One Half of Patients with Waldenström Macroglobulinemia Have Large Deletions of Chromosome 6q.

Background: Waldenström macroglobulinemia (WM) is a plasma cell dyscrasia characterized by a monoclonal IgM paraproteinemia. The most common genomic abnormality in the clonal cells of WM is loss of portions of the long arm of chromosome 6 (6q-), however a minimal area of deletion has yet to be identified. We decided to further clarify the area of deletion on the 6q-arm in WM along with its prevalence.

Methods: Patients (n=41) were required to have a serum IgM paraproteinemia of 1.5 g/dl or greater and a monoclonal lymphoplasmacytic infiltrate. We used interphase fluorescence in situ hybridization (FISH) combined with cytoplasmic immunoglobulin (cIg) staining with an AMCA conjugated anti-IgM antibody along with cytomorphology for the detection of the clonal cells on cytospin slides. Ten probes were used including two commercial (CEP 6 and Telomere 6q) and eight Bacteria Artificial Chromosome (BAC). BAC clones were selected based on a specific gene encompassed in the insert and/or localization on the q arm. Genes of interest included BLIMP1, c-myb, BTF, SHPRH, LATS1 and FOP. BAC’s were grown in selective media; DNA was extracted and directly labeled with a SpectrumGreen or SpectrumOrange fluorophore by standard nick translation. Primers were designed to confirm the insert contained the gene of interest and probes were tested on normal metaphases to reaffirm localization to the correct area of the chromosome and rule out any cross hybridization.

Results: Deletions of 6q were noted in 21 (51%) patients. Of the 19 abnormal patients with all probes tested 1 (5%) case had ten probes deleted, 7 (37%) cases had nine probes, 1 (5%) had eight, 4 (21%) had seven, 3 (16%) had six and 3 (16%) had four probes deleted. Only 3 cases retained the end of the q arm indicating an interstitial deletion while the remaining 16 cases indicate terminal deletions. Deletions of the BAC containing the gene SHPRH at 6q24 were seen in each of the 19 abnormal cases. The majority of patients exhibited little heterogeneity for any given deletion having greater than 80% involvement of the clone.

Conclusion: This study demonstrates that large deletions of the long arm of chromosome 6 in WM are frequent and are usually terminal deletions rather than interstitial and have a common area of deletion of the q24 portion of the q arm. Using the given number of probes localizing along the q arm makes it highly unlikely of any areas of noncontiguous deletion. The presence of deletions in the majority of the clonal cells may suggest that clones with 6q- emerge early in the course of the disease and is selected because of growth advantage or apoptosis resistance. Whether 6q deletions are important for disease pathogenesis has yet to be determined. Clinical prognosis studies and microarray data analysis are pending.