Hypermutation in Mantle Cell Lymphoma Does Not Indicate a Clinical or Biological Sub Entity.
Authors: Schraders Margit , Oeshker Sabine , Kluin Philip M. , Hebeda Konnie , Schuuring Ed , Hansmann Martin-Leo , Groenen Patricia J.T.A. , Van Krieken Joannes H.J.M. 
Published: October 2019

Abstract
Mantle cell lymphoma (MCL) is a prime example of a well-defined entity based on morphology, phenotype, genetics and also clinical features. Although, most patients have an adverse clinical course, some have a better survival than others. The most consistently reported adverse histopathological prognostic parameter is a high mitotic rate. Recently, it has been shown that hypermutation in the immunoglobulin heavy chain gene occurs in a subset of mantle cell lymphomas. It is, however, unclear, how hypermutation needs to be defined and whether the mutational status is stable over time within a given case. Also it is not clearwhether hypermutation might be influenced by therapy and how it is related to other relevant biological features of MCL, like genetic imbalances, breakpoint of the cyclin D1 gene, and proliferative and apoptotic rate. In the present study we analyzed a series of typical MCL with respect to mutational status, and compared the results with clinicopathological and genetic data to determine whether the presence of mutation does indicate a sub entity with clinical or pathological relevance. For this study 28 specimens from 24 patients were selected and stained for cyclin D1 and CD5 and were reviewed by four experienced pathologists at a multiheaded microscope. Morphological features were assessed, mitotic figures were counted, clonal IGH-VJ gene rearrangements of the MCL were identified according to the Biomed-2-protocol. All, but one case, were clonal by PCR. The negative case was excluded from further molecular studies. Mutation in the VH gene segment was absent in 7 cases, another 6 cases carried 0.5% mutation frequencies; in 10 cases more than 1.5% mutation frequencies were present, in 6 of these more than 2%. We found a preferential usage of VH3-21 (26%) and VH4-34 (17%) in tumor cells of both, mutated and unmutated cases. Of two cases from our series in which sequential biopsies were available tumor cells harbored mutations in the VH genes (1.5% mutation frequency and 2.2% mutation frequency, respectively), but there was no change of mutation status over time, the duration being as long as 7 years in one case. In our study there was a statistically significant difference between MCL with a Ki-67-positivity <35% and MCL with a Ki-67-positivity >=35% in regard to a longer survival time in the former whereas mutation status of both groups did not correlate with Ki-67-positivity. Further, no significant correlations were found between mutation status and the other morphologic and genetic features analyzed. In conclusion, our results provide additional evidence that mutation status does not permit definition of sub entities in MCL that are associated invariably with a certain prognosis. Mutation status in MCL is better interpreted as a feature within the spectrum of disease that seems to have little clinical or pathological relevance. Margit Schraders and Sabine Oeschker contributed equally to this work.