SNP-A Based Karyotyping Facilitates Improved Mapping of Deletions and Uniparental Disomy within the Long Arm of Chromosome 5 in Myeloid Disorders.

In MDS, cytogenetics has a major prognostic influence on the phenotype of the malignant clone and specific defects may point towards potential therapeutic targets. However, traditional metaphase cytogenetics (MC) has limited resolution and does not allow for detection of uniparental disomy (UPD). Defects on chromosome 5q have been studied using various methods to identify a minimal commonly deleted region (CDR). SNP-array karyoptyping (SNP-A) allows for precise detection of copy number changes as well as UPD. We hypothesized that SNP-A may reveal new lesions on chromosome 5 and allow for better definition of CDRs and pathogenic genes. Of 512 patients, 15% showed a 5q abnormality as a sole or associated aberration by MC. DNA was available in 189 patients and was subjected to 250K SNP-A. In 7 patients with normal/non-informative MC, a deletion on 5q was clearly detectable by SNP-A; in total, SNP-A identified 5q abnormalities in 14% patients in this group (vs. 11% by MC). UPD 5q was found in one patient with CMML. By SNP-A, 6/27 patients showed an isolated 5q deletion. SNP-A can also be used to construct precise cytogenetic maps. The commonly deleted region (CDR1,5q31.2, 137,472,900–139,451,900) was present in 24/27 patients. Significant overlap occurs with the CDR previously defined by Fairman, Zhao, Horrigan et al. This region includes important genes such as Cdc25C and EGR1. Of 24 patients with a deletion in CDR1, 21 had multilineage dysplasia predominantly in the megakaryocytic line (92%). While elevated platelet counts occurred in 3 patients, increased levels of megakaryocytes were common (83%). Previous studies by Bouldwood/Jaju suggested that the minimal CDR among patients with 5q- syndrome (CDR2, 5q33.1-33.2) differs slightly from that associated with secondary AML/MDS (CDR1). However, when patients (5/27) with classical 5q- syndrome were analyzed, all displayed single deletions spanning both CDR1 and CDR2. Other areas of partial overlaps were also identified (5q12.1; 5q13.3) more centromeric to CDR1 and present in 7/27 patients. 2 cases were particularly interesting: 1 with segmental UPD involving the CDR, the other showing a small deletion defining the CDR itself. In the latter patient, marked thrombocytosis was present and SNP-A demonstrated a complex chromosomal rearrangement. While MC revealed a segmental deletion of 5q and a concomitant duplication of this abnormal homolog, SNP-A showed that while the p arm portion had been duplicated, the q arm, with the exception of two small deletions (1.35 and 1.98Mb in length, confirmed by FISH), had a normal diploid set. SKY clarified that chr. 5 material had indeed been displaced to both chr. 3 and 7 with a reciprocal translocation of chr. 3 material occurring on the abnormal chr. 5. In sum, our studies demonstrate the utility of SNP-A as a karyotyping tool that can detect previously cryptic areas of LOH on chr. 5 and facilitate definition of shared 5q defects. We also show that our patients with 5q- syndrome had lesions spanning both 5q33 and the more proximal 5q31.2 area, making pathogenic distinction based on cytogenetics difficult.