Peculiar nuclear atypia associated with MDM2 gene amplification in carcinoma ex‐pleomorphic adenoma harbouring an alteration of HMGA2

Aims Salivary gland neoplasms (SGN) exhibiting the HMGA2 :: WIF1 fusion are recognized by their resemblance to histology found in canalicular adenoma. Recently, ~20% of cases among 28 HMGA2 :: WIF1 ‐rearranged‐SGN showed malignancy and adverse outcomes (recurrence, distant metastasis, and disease‐specific mortality). Among them, MDM2/CDK4 amplifications were identified in one case. This outcome suggests that the MDM2/CDK4 amplifications could be useful to predict an aggressive course of carcinoma ex‐pleomorphic adenoma (CEPA). Methods and Results We investigated the correlation between HMGA2 fusion and MDM2 amplification in four salivary gland neoplasms, providing detailed clinicopathological features and outcomes. Cases were selected from different institutions. Histological examination, immunohistochemistry, fluorescence in situ hybridization (FISH), RNA sequencing, and whole‐exome capture were performed. The cohort included four CEPA cases, all female, aged between 32 and 89 years. Tumours arose from the parotid gland with an average size of 24.5 mm. None exhibited recurrence or distant metastases during the 4–5 months of follow‐up. Pathologically, all cases displayed a peculiar atypical nuclei with ‘gear‐like appearance’. Immunohistochemically, tumours exhibited a biphasic pattern with myoepithelial and ductal differentiation markers. All cases showed HMGA2 overexpression and MDM2 amplification by FISH and RNA sequencing. In a control cohort of MDM2 nonamplified CEPA cases, not exhibiting the peculiar nuclear atypia. Conclusions Our findings suggest a strong correlation between HMGA2 alteration/ MDM2 amplification and a peculiar nuclear atypia, advocating for their evaluation in biphasic tumours to facilitate accurate diagnosis and tailored posttumour removal monitoring. Further studies are warranted to validate these observations and elucidate their prognostic implications.