Assessment of HMGA2 and PLAG1 rearrangements in breast adenomyoepitheliomas.

Breast adenomyoepitheliomas (AMEs) are rare epithelial-myoepithelial neoplasms that may occasionally produce myxochondroid matrix, akin to pleomorphic adenomas (PAs). Regardless of their anatomic location, PAs often harbor rearrangements involving HMGA2 or PLAG1 . We have recently shown that the repertoire of somatic genetic alterations of AMEs varies according to their estrogen receptor (ER) status; whilst the majority of ER-positive AMEs display mutually exclusive PIK3CA or AKT1 hotspot mutations, up to 60% of ER-negative AMEs harbor concurrent HRAS Q61 hotspot mutations and mutations affecting either PIK3CA or PIK3R1 . Here, we hypothesized that a subset of AMEs lacking these somatic genetic alterations could be underpinned by oncogenic fusion genes, in particular those involving HMGA2 or PLAG1 . Therefore, we subjected 13 AMEs to RNA-sequencing for fusion discovery ( n  = 5) and/or fluorescence in situ hybridization (FISH) analysis for HMGA2 and PLAG1 rearrangements ( n  = 13). RNA-sequencing revealed an HMGA2 - WIF1 fusion gene in an ER-positive AME lacking HRAS, PIK3CA and AKT1 somatic mutations. This fusion gene, which has been previously described in salivary gland PAs, results in a chimeric transcript composed of exons 1-5 of HMGA2 and exons 3-10 of WIF1 . No additional in-frame fusion genes or HMGA2 or PLAG1 rearrangements were identified in the remaining AMEs analyzed. Our results demonstrate that a subset of AMEs lacking mutations affecting HRAS and PI3K pathway-related genes may harbor HMGA2 - WIF1 fusion genes, suggesting that a subset of breast AMEs may be genetically related to PAs or that a subset of AMEs may originate in the context of a PA.