Molecular Profile of a Pituitary Rhabdomyosarcoma Arising From a Pituitary Macroadenoma: A Case Report

LZTR1 Mutation Mediates Oncogenesis through Stabilization of EGFR and AXL

LZTR1 is the substrate-specific adaptor of a CUL3-dependent ubiquitin ligase frequently mutated in sporadic and syndromic cancer. We combined biochemical and genetic studies to identify LZTR1 substrates and interrogated their tumor-driving function in the context of LZTR1 loss-of-function mutations. Unbiased screens converged on EGFR and AXL receptor tyrosine kinases as LZTR1 interactors targeted for ubiquitin-dependent degradation in the lysosome. Pathogenic cancer-associated mutations of LZTR1 failed to promote EGFR and AXL degradation, resulting in dysregulated growth factor signaling. Conditional inactivation of Lztr1 and Cdkn2a in the mouse nervous system caused tumors in the peripheral nervous system including schwannoma-like tumors, thus recapitulating aspects of schwannomatosis, the prototype tumor predisposition syndrome sustained by LZTR1 germline mutations. Lztr1- and Cdkn2a-deleted tumors aberrantly accumulated EGFR and AXL and exhibited specific vulnerability to EGFR and AXL coinhibition. These findings explain tumorigenesis by LZTR1 inactivation and offer therapeutic opportunities to patients with LZTR1-mutant cancer.

SIGNIFICANCE

EGFR and AXL are substrates of LZTR1-CUL3 ubiquitin ligase. The frequent somatic and germline mutations of LZTR1 in human cancer cause EGFR and AXL accumulation and deregulated signaling. LZTR1-mutant tumors show vulnerability to concurrent inhibition of EGFR and AXL, thus providing precision targeting to patients affected by LZTR1-mutant cancer. This article is highlighted in the In This Issue feature, p. 517.

Teratocarcinosarcoma-Like and Adamantinoma-Like Head and Neck Neoplasms Harboring NAB2::STAT6: Unusual Variants of Solitary Fibrous Tumor or Novel Tumor Entities?

The archetypal solitary fibrous tumor (SFT) features fibroblastic cells with varying cellularity without any particular architectural arrangement in a collagenous matrix, with staghorn vessels, CD34 and STAT6 expression, and NAB2::STAT6. To date, this fusion is thought to be specific for SFT. With more routine use of fusion gene panels, the histologic diversity of NAB2::STAT6-positive tumors is increasingly appreciated. Here we present four head and neck tumors harboring NAB2::STAT6 but exhibiting remarkably unusual morphologic features for SFT. All cases were pulled from the authors' consultation files. Immunohistochemistry was performed, along with targeted RNA sequencing in all cases, plus DNA next-generation sequencing on two. The cases arose in the nasal cavity (n = 2), retromolar trigone (n = 1) and parapharynx (n = 1), in patients ranging from 39 to 54 (mean, 44). Both nasal cases were biphasic, with a variably cellular collagenized stroma that resembled SFT but also interspersed malignant epithelial and neuroepithelial nests. One of the nasal cases also exhibited overt rhabdomyoblastic differentiation within both components. The two non-nasal cases were comprised of plump, epithelioid cells that were diffusely positive for pan-cytokeratin. One of these cases had prominent cystic change lined by overtly squamous epithelium. STAT6 immunostaining was positive in all cases, although the epithelial/neuroepithelial nests in the sinonasal cases were negative. All cases were confirmed to harbor NAB2::STAT6 by RNA sequencing. The two sinonasal cases were also found to harbor oncogenic mutations. The presented cases highlight a much broader histologic diversity than previously known for neoplasms with NAB2::STAT6. The biphasic nasal cases closely resemble teratocarcinosarcoma, while the epithelioid, cytokeratin-positive cases could be conceptualized as "adamantinoma-like," to borrow terminology already in use for Ewing sarcomas with complex epithelial differentiation. To identify similar cases, pathologists should have a low threshold for using STAT6 immunohistochemistry on any difficult-to-characterize head and neck tumor.