Low-grade non-intestinal-type sinonasal adenocarcinoma: a histologically distinctive but molecularly heterogeneous entity.

Although low-grade non-intestinal-type sinonasal adenocarcinoma (SNAC) is formally a diagnosis of exclusion defined by the absence of salivary or intestinal differentiation, most tumors in this category comprise a distinctive histologic group that are increasingly thought to derive from seromucinous glands. However, the molecular underpinnings of SNAC remain poorly understood, and it is unclear if diverse genetic alterations recently reported in isolated cases should delineate separate subgroups. This study aims to perform comprehensive evaluation of gene fusions and mutations and their histologic correlates in low-grade SNAC to clarify its pathogenesis and classification. We identified 18 non-intestinal-type SNAC that all displayed characteristic tubulopapillary architecture and low-grade cytology, although several cases had other unique histologic features and 3 showed intermixed high-grade areas. Among tumors stained with S100 protein, SOX10, and DOG1, 86% expressed at least one of these seromucinous markers. Of 17 cases with sufficient RNA or DNA available for analysis, likely oncogenic molecular alterations were identified in 76% of cases, most notably including CTNNB1 p.S33F mutations in 2 cases, concomitant BRAF p.V600E and AKT1 p.E17K mutations in 2 cases, and ETV6::NTRK3, PRKAR1A::MET, FN1::NRG1, and DNAJB1::PRKACA fusions in 1 case each. While tumors with most genetic alterations were histologically indistinguishable, cases with CTNNB1 mutations had intermixed squamoid morules and cases with BRAF and AKT1 mutations showed a myoepithelial cell population and prominent papillary to micropapillary architecture. Overall, these findings confirm previous reports of frequent seromucinous differentiation in low-grade SNAC. However, these tumors display striking molecular diversity with involvement of multiple kinase fusions, leading to frequent activation of signaling cascades including the MAPK pathway. While most genetic alterations are not associated with sufficiently distinctive histologic features to suggest separate classification, biphasic tumors with BRAF p.V600E mutations are more unique and may represent a distinctive subgroup.