Page 151 - 2021_10-Haematologica-web

Evolution of ISFN to aggressive B-cell lymphoma
ABC
Figure 3. Different patterns of clonal evolution from in situ follicular neoplasia to aggressive B-cell lymphoma based on the distribution of mutations. Variants are depicted at protein level. Mutations highlighted in red were gained during the evolution. Synonymous and 5’UTR variants of BCL2 are not shown, but were also taken into account for the construction. The existence of “Progenitor clones” was assumed on the basis of shared mutations. (A) In situ follicular neoplasia (ISFN) and dif- fuse large B-cell lymphoma (DLBCL) evolved divergently from a common progenitor clone. (B) ISFN and DLBCL evolved divergently from a common progenitor, with the ISFN subsequently progressing to follicular lymphoma (FL). (C) FL and DLBCL evolved directly from the ISFN and gained additional alterations.
unable to amplify a clonal B-cell rearrangement or the BCL2 translocation sequence. Both samples, however, demonstrated a BCL2 translocation only detectable by FISH using an IGH/BCL2 dual-color, double fusion probe, possibly the result of a cryptic, non-canonical BCL2 rearrangement.32 This, in combination with shared muta- tions of KMT2D and IGLL5, serves as evidence of a com- mon clonal origin.
Clonally related ISFN can be identified not only before or simultaneously with manifest lymphoma, but may also be present even years after the malignant transforma- tion took place, most likely representing a subclone that diverged at an earlier stage of the disease.14,33 Persisting precursors, presumably more resistant to chemotherapy, may therefore play a role in lymphoma relapse as well. Indeed, studies of FL and DLBCL relapses have shown that both the primary and the recurrent lymphoma often represent divergent subclones that arose independently from a common progenitor, which again supports our hypothesis that aggressive BCL can develop directly from a premalignant precursor.22,34 The existence of such pro- genitor populations has been exemplified in two reports of clonally related FL and clonally related DLBCL arising in both donor and recipient after hematopoietic stem cell transplantation.35,36 In both studies, the related lym- phomas exhibited multiple shared alterations, which were therefore acquired prior to transplantation.35,36 Likewise, FL and transformed FL have been shown to often arise by divergent evolution.37,38 The complexity of this evolutionary process is also reflected in our paired ISFN and aggressive BCL cases, since the distribution of private and shared mutations suggests an early subclonal divergence for the majority of cases. This is supported further when IG data are taken into account, given the high frequency of different glycosylation sites in both components, and the phylogenetic trees based on SHM patterns of rearranged IGH sequences, which are compat-
ible with a divergent evolution. Linear evolution from ISFN to aggressive BCL, at least based on the available mutational data, seems to be less frequent, but similar findings have been reported regarding the transformation of FL.38 Our IGH data also provide insight into the process of early clonal selection. The more balanced distribution of subclones in the ISFN implies that at this point, no sub- clone has acquired a decisive selection advantage. In con- trast, the aggressive BCL lesions demonstrated one or two highly dominant subclones, which possibly emerged after obtaining crucial secondary genetic alterations that improved clonal fitness and further paved the way towards high-grade malignancy.
The most commonly mutated gene in our study was BCL2, likely because BCL2 juxtaposed to an IG promotor results in a significantly higher number of mutations com- pared to non-translocated counterparts, as a result of tar- geting by AID.39,40 The abundance of BCL2 mutations across ISFN lesions, as well as the intraclonal heterogene- ity revealed by the IGH sequence analysis and the detec- tion of novel N-glycosylation sites confirm that the process of SHM is ongoing in ISFN.15,19 Prolonged AID activity is regarded as an important factor in the patho- genesis of GC-derived lymphomas and especially FL.21,22,41 In a mouse model, multiple re-entries of long-lived BCL2+ B-cell clones into the GC environment resulted in the accumulation of secondary alterations with a muta- tional signature consistent with AID-mediated mutagen- esis.42 The same concept has been proposed for human FL development, where FLLC are subject to similar dynam- ics with an extensive dissemination throughout the body, leading to a multitude of subclones exhibiting different SHM signatures as evidence of their GC passage.17,42 Since ISFN is considered the tissue-based counterpart of FLLC, our data also indicate that circulating t(14;18)+ FLLC
clones might serve as precursors to aggressive BCL as well.42,43
haematologica | 2021; 106(10)
2679