SNP Array Karyotyping Improves Detection Rate of Clonal Chromosomal Abnormalities in Refractory Anemia with Ringed Sideroblasts.

Among WHO low-risk categories of MDS, refractory anemia with ringed sideroblasts (RARS) can be more accurately diagnosed by characteristic pathomorphology. Clonal hematopoiesis and chromosomal abnormalities exemplify a close pathogenetic relationship to other forms of MDS. RARS shows considerable clinical variability even for patients (pts) with identical cytogenetic defects. Due to the low resolution of metaphase cytogenetics (MC) and its dependence on cell growth in vitro, this test is often non-informative in MDS. High-density SNP arrays (SNP-A) allow for a precise identification of unbalanced genomic lesions and copy-neutral loss of heterozygozity. We hypothesize that cryptic chromosomal (chr) aberrations exist in most, if not all, pts with RARS. Their detection may help to improve prognostication, distinguish distinct phenotypes and point towards unifying pathogenic defects. Initially, we analyzed the results of MC in pts with MDS and MDS/MPD (N=455) and in a sub-cohort of RARS, RCMD-RS, RARSt and other MDS subtypes with >15% RS. When we compared pts with/without RS, chr defects were found at comparable frequencies (∼50%). The most commonly occurring defects associated with RS, compared to other forms of MDS, included those of chr 5 (9% vs. 16%, 7 (8% vs. 12%) and 20 (3% vs. 8%). DNA was available for 36 pts with RS and was subjected to 250K SNP-A karyotyping. Pathologic lesions were defined upon exclusion of normal copy number polymorphisms identified in 81 controls (O’Keefe at al ASH 2007), as well as the Database of Genomic Variants (http://projects.tcag.ca/variation). By MC, a defective karyotype was present in 16/36 pts (44%). Deletions involving chr 5, 7 and complex MC were found in 3, 5, and 2pts, respectively. However, when SNP-A was applied as a karyotyping tool (copy number and LOH analysis), all aberrations found by MC were confirmed, but also new lesions were detected so that an abnormal karyotype was established in 62% of pts. Several previously cryptic/recurrent lesions included losses of a portion of chr. 2 (N=2; 2p16.2, 2p16.3), and deletions (N=4; 7p11.1–14.1, 7p21.3, 7q11.23–21.11, 7q21.12-qter) as well as gains (N=1; 7q33) on chr 7. We have also detected segmental uniparental disomy (UPD) in chr 1 (N=2; 1p21.3–22.2, 1p). This type of lesion cannot be detected using MC and provides an additional mechanism leading to LOH. When both bone marrow and blood of 5 RARS patient were tested using SNP-A, blood analysis had 100% accuracy rate as compared to marrow; all defects seen in the marrow were also found in blood. We conclude that chromosomal defects are present in a majority of RARS patients and arrays with higher resolution will identify defects in most, if not all of the patients. Our study also demonstrates testing of peripheral blood by SNP-A can complement marrow MC, especially in cases in which marrow is not available. Detection of clonal marker aberrations in blood of RARS patients suggests that mostly clonal dysplastic progenitor cells contribute to blood production rather than residual “normal” progenitors.