Retinoblastoma gene mutations detected by whole exome sequencing of Merkel cell carcinoma.

Merkel cell carcinoma is a highly aggressive cutaneous neuroendocrine tumor that has been associated with Merkel cell polyomavirus in up to 80% of cases. Merkel cell polyomavirus is believed to influence pathogenesis, at least in part, through expression of the large T antigen, which includes a retinoblastoma protein-binding domain. However, there appears to be significant clinical and morphological overlap between polyomavirus-positive and polyomavirus-negative Merkel cell carcinoma cases. Although much of the recent focus of Merkel cell carcinoma pathogenesis has been on polyomavirus, the pathogenesis of polyomavirus-negative cases is still poorly understood. We hypothesized that there are underlying human somatic mutations that unify Merkel cell carcinoma pathogenesis across polyomavirus status, and to investigate we performed whole exome sequencing on five polyomavirus-positive cases and three polyomavirus-negative cases. We found that there were no significant differences in the overall number of single-nucleotide variations, copy number variations, insertion/deletions, and chromosomal rearrangements when comparing polyomavirus-positive to polyomavirus-negative cases. However, we did find that the retinoblastoma pathway genes harbored a high number of mutations in Merkel cell carcinoma. Furthermore, the retinoblastoma gene (RB1) was found to have nonsense truncating protein mutations in all three polyomavirus-negative cases; no such mutations were found in the polyomavirus-positive cases. In all eight cases, the retinoblastoma pathway dysregulation was confirmed by immunohistochemistry. Although polyomavirus-positive Merkel cell carcinoma is believed to undergo retinoblastoma dysregulation through viral large T antigen expression, our findings demonstrate that somatic mutations in polyomavirus-negative Merkel cell carcinoma lead to retinoblastoma dysregulation through an alternative pathway. This novel finding suggests that the retinoblastoma pathway dysregulation leads to an overlapping Merkel cell carcinoma phenotype and that oncogenesis occurs through either a polyomavirus-dependent (viral large T antigen expression) or polyomavirus-independent (host somatic mutation) mechanism.

T-cell prolymphocytic leukemia frequently shows cutaneous involvement and is associated with gains of MYC, loss of ATM, and TCL1A rearrangement.

T-cell prolymphocytic leukemia (T-PLL) is a rare aggressive mature T-cell leukemia with frequent cutaneous presentation, which has not been well characterized. Among the 25 T-PLLs diagnosed between 1990 and 2013 at our institution, 32% (8/25) showed cutaneous manifestations, presenting as rash, purpura, papules, and ulcers. The skin biopsies showed leukemia cutis with perivascular and periadnexal irregular, small to medium-sized lymphoid infiltrates without epidermotropism. The lymphoid infiltrates were composed of mature CD4+ T cells expressing other T-cell antigens, and a subset (48%) showed dual CD4+/CD8+ coexpression. Higher median absolute peripheral blood lymphocyte count (43.0 vs. 13.0 k/mm; P=0.031) and elevated lactate dehydrogenase levels (P=0.00018) at the time of diagnosis were significantly associated with T-PLLs with skin involvement compared with those without. The extent of bone marrow involvement (P=0.849) and overall survival (P=0.144) was similar in the 2 groups. Fluorescence in situ hybridization or karyotype revealed frequent gains of MYC (67%; n=9), loss of ATM (64%; n=11), and TCL1A rearrangement or inversion 14q (75%; n=12). Gains of TCL1A was also seen (78%; n=9), including in some cases that had concurrent TCL1A rearrangement, whereas TP53 loss was less common (30%; n=10). No correlation was seen between the immunophenotype and morphology versus the presence or absence of skin involvement. These data suggest that cutaneous involvement by T-PLL is relatively common and often associated with significant peripheral blood involvement. The frequent MYC, ATM, and TCL1A alterations identified support that these genes are integral to the pathogenesis of T-PLL.