Dissecting the Role of Acquired Regions of Homozygosity Detected By Microarray Along the Genome of a Large Cohort of Adult Patients with B-Cell Precursor Acute Lymphoblastic Leukemia
Authors: Lapietra Gianfranco , Ribera Jordi , Morgades Mireia , Ruiz Xiville Neus , Torrent Anna , Mallo Mar , Hernandez Rivas Jesus Maria , Hernández Sánchez Alberto , Orfao Alberto , Granada Isabel , Navarro Jose Tomas , Del Giudice Ilaria , Martelli Maurizio , Chiaretti Sabina , Ribera Josep-Maria 
Published: December 2024

INTRODUCTION
Acquired regions of homozygosity (aROHs) detected along the human genome are due to mitotic recombination between homologous chromosomes. This mechanism may contribute to the growth of a neoplastic clone if the involved region embraces tumor suppressor or onco-genes harbouring a mutation, with subsequent loss of the wild-type allele and duplication of the aberrant one. Cytogenetic diagnostic tools based on allelic load assay have allowed identification of aROHs also in B-cell acute lymphoblastic leukemia (B-ALL). However, real-life data about contribution of these aberrations to leukemogenesis are scarce and controversial. Therefore, we investigated aROHs detected in a large and uniform longitudinal cohort of adult B-ALL patients (pts).
METHODS
The study was carried out at the Cytogenetic Unit of Catalan Institute of Oncology-Josep Carreras Leukemia Research Institute (Badalona, ES). Pts enrolled in clinical trial NCT04179929 (LAL-19) with available single nucleotide polymorphisms (SNP)-array analysis performed at baseline for genetic risk-stratification as per protocol were considered evaluable. For each patient, DNA extracted from infiltrated bone marrow (BM) and amplified by PCR was hybridized to Cytoscan 750 K array (Affymetrix, Santa Clara, CA, US). Hybridization to each probe was assessed using a GeneChip Scanner (Affymetrix) and results scored using CHAS v4.5 software (Affymetrix). For the aim of the study, we first selected ROHs with size≥3 Mb and mapping in regions covered with≥20 consecutive probes. In order to identify only acquired non-polymorphic forms, we excluded ROHs overlapping ≥50% of their length with regions included in public databases of healthy controls and with variant of allele frequency (VAF)&gt;BM infiltration. Finally, eligible calls were cross-referenced with pts' biological and clinical data for statistical analysis.
RESULTS
From December 2019 to March 2024, 371 pts were included in LAL-19 trial. Two hundred and seventy seven cases (75%) were eligible for our study. Median age was 40 years (range 18-60). The male/female ratio was 1:1. Overall, 332 aROHs were detected in 156 pts (56%). The median number of aROHs per pt was one (range 1-9); the median size was 6.7 Mb (range 3-27.5). Ninety-four % of aROHs were segmental. aROHs were distributed across the whole genome, 5q31.1 and 7q11.23 being the most frequently involved regions, without affecting any gene clearly involved in the leukemogenesis. No difference was observed between pts with and without aROHs in terms of age, sex and clinical features at the onset. However, the presence of aROHs was associated with the absence of hypodiploidy (&lt;44 chromosomes) and absence of mutations in the survival regulatory pathways detected by NGS standard panel (p=0.029 and p=0.043, respectively). Based on follow-up (FU) data, survival analysis was possible in 192/277 pts (69%). After a median FU of one year, the presence of aROHs had no impact on either overall survival (OS) (p=0.594) or cumulative incidence of relapse (CIR) (p=0.893). Within the population with aROHs, no difference emerged in terms of OS and CIR stratifying by number (1 versus ˃1) and size of lesions (&lt;20 Mb versus &gt;20 Mb, being 20 Mb the most employed cut-off in literature).
CONCLUSIONS
Our study confirms recurrence of non-polymorphic aROHs along the genome of adult B-ALL pts, without affecting key-genes for leukemogenesis. These aberrations do not impact the prognosis. On the contrary, a protective role for the genome could be hypothesized, due to their association with a low rate of mutation and with preserved diploidy.