One extra chromosome copy (i.e., trisomy) is the most common type of chromosome aberration in cancer cells. The mechanisms behind the generation of trisomies in tumor cells are largely unknown, although it has been suggested that dysfunction of the spindle assembly checkpoint (SAC) leads to an accumulation of trisomies through failure to correctly segregate sister chromatids in successive cell divisions. By using Wilms tumor as a model for cancers with trisomies, we now show that trisomic cells can form even in the presence of a functional SAC through tripolar cell divisions in which sister chromatid separation proceeds in a regular fashion, but cytokinesis failure nevertheless leads to an asymmetrical segregation of chromosomes into two daughter cells. A model for the generation of trisomies by such asymmetrical cell division accurately predicted several features of clones having extra chromosomes in vivo, including the ratio between trisomies and tetrasomies and the observation that different trisomies found in the same tumor occupy identical proportions of cells and colocalize in tumor tissue. Our findings provide an experimentally validated model explaining how multiple trisomies can occur in tumor cells that still maintain accurate sister chromatid separation at metaphase-anaphase transition and thereby physiologically satisfy the SAC. It is well established that chromosomal alterations in cancer can arise as a consequence of abnormal segregation of chromosomes at mitosis, but it remains to be shown precisely how extra copies of whole chromosomes are gained. It has been suggested that deficiency of the spindle assembly checkpoint (SAC) or other key mechanisms controlling sister chromatid separation could promote the generation of trisomies in cancer cells through a continuously elevated rate of concurrent chromosome gain and loss (i.e., nondisjunction) at metaphase-anaphase transition (1-4). The SAC deficiency model has been challenged by the fact that mutations in mitotic checkpoint genes have been found only in a minority of human cancers (3-7), but the absence of such mutations could still be explained by epigenetic modifications of mitotic control genes or by mutations in SAC genes that are not yet characterized. Therefore, it has remained difficult to experimentally validate the association between SAC deficiency and trisomies. This problem could be circumvented by estimating directly the rate of sister chromatid separation failure at mitosis. To do this, we used FISH to monitor the segregation of individual chromosomes in anatelophase cells. This method was then applied to Wilms tumor (WT), which is a prototypical model for cancers with whole chromosome gains, showing polysomies in the majority of cases with abnormal karyotypes, of which 62% have two or more coexisting trisomies and 16% have tetrasomies (Mitelman Database of Chromosome Aberrations in Cancer, 2010). In contrast to previous assumptions, we find that continuous generation of trisomies through SAC deficiency is unlikely to explain the gen...

Telomere length alterations are known to cause genomic instability and influence clinical course in several tumor types, but have been little investigated in neuroblastoma (NB), one of the most common childhood tumors. In the present study, telomere-dependent chromosomal instability and telomere length were determined in six NB cell lines and fifty tumor biopsies. The alternative lengthening of telomeres (ALT) pathway was assayed by scoring ALT-associated promyelocytic leukemia (PML) bodies (APBs). We found a reduced probability of overall survival for tumors with increased telomere length compared to cases with reduced or unchanged telomere length. In non-MYCN amplified tumors, a reduced or unchanged telomere length was associated with 100% overall survival. Tumor cells with increased telomere length had an elevated frequency of APBs, consistent with activation of the ALT pathway. The vast majority of tumor biopsies and cell lines exhibited an elevated rate of anaphase bridges, suggesting telomere-dependent chromosomal instability. This was more pronounced in tumors with increased telomere length. In cell lines, there was a close correlation between lack of telomere-protective TTAGGG-repeats, anaphase bridging, and remodeling of oncogene sequences. Thus, telomere-dependent chromosomal instability is highly prevalent in NB, and may contribute to the complexity of genomic alterations as well as therapy resistance in the absence of MYCN amplification and in this tumor type.

From the three tested cell lines, orthotopic WiT49 xenografts best reflect the triphasic pattern of classical WT.

Genes highly expressed in WT compared to fetal kidney are generally overexpressed in all of the three common WT tissue elements. An exception is the predominant expression of SIX1 in blastemal cells, hereby identifying this protein as a candidate marker for blastema.