Low Frequency Of IGHV4-39/IGHD6-13/IGHJ5 Rearrangement With Stereotyped HCDR3 (subset #8) In Russian and Ukrainian Chronic Lymphocytic Leukemia Patients
Authors: Biderman Bella , Bilous Nadiia , Severina Nataliya , Sudarikov Andrey , Nikitin Eugene , Kryachok Iryna , Abramenko Iryna , Chumak Anatoliy 
Published: October 2019

Abstract
The leukemia cells of unrelated patients with chronic lymphocytic leukemia (CLL) display a restricted repertoire of immunoglobulin gene rearrangements and very similar structure of B-cell receptor (stereotyped BCR). Numerous different subsets of stereotyped BCRs were identified. Some associations between specific BCRs subsets, clinical course of CLL, and molecular abnormalities were found. Namely, short overall survival, accumulation of SF3B1 mutations in subset #2, high risk for Richter’s transformation, trisomy 12, NOTCH1 mutations in subset #8, increased frequency of NOTCH1 mutations in subset #1 were revealed (Rossi et al., 2009, 2013; Streffoed et al., 2013).
The aim of this study was to evaluate distribution of different stereotyped BCR subsets in Russian and Ukrainian CLL cohort with special attention on prognostic unfavorable subsets.

Methods
Immunoglobulin heavy chain variable (IGHV) genes of 969 CLL patients (564 from Russia and 405 from Ukraine) were amplified according to BIOMED-2 rules, sequenced and analyzed with IMGT and IgBlast databases. Additionally, NOTCH1 c.7544_7545delCT mutations were analyzed in 301 patients (randomly collected and in the prognostic unfavorable subsets) by amplification refractory mutation system (ARMS) PCR as proposed by Rossi D. et al., 2012. In 19 cases NOTCH1 mutational status was confirmed by direct DNA Sanger sequencing.

Results
Joined Russian-Ukrainian CLL cohort was presented mainly by unmutated (UM) IGHV gene cases (64.2%). The distribution of major clusters of stereotyped BCR did not differ from those presented in the literature (comparison was made separately for cases with mutated (M) and UM IGHV genes, taking into account the differences in the mutational status of IGHV genes between our CLL cohort and CLL cohorts from European countries and USA), except for subset #8, which was found in one case only (0.16% of UM IGHV gene cases). At the same time, subset #8 (IGHV4-39/IGHD6-13/IGHJ5 rearrangement) belongs to the group of “major” subsets in CLL (Agathangelidis et al., 2012), and its frequency varies from 0.9% to 2% among cases with UM IGHV genes in the most of presented CLL cohorts (Murray et al., 2008; Messmer et al., 2009; Maura et al., 2011). Frequency of subset #2 had tendency to be lower among Russian CLL patients (1.4% of M CLL cases), but not among Ukrainian patients (4.5% of M CLL cases) comparing with European CLL cohorts.
NOTCH1 c.7544_7545delCT was identified in 27 out of 301 patients (9.0%), which was in accordance with other CLL studies. Twenty five cases with NOTCH1 mutations had UM IGHV genes and 2 cases expressed M IGHV3-21 gene (both subset #2). As well, majority of NOTCH1 mutated cases (17/27, 63%) showed stereotyped BCRs, which belonged to the following subsets: #1 (2 of 12), #2 (2 of 7), #3 (1 of 6), #7 (2 of 10), #9 (2 of 10), #25 (1 of 2), #59 (1 of 3) (subset’s designation according Murray et al., 2008), UA/ref2 (1 of 2), UA/ref11 (2 of 2) (subset’s designation according Bilous et al., 2010), or having a homology with the CLL sequences of the public CLL database. No NOTCH1 mutations were found in 11 IGHV4-39-used cases, including one case from subset #8. In 10 of 27 (37.0%) NOTCH1 mutated cases IgHV1-69 gene was used, while among wild type NOTCH1 patients IgHV1-69 was presented in 65/274 (23.7%) cases (p=0.12).

Conclusion
The distribution of the major subsets of stereotyped BCRs in CLL patients from Russia and Ukraine (eastern slavic countries) was similar to the western European countries and USA, except for low frequency of subset #8, associated with unfavorable clinical course. Our preliminary data suggested that NOTCH1 c.7544_7545delCT mutation is typical for UM CLL cases with stereotyped BCRs, but no clear association with specific subsets was found in the absence of subset #8.

Disclosures:
No relevant conflicts of interest to declare.