In this study, we analyzed the targeting of the somatic hypermutation
(SHM) mechanism at specific hotspot sequence motifs in the V-H and V
kappa genes of 10 follicular lymphoma (FL) cases and the V kappa and V
lambda genes of 11 kappa- and six lambda -light chain expressing
multiple myeloma (MM) cases. These sequences were analyzed for targeting
of specific motifs, ie certain highly mutable trinucleotides (3-NTPs),
the tetranucleotide (4-NTP) RGYW and its complementary, WRCY (where R =
purine, Y = pyrimidine and W = A or T). Comparisons were carried out
between mutation frequencies in RGYW vs WRCY and the incidence of
mutations in complementarity determining region (CDR)-1 vs CDR2 vs CDR3.
Statistically significant differences were obtained when comparing: (1)
the ratio of mutations in 4-NTPs (RGYW, WRCY, RGYW+WRCY)/mutations in
the whole V sequence in MM-V kappa vs MM-V lambda; (2) the total number
of mutated 4-NTPs in MM-V kappa vs FL-V kappa; (3) the number of mutated
RGYW 4-NTPs in MM-V kappa vs FL-V kappa and FL-V-H vs FL-V kappa; (4)
the number of mutated WRCY 4-NTPs in MM-V kappa vs FL-V kappa (P =
0.006) and FL-V-H vs FL-V kappa; (5) the targeting of RGYW vs WRCY in
the CDRs of FL-V-H genes. Similar results (regarding statistical
significance) were obtained when undertaking intergroup comparisons for
3-NTPs. These findings conform well with relevant data derived from
normal peripheral B cells. The differences observed in favor of 4-NTP
(RGYW and WRCY) targeting in FL-V-H vs FL-V kappa and MM-V kappa vs FL-V
kappa may implicate differences in the evolution of SHM coupled with
selection in different stages of B cell ontogeny. Several explanations
can be offered for the fact that hotspot sequences were not always
targeted by SHM in FIL and MM: (1) other unrecognized motifs may be
targets of SHM; (2) ‘inappropriately’ introduced mutations were fixed
and propagated by the neoplastic process; (3) certain FL and MM cases
might have lost their ability to correct mutations introduced in classic
hotspots due to deficient mismatch-repair (MMR) mechanisms; conversely,
in other cases with intact MMR function, the hotspot to non-hotspot
targeting of somatic hypermutation is balanced
