High Resolution Karyotyping of Large Granular Lymphocyte Leukemia by SNP Arrays Reveals Clonal Defects Leading to LOH at Loci Integral to Lymphocyte Function.

Large granular lymphocyte leukemia (LGL leukemia) is a semiautonomous clonal lymphoproliferation of cytotoxic T cells associated with various immune cytopenias. Within the spectrum of acquired immune-mediated bone marrow failure states, LGL leukemia can serve as monoclonal model of usually polyclonal T cell-mediated pathology. Mechanisms of unopposed clonal expansion of LGL cells in many aspects resemble true lymphoma and are not well understood. In addition to its reactive character, intrinsic clonal defects may be present in some patient with LGL, in particular those with more pronounced lymphoproliferative features. However, unlike in B-cell lymphomas, recurrent chromosomal abnormalities have not been frequently identified in LGL leukemia using traditional metaphase karyotyping techniques. We have applied Affymetrix 250K single nucleotide polymorphism-arrays (SNP-A) and 6.0 SNP-A in 28 patients with LGL leukemia, to elicit a far higher resolution of chromosomal content through genomic mapping of individual SNP and respective copy number analysis in LGL leukemia. SNP-A based cytogenetics have been applied successfully to MDS patients and has increased prognostic reliability. Blood mononuclear cells containing high proportion of clonal cells as determined by TCR Vβ flow cytometry were used as a source of DNA. For comparison, a large number of control blood and marrow specimens (N=119 for 6.0 and 124 for 250K SNP arrays) were analyzed. Data were processed using Genotyping Console v2.1 software (Affymetrix, Santa Clara, CA). After exclusion of known copy number variants (CNV) referenced in public databases and our own set of 178 normal controls, we found distinct chromosomal changes in 16/28 (57%) of LGL leukemia patients. Consensus regions of deletion/gain or uniparental disomy (UPD) were identified. The most common abnormalities included either UPD or copy number loss of chromosome 3q21.2–q21.31, which was identified in 6 (21%) patients. This region harbors CD86, the gene encoding the B7.2 protein responsible for T cell activation and regulation through costimulatory mechanisms. Copy number loss/UPD was also identified at chromosome 1p31.1–p32.3 in 4 (18%) patients. Interestingly, copy number gain in this same region was identified in 2 (9%) patients suggesting that this region may correspond to either a new, infrequent germ line encoded CNV or represents a somatic microdeletion. Recent studies indicated a role for SIL/SCL, which resides at this locus, in V(D)J recombination, lymphocyte development, and maturation. Additional conserved chromosomal abnormalities included copy number gains in 14q (11%) and copy number loss at 11p15 (14%), all possibly representing germ line or somatic CNV physiologically acquired during lymphocyte ontogenesis. We compared chromosomal lesions identified in our LGL cohort with clinical features including age, presence of neutropenia, anemia, thrombocytopenia, splenomegaly, immunophenotype, and degree of clonal expansion. No difference in clinical course was found between patients with and without cytogenetic abnormalities with regard to type and severity of cytopenias or size of the LGL clone. However, patients with 1p31.1–p32.3 deletions were found to have lesser degree of neutropenia compared to patients without 1p31.1–p32.3 deletions (p<0.001). While these data may reflect chromosomal variants and random somatic abnormalities, the recurrent loss of heterozygosity and gains within several loci with known regulatory function for lymphocyte biology suggests the involvement of these defects in the mechanism of clonal evolution.