Update on genetically defined lung neoplasms: NUT carcinoma and thoracic SMARCA4-deficient undifferentiated tumors.

NUT carcinoma, also known as NUT midline carcinoma, is an aggressive malignancy mainly affecting the midline structures of younger patients and almost invariably leading to death within a few months of the diagnosis. Morphologically, NUT carcinoma consists of sheets of monomorphous small or medium size cells with scant cytoplasm, commonly featuring areas of abrupt squamous differentiation with keratinization. Immunohistochemistry for NUT protein is sensitive and specific, typically showing a speckled nuclear reactivity, assisting in diagnosis. The molecular background of NUT carcinoma includes the reciprocal translocation t(15;19) leading to expression of the BRD4-NUT fusion transcript with oncogenic properties. Other less common genes may occasionally be fused with NUT not only in NUT carcinoma but also in other soft tissue tumors, highlighting the fact that NUT-rearranged tumors may represent a larger and more diverse family of neoplasms. Thoracic SMARCA4-deficient undifferentiated tumors are aggressive malignancies diagnosed more often in young male smokers, which often lead to death within a few months. SMARCA4-deficient tumors show undifferentiated morphology with occasional hepatoid and rhabdoid features. Immunohistochemically, the hallmark of diagnosis is loss of expression of SMARCA4 (BRG1). Concurrent loss of SMARCA2 expression, as well as expression of one or more stem cell markers SOX2, CD34, or SALL4 is common. Truncating mutations in SMARCA4, a catalytic subunit of the mammalian BAF (SWI/SNF) complex, are the dominant oncogenic molecular event underlying the pathogenesis of these tumors. SMARCA4 deficiency can also be seen as a passenger somatic event in multiple solid neoplasms manifesting as focal dedifferentiation and rhabdoid morphology.