Ribosomal Proteins Are Overexpressed in Hyperdiploid Multiple Myeloma.

Multiple myeloma (MM) is proposed to consist of two main pathogenetic groups. While hyperdiploidy (HD) is characterized by multiple trisomies of odd-numbered chromosomes (i.e. 3, 5, 9, 11, 15, and 19), non-hyperdiploid MM (NHD) show frequently one of the several recurrent IgH-translocations. The aim was to compare HD versus NHD by gene expression profiling (GEP). CD138-positive multiple myeloma cells from 74 newly diagnosed MM patients (42 GEP training group (TG), 32 GEP validation group (VG)) were purified by autoMACS-sorting. Sorted cells were analyzed by interphase-FISH with probes specific for 6q21, 8p21, 9q34, 11q23, 13q14, 15q22, 17p13, 19q13 and translocations t(4;14) and t(11;14). HD and NHD were defined by using a copy number score (CS), which was calculated by subtracting the number of probes indicating losses from the number of probes detecting additional copies (CS >0: HD; CS ≤0: NHD). GEP was performed with Affymetrix DNA-microarrays. Nearest shrunken centroid classification (NSCC) was used to discriminate the different groups, using GCRMA-normalized gene expression values. The prediction error was estimated by means of nested cross-validation using 10 repetitions of 10-fold cross-validation within the training set and separately calculated by use of the NSCC classifier of the training set to predict the validation set. Goeman’s global test was used to check the influence of ribosomal protein expression between HD and NHD. In the TG, both HD and NHD were found in 21 patients. The VG comprised 13 patients with NHD and 19 patients with HD. NSCC resulted in a predictor for HD versus NHD of 81 probe sets with a cross-validated misclassification rate of 14.2% for the TG and 26.5% for the VG. Three of the top ten genes were ribosomal proteins, overexpressed in patients with HD. Goeman’s global test further showed that ribosomal proteins are overexpressed in HD (TG: p<0.001; VG: p=0.03). Interestingly, ribosomal protein genes located on even-numbered chromosomes were also overexpressed. In conclusion, overexpression of ribosomal proteins independent of their location on odd- or even-numbered chromosomes indicates more than just a gene dosage effect and therefore a pathogenetic role of the upregulation of the ribosomal machinery in HD.