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Page 94 Oquendo et al. J Transl Genet Genom 2021;5:89-111 https://dx.doi.org/10.20517/jtgg.2021.04

Recurring translocations are much less frequent than in other low-grade lymphomas such as MCL, FL and
mucosa associated lymphoid tissue lymphoma, and usually involve the IGHV locus. These include
t(14;19)(q32;q13), t(6;14)(p21;q32), t(9;14)(p13;q32), t(1;14)(q21;q32) which target the genes BCL3, CCND3,
PAX5 and BCL9/MUC1, respectively. Translocations of 7q21, have been documented in some cases,
predominantly with the immunoglobulin kappa locus resulting in upregulation of CDK6 .
[47]

MUTATIONS IN KEY GENES AND PATHWAYS
Whole genome/exome sequencing (WGS/WES) has been employed to scan the entire cancer genome at
base-pair resolution and identify somatically acquired gene mutations across a broad spectrum of mature B-
cell tumours [48-52] . The analysis of large patient cohorts, often conducted as part of large international
sequencing consortia, has provided a wealth of genomic information on these neoplasms, identifying a
plethora of germ-line SNPs associated with disease-risk, panels of genes targeted by somatic mutations and
a number of acquired mutational mechanisms. Unlike more prevalent mature B-cell malignancies, SMZL is
currently precluded from these international sequencing projects, the result being an incomplete catalogue
of tumour associated genomic lesions and mutational processes, drawn from a paucity of published
[53]
genomic data. Only six genome-wide studies have been conducted on only 35 patients . The only study
employing WGS was performed by Kiel et al. but was limited to six cases without matched germline
[54]
DNA. Five WES studies have been carried out on discovery cases, often with subsequent targeted re-
sequencing of relevant genes in additional samples. To date, none of these studies have reported mutational
[55]
signatures nor mechanisms, such as kataegis and chromothripsis . Even the somatic mutational burden
remains disputed, with a range of somatic mutations per patients of between 9 and 82 (mean 25) [23,56,57] . This
limited agreement is likely due to the low patient numbers, and experimental and computational differences
in WGS/WES processing, but could also allude to disease heterogeneity and statistical power insufficient to
catalogue the complete mutational landscape of the disease [23,53] . Targeted re-sequencing approaches have
helped elucidate recurrently mutated genes, but these studies have often included only small numbers of
matched germ-line material for analysis. Whilst we currently have only a limited picture of the somatic
landscape of SMZL, several recurrently mutated genes have been identified, which preferentially target
physiologically important cellular processes, such as MZ B-cell maturation and migration, and cell cycle
control [Figure 2]. A number of genomic studies, with limited functional validation, all agree that the most
important recurrently mutated genes are KLF2, NOTCH2 and TP53.

KLF2 mutations are present in up to 40% of SMZL cases
KLF2 is the most frequently mutated gene in SMZL (20%-40% of cases) [24,33,58,59] . The gene belongs to the
family of Kruppel-like transcription factors, a subfamily of the zinc-finger class of DNA binding
transcriptional regulators . KLF2 directly binds to promoters regulating the expression of genes involved
[60]
in cell homing, NF-κB signalling and cell cycle control [23,61] . In murine systems, loss of KLF2 drives the
germinal cells to a MZ-like phenotype and preclusion of migration to the splenic MZ [62-64] , thereby
preventing germinal centre B-cell responses to antigens in the MZ. Nuclear localization of the KLF2 protein
and consequent DNA binding require three C-terminal highly conserved zinc finger domains and two
nuclear localization sequences, respectively. Gene mutations can be missense substitutions or truncating
events [Figure 3], where the latter often results in the removal of the nuclear localization sequences [23,24,33,58] .
Missense substitutions result in amino acid changes within the nuclear localization sequences of KLF2 or
within highly conserved regions of the first zinc finger domain. These KLF2 mutants render the protein
unable to elicit its transcriptional activity by displacement from the nucleus thereby preventing the ability of
KLF2 to supress NF-κB induction by upstream signalling pathways [23,33] . The p.Q24X (stopgain) variant is a
hotspot mutation [23,24,33,59] . Although no functional evidence on this specific variant is available, it is very
likely that due to its position on the first exon it would result in a truncated and non-functional protein.
Furthermore, this variant has a scaled Combined Annotation Dependent depletion score of 36 indicating
[65]