BLIMP1 Is Commonly Inactivated In Anaplastic Large T-Cell Lymphomas (ALCL)
Authors: Boi Michela , Rinaldi Andrea , Piva Roberto , Rancoita Paola MV , Bonetti Paola , Matolcsy Andras , Tousseyn Thomas , Rodríguez-Pinilla Socorro Maria , Piris Miguel , Beà Sílvia , Campo Elias , Bhagat Govind , Swerdlow Steven H. , Rosenwald Andreas , Ponzoni Maurilio , Young Ken H. , Piccaluga Pier Paolo , Pileri Stefano , Neri Antonino , Medico Enzo , Zucca Emanuele , Kwee Ivo , Inghirami Giorgio , Bertoni Francesco 
Published: October 2019

Abstract
Abstract 2634
ALCL has been separated in two distinct subtypes based on the presence or absence of translocations involving the ALK gene. It is accepted that ALK+ALCL is a distinct subgroup which shares a unique phenotype, with well defined genetic and clinical features. Although the clinical presentations, translocations and genetic events vary between ALK+ and ALK-ALCL, the relationship between these two ALCL subtypes and whether ALK-ALCL may represent a subset of peripheral T-cell lymphomas, not otherwise specified (PTCL, NOS), remains unclear. In this regard, the WHO Classification classifies ALK-ALCL as a provisional entity. A better understanding of the underlying genetics would provide critical explanations to answer some of these questions. With the aim of identifying the genetic events underlying the pathogenesis of ALCL, we studied a series of 69 cases of ALCL (34 ALK-, 35 ALK+) with high-density genome wide SNP-based arrays.

Methods.
DNA was extracted from frozen biopsies. DNA profiles were obtained using the Affymetrix GeneChip Human Mapping SNP6 arrays. Differences in frequencies between subgroups were evaluated using Fisher's exact test. A subset of cases also had available gene expression profiles. Clinical data were available in half of the cases and genomic lesions were evaluated for their impact on clinical outcome with the log-rank test.

Results.
The most common losses were at 6q21, 17p13 (19%), 13q22.3 (15%), 3p21.31, 13q32.3 (14%), 1p13.3, 16q23.1 (WWOX) (13%), 16q23.3–24.1 (12%), 1p33 and 16q22.1 (10%). The most common gains occurred at 8q22 (20%), 1q (13%), 7q (10–15%; CDK6, 15%), 8q24 and 9p24.1 (10%). ALK-ALCL displayed a higher number of genomic aberrations in comparison with ALK+ALCL. The lesions presenting major differences included: -6q21 (35% vs 6%; P=0.002), -1p13 (26% vs 3%, P=0.001), -3q22 (26% vs 0%, P=0.001), -4q12-q26 (18% vs 0%; P=0.009), +9p21 (17% vs 0%, P=0.009), -17p13 (TP53, 26% vs 6%, P=0.019). The deletions at 6q21 targeted the gene PRDM1, coding for BLIMP1. The whole coding sequence of PRDM1 has been sequenced in 33 ALK- ALCL samples. Only one somatic mutation, inducing a stop codon, was identified, in one case bearing copy neutral loss of heterozygosity (cnLOH) spanning PRDM1 locus, suggesting a loss of functional protein in this patient. As a whole, 38% of ALK-ALCL presented loss of at least one allele of PRDM1. Only two cases were observed with complete gene loss: the ALK-case with somatic mutation plus cnLOH, and one ALK+ case with homozygous deletion. The presence of 6q21 deletion had an impact on progression free survival among all ALCL (P=0.048), likely reflecting its association with ALK-ALCL, but not when considering ALK- patients only. Xenografts derived from primary ALCL samples bearing 6q21 loss presented decreased BLIMP1 expression level. The detection of PRDM1 loss was present also in cell lines, in which also a decreased level of BLIMP1 RNA and protein was observed. Additional genes, members of PRDM1 pathway, were identified as targets of focal deletions.

Conclusions.
A series of recurrent lesions has been identified in ALCL. Alongside TP53 loss, inactivation of PRDM1 by genomic losses or somatic mutations was the most common detected lesion, and was more frequently inactivated in ALK-ALCL. PRDM1, encoding BLIMP1, a master regulator of T-cells differentiation, appears as a central gene in ALCL pathogenesis. Other genes, belonging to the same pathway, were found to have focal genomic aberrations in a smaller number of cases.

Disclosures:
No relevant conflicts of interest to declare.