Espressione del sistema di riparazione dei mismatch (MMR) nei tumori neuroendocrini ipofisari (PitNETs)

Purpose: Defects in the mismatch repair (MMR) system have been occasionally described in aggressive/metastatic pituitary neuroendocrine tumors (PitNETs), which have been reported in the setting of Lynch syndrome (LS). Recent studies suggest a predictive value of MMR expression in the response of aggressive/metastatic PitNETs to temozolomide and/or or immunotherapy (TMZ/ICI). Yet, the potential role of the MMR system in pituitary tumorigenesis and its potential prognostic implications are largely unknown. The aim of this thesis is to investigate the expression of MMR in a large cohort of PitNETs according to: a) bio-clinical factors, including tumor histotypes, b) the presence of hotspot somatic mutations in PitNETs and/or a potential genetic background, c) the MMR promoter methylation pattern and the potential regulation of the corresponding gene expression d) in the presence of a documented loss of MMR protein expression/MMR defect (MMRd), to identify the demographic and clinical characteristics of affected patients, including the individual/familial oncological context, and e) based on data reported in extra-pituitary neoplasia with MMRd, the potential relationship between the expression of MMR, PDL1 and selected marker of immune tumor microenvironment (iTME). Methods: The expression of the MMR genes (MSH2, MSH6, MLH1, PMS2) was studied by RT-qPCR in 113 PitNETs, classified according to pathological criteria. Tumor characteristics were defined pre-operatively. In a subset of Pit1 and Tpit PitNETs (n=58), RT-qPCR studies were extended to PDL1 and iTME markers (CD68, CD163, CD3, CD8 and FoxP3). The methylation pattern of MSH2, MSH6 and MLH1 promoters was also studied, as well as hotspot mutations (GNAS1, SF3B1 and USP8/48), and AIP loss of heterozygosity in case of known germline mutation. MMRd was defined by the loss of almost one MMR protein expression. Results: MMR gene expression, except MLH1, was significantly influenced by their lineage of origin (P=0.005, P<0001 and P=0.030 for MSH2, MSH6 and PMS2, respectively). The highest expression was found in SF1 tumors. In Pit1 tumors, MMR expression was significantly lower in large and/or invasive cases, whereas in Tpit tumors it was significantly lower in the presence of hypercortisolism and in microadenomas. No clear role for MMR gene methylation was found. MMRd was documented in 11/41 cases. Most MMRd tumors were lactotroph in young patients or corticotroph associated with Cushing’s disease. MSH6 loss was the most frequent. Preliminary data on iTME markers expression indicated macrophages as the most prevalent population (M1>M2 subtype), while T regulatory cells were the less represented. All iTME markers positively correlated with each other (P<0.02) and, except FoxP3, were significantly more abundant in Pit1 vs Tpit tumors. The latter also showed a significantly lower PDL1 expression than Pit1 tumors (P<0.001). Of note, some correlations were also found between MMR and iTME markers. Conclusions: The MMR system appears to be a new player in pituitary tumorigenesis and is differentially regulated according to tumor lineage of origin. MMRd is not limited to ag/met PitNETs and the underlying mechanisms should be further investigated, not excluding somatic/germline defects in a minority of cases. The lower expression of PDL1 might be a characteristic of the lineage rather than a peculiarity of tumors at an advanced stage. The potential relationship between MMR and the iTME should also be further investigated at a protein level, because MMRd have been shown to indirectly stimulate the immune response in other solid neoplasia. Thus, the characterization of iTME and MMR status may have future implications in the understanding of pituitary tumorigenesis and progression, as well as in the selection of patients for TMZ/ICI in rare ag/met PitNETs.