as associated with a variety of kinase gene fusions. We report two cases of GAB1‐ABL1 fusions in spindle cell tumors that histologically overlap with neurotrophic tyrosine receptor kinase (NTRK)‐rearranged spindle cell tumors. The first case occurred in a 76‐year‐old female who had a large deep‐seated spindle cell tumor composed of monotonous ovoid to spindle cells in a background of thick stromal collagen bands with prominent hyalinized vessels and inconspicuous mitoses (<1/10 HPF). Immunohistochemical stains showed co‐expression of S100 and CD34. A GAB1‐ABL1 fusion was detected by whole transcriptome RNA sequencing. The patient had a partial response to imatinib. The second case was previously described as a solitary fibrous tumor, occurring in a 9‐year‐old female with a cellular spindle cell tumor with patchy CD34 immunoexpression but no expression of S100. Upon clinicopathologic re‐review, including anchored multiplex next‐generation sequencing, a GAB1‐ABL1 fusion was identified. In summary, we report the first two cases of spindle cell tumors with variable expression of CD34 and/or S100, driven by GAB1‐ABL1 gene fusions with histologic overlap with NTRK‐rearranged spindle cell tumors, suggesting that ABL‐fusions may also be oncogenic drivers within this spectrum of tumors. These cases highlight the evolving understanding of fibroblastic spindle cell tumor biology and the utility of sequencing in identifying a targetable alteration.

Spindle cell/sclerosing rhabdomyosarcoma (ssRMS) is a rare subtype of rhabdomyosarcoma, commonly harboring a gain-of-function L122R mutation in the muscle-specific master transcription factor MYOD1. MYOD1-mutated ssRMS is almost invariably fatal, and development of novel therapeutic approaches based on the biology of the disease is urgently needed. MYOD1 L122R affects the DNA-binding domain and is believed to confer MYC-like properties to MYOD1, driving oncogenesis. Moreover, the majority of the MYOD1-mutated ssRMS harbor additional alterations activating the PI3K/AKT pathway. It is postulated that the PI3K/AKT pathway cooperates with MYOD1 L122R. To address this biological entity, we established and characterized a new patient-derived ssRMS cell line OHSU-SARC001, harboring MYOD1 L122R as well as alterations in PTEN, PIK3CA, and GNAS. We explored the functional impact of these aberrations on oncogenic signaling with gain-of-function experiments in C2C12 murine muscle lineage cells. These data reveal that PIK3CAI459_T462del, the novel PIK3CA variant discovered in this patient specimen, is a constitutively active kinase, albeit to a lesser extent than PI3KCAE545K, a hotspot oncogenic mutation. Furthermore, we examined the effectiveness of molecularly targeted PI3K/AKT/mTOR and RAS/MAPK inhibitors to block oncogenic signaling and suppress the growth of OHSU-SARC001 cells. Dual PI3K/mTOR (LY3023414, bimiralisib) and AKT inhibitors (ipatasertib, afuresertib) induced dose-dependent reductions in cell growth. However, mTOR-selective inhibitors (everolimus, rapamycin) alone did not exert cytotoxic effects. The MEK1/2 inhibitor trametinib did not impact proliferation even at the highest doses tested. Our data suggest that molecularly targeted strategies may be effective in PI3K/AKT/mTOR-activated ssRMS. Taken together, these data highlight the importance of utilizing patient-derived models to assess molecularly targetable treatments and their potential as future treatment options.

DNMT3A Overgrowth Syndrome (DOS, also known as Tatton-Brown Rahman Syndrome/TBRS) is one of several overgrowth syndromes with complex phenotypes caused by constitutional mutations in genes encoding epigenetic regulators. The clinical features of DOS are variable but include overgrowth (tall stature and/or obesity) and intellectual disability. DNMT3A is essential for de novo DNA methylation and plays an important role in hematopoiesis. Somatic mutations in DNMT3A are among the most common initiating mutations in normal karyotype acute myeloid leukemia (AML) patients and in elderly people with clonal hematopoiesis. The natural history of DOS has not been fully explored since the first description of this rare condition in 2014. Because of the association of somatic DNMT3A mutations and leukemia development, we assessed information from the ~200 known DOS patients world-wide and were able to document eight with hematologic malignancies. Based on this prevalence, we suggest DOS is a cancer predisposition syndrome, especially for hematologic malignancies. Using recommendations from an expert panel, we suggest DOS patients should be prospectively monitored for hematologic malignancies, which may allow for early intervention and permit its natural history to be better defined.