Elevated Microsatellite Alterations at Selected Tetranucleotide Repeats (EMAST) in Penile Squamous Cell Carcinoma-No Evidence for a Role in Carcinogenesis.

Penile squamous cell carcinoma (pSCC) is a rare malignancy with a global incidence ranging from 0.1 to 0.7 per 100,000 males. Prognosis is generally favorable for localized tumors, but metastatic pSCC remains challenging, with low survival rates. The role of novel biomarkers, such as tumor mutational burden (TMB) and microsatellite instability (MSI), in predicting the response to immune checkpoint inhibitors (ICIs) has been investigated in various cancers. However, MSI has not been observed in pSCC, limiting immunotherapy options for this patient subgroup. Elevated microsatellite alterations at selected tetranucleotide repeats (EMAST) are a distinct form of genomic instability associated with deficient MSH3 expression, which has been proposed as a potential biomarker in several cancers. This study investigates EMAST and MSH3 expression in a cohort of 78 pSCC cases using PCR, fragment analysis and immunohistochemistry. For the detection of EMAST, the stability of five microsatellite markers (D9S242, D20S82, MYCL1, D8S321 and D20S85) was analyzed. None of the cases showed an instability. As for MSH3 immunohistochemistry, all analyzable cases showed retained MSH3 expression. These results strongly suggest that neither EMAST nor MSH3 deficiency is involved in the carcinogenesis of pSCC and do not represent reliable predictive biomarkers in this entity. Furthermore, these findings are in full agreement with our previous study showing a very low frequency of MSI and further support the thesis that EMAST and MSI are strongly interconnected forms of genomic instability. Further research is needed to explore novel therapeutic targets and predictive biomarkers for immunotherapy in this patient population.