Clinicopathological and molecular characterization of biphasic hyalinizing psammomatous renal cell carcinoma: further support for the newly proposed entity

The classification of renal neoplasms continues to evolve with novel, emerging, and provisional entities being described constantly. Biphasic hyalinizing psammomatous renal cell carcinoma (BHP RCC) associated with somatic NF2 mutations is one such new renal entity and is considered as a provisional category of RCC due to its very limited data. To provide further support for the newly proposed entity, we identified three additional cases of BHP RCC, with clinicopathological, immunohistochemical, and various molecular analyses. There were 2 males and 1 female, aged 65, 56, and 69 years, respectively. The neoplasms were unencapsulated, and all had a characteristic biphasic appearance of smaller cells clustering around basement membrane material within larger acini, forming pseudorosettes or a glomeruloid pattern. Hyalinized sclerotic stroma and psammoma bodies were abundant in two cases and focally present in one case. Focal areas of a less distinctive appearance were also noted; one additionally had an elongated tubular pattern in the myxoid stroma that is reminiscent of mucinous tubular and spindle cell carcinoma; one consisted solid alveolar architectures of epithelioid clear cells, bearing some resemblance to clear cell RCC. The neoplasms did not have a distinctive immunohistochemistry (IHC) profile, though all labeled for vimentin and CK7. Targeted DNA sequencing revealed that one case harbored a pathogenic somatic frameshift mutation in the NF2 gene, which was further confirmed by Sanger sequencing. The other two cases lacked NF2 mutations and instead demonstrated NF2 promoter methylation by methylation-specific polymerase chain reaction. Subsequent IHC assessment showed loss of expression of NF2 in all 3 cases, which evaluated NF2 status at the protein level. According to RNA sequencing-based clustering analysis, the 3 cases formed a distinct group with a shared specific transcriptional profile different from that of other established renal tumor types. In addition, phosphate inositide 3-kinase (PI3K)/AKT pathway was enriched significantly and on the top of all enriched pathways. Clinically, one patient developed bone metastases and died of disease two years after diagnosis. The other two patients had no evidence of recurrence or metastases, at 4- and 5-year follow-up. These findings not only validate previously described clinicopathological features but also expand the potentially genetic alterations and available clinical outcome data.