Telomere shortening and chromosomal instability are believed to play an important role in the development of myeloid neoplasia. So far, published data are only available on the average telomere length in myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML), but not on the telomere length of individual chromosomes. We used a new technique, telomere/centromere-fluorescence in situ hybridization (T/C-FISH), which combines fluorescence R-banding and FISH using a probe against the telomere repeats to measure the telomere length of each chromosome arm in 78 patients with MDS. In line with the previous results, patients with MDS showed significantly shorter telomeres than those of healthy controls. Telomere lengths did not differ significantly between distinct morphological subtypes of MDS. However, there was a significant difference in telomere length between patients with an isolated monosomy 7 and patients with a normal karyotype (P < 0.05). Notably, patients with an isolated monosomy 7 showed significantly longer telomeres than patients with a normal karyotype in many chromosome arms, among them 7p and 7q. Neo-telomeres were found in two patients with a complex karyotype, in one case at the fusion site of a dic(14;20). Normal and aberrant metaphases of the same patient did not differ in telomere length, thus indicating to telomere shortening as a basic mechanism affecting all hematopoietic cells in patients with MDS. In some MDS subtypes, like MDS with isolated monosomy 7, telomeres may be stabilized and even increase in length because of the activation of telomerase or alternative mechanisms.

The Sleeping Beauty (SB) transposase and its newly developed hyperactive variant, SB100X, are of increasing interest for genome modification in experimental models and gene therapy. The potential cytotoxicity of transposases requires careful assessment, considering that residual integration events of transposase expression vectors delivered by physicochemical transfection or episomal retroviral vectors may lead to permanent transposase expression and resulting uncontrollable transposition. Comparing retrovirus-based approaches for delivery of mRNA, episomal DNA or integrating DNA, we found that conventional SB transposase, SB100X and a newly developed codon-optimized SB100Xo may trigger premitotic arrest and apoptosis. Cell stress induced by continued SB overexpression was self-limiting due to the induction of cell death, which occurred even in the absence of a co-transfected transposable element. The cytotoxic effects of SB transposase were strictly dose dependent and heralded by induction of p53 and c-Jun. Inactivating mutations in SB’s catalytic domain could not abrogate cytotoxicity, suggesting a mechanism independent of DNA cleavage activity. An improved approach of retrovirus particle-mediated mRNA transfer allowed transient and dose-controlled expression of SB100X, supported efficient transposition and prevented cytotoxicity. Transposase-mediated gene transfer can thus be tuned to maintain high efficiency in the absence of overt cell damage.