JAK2, As Well As PDL1 and PDL2, are Recurrently Targeted by 9p24 Structural and Numerical Aberrations in Lymphoid Neoplasms of Both B- and T-Cell Origin

Genetic aberrations of the 9p terminal region are recurrent in hematological malignancies. Many of them involve the JAK2 gene located at 9p24, which is non-randomly targeted by chromosomal translocations, amplifications and mutations in various types of leukemia, lymphoma and myeloproliferative neoplasms. Of interest, recent reports showed that also PDL1 and PDL2, two genes located 350kb centromeric to JAK2, are involved in 9p24 aberrations. These genes, coding for the programmed death ligands, were identified as key targets of 9p24 amplification in primary mediastinal B-cell lymphoma (PMBCL) and classical Hodgkin lymphoma (cHL). Moreover, CIITA-PDL1 and CIITA-PDL2 fusions, leading to overexpression of PDL1 and PDL2, respectively, have been reported in two PMBCL cases.

The objective of this study is to characterize structural and numerical 9p24 aberrations in lymphoid malignancies and to assess the molecular consequences of these abnormalities.

Eighty-four cases of B-cell non-Hodgkin lymphoma (B-NHL) and 4 cases of T-cell NHL with 9p24 aberrations were selected for this study. In addition, we included 130 cases of cHL with unknown 9p24 status. All cases were screened by FISH with a dual color break apart assay for JAK2 containing probes covering PDL1 and PDL2. Cases with rearrangements of these loci were further characterized by FISH with BAC and fosmid probes, and by aCGH. In available cases, qRT-PCR and/or IHC was used to determine the expression level of candidate genes.

Structural aberrations of 9p24 were identified in 8 patients presenting with CLL in Richter transformation (1), MALT/MZL (2), FL (1), DLBCL (2) and ALL/LL (2). The 9p24 breakpoints were further mapped to a 200 kb region centromeric to JAK2 and harboring PDL1 and PDL2. These aberrations (translocations, insertion, inversion) involved either immunoglobulin (IG) genes (3 cases), or non-IG sequences at 1p34/PHACTR4, 4p13, 3p25, 6q13/EEF1A1 and 13q11 (5 cases, including one with a three-way translocation also involving IGH). In one case with inv(9)(p24p23), a microdeletion affecting the 3' region of JAK2 and the 5' region of PDL1 was detected by FISH and aCGH. The expression pattern of 5 candidate genes analyzed in 5 available cases showed upregulation of PDL1 and PDL2 in 1 case, overexpression of sole PDL2 in 2 cases and a normal level of analyzed transcripts in 2 cases. In both cases with t(9;14)(p24;q32) translocation of JAK2 to der(14) was demonstrated by FISH. Unfortunately, these cases were not available for qRT-PCR.

9p24 amplification hallmarked by homogeneously staining region (hsr) was identified in 10 cases of PMBCL and 4 cases of T-NHL. FISH screening of 130 cases of cHL identified 54 lymphomas with low copy number gains (5–10 copies) and 30 cases with 9p24 amplification, including 6 cases with a predominant amplification of the region covering PDL1 and PDL2. In addition, putative involvement of PDL1/PDL2 in unbalanced translocations associated with loss of JAK2 and telomeric 9p24 sequences was found in 3 cases of cHL.

Expression of the JAK2, PDL1 and PDL2 mRNA was analyzed in 7 cases of PMBCL and 2 cases of T-NHL. Among the PMBCL samples, 4 showed overexpression of all 3 transcripts, 2 showed upregulation of PDL1 and PDL2, and one showed overexpression of sole PDL1. JAK2 and PDL1 were found to be highly expressed in one case of T-NHL, while a case of ALK+ ALCL revealed only upregulation of PDL1.

IHC analysis of P-JAK2, PDL1 and PDL2 proteins in available cases is ongoing.

Our study demonstrates that the 9p24 region is recurrently targeted by structural and numerical aberrations in B- and T-cell leukemia/lymphoma. These abnormalities may represent primary as well as secondary genetic changes acquired during progression of the disease. Structural aberrations predominantly involve PDL1 and/or PDL2 that seem to be deregulated by their juxtaposition with regulatory sequences provided by partner genes. Five postulated PDL1/PDL2 partner genes include IGH/14q32, IGL/22q11, PHACTR4/1p34.3 EEF1A1/6q13 and JAK2/9p24. In the analyzed PMBCL and T-NHL cases, 9p24 amplification constantly targets PDL1 and is randomly associated with an aberrant co-expression of either JAK2 or PDL2. Given the role of PDL1/PDL2 in inhibition of T-cell activation, their impaired expression in B- and T-cell lymphoma may lead to “T-cell exhaustion” and deficient cellular immunity favoring tumor progression.

No relevant conflicts of interest to declare.