Chromothripsis: chromosomes in crisis

Criteria for inference of chromothripsis in cancer genomes

Chromothripsis scars the genome when localized chromosome shattering and repair occurs in a one-off catastrophe. Outcomes of this process are detectable as massive DNA rearrangements affecting one or a few chromosomes. Although recent findings suggest a crucial role of chromothripsis in cancer development, the reproducible inference of this process remains challenging, requiring that cataclysmic one-off rearrangements be distinguished from localized lesions that occur progressively. We describe conceptual criteria for the inference of chromothripsis, based on ruling out the alternative hypothesis that stepwise rearrangements occurred. Robust means of inference may facilitate in-depth studies on the impact of, and the mechanisms underlying, chromothripsis.

Gene fusions by chromothripsis of chromosome 5q in the VCaP prostate cancer cell line

The VCaP cell line is widely used in prostate cancer research as it is a unique model to study castrate resistant disease expressing high levels of the wild type androgen receptor and the TMPRSS2-ERG fusion transcript. Using next generation sequencing, we assembled the structural variations in VCaP genomic DNA and observed a massive number of genomic rearrangements along the q arm of chromosome 5, characteristic of chromothripsis. Chromothripsis is a recently recognized phenomenon characterized by extensive chromosomal shattering in a single catastrophic event, mainly detected in cancer cells. Various structural events identified on chromosome 5q of VCaP resulted in gene fusions. Out of the 18 gene fusion candidates tested, 15 were confirmed on genomic level. In our set of gene fusions, only rarely we observe microhomology flanking the breakpoints. On RNA level, only five transcripts were detected and NDUFAF2-MAST4 was the only resulting in an in-frame fusion transcript. Our data indicate that although a marker of genomic instability, chromothripsis might lead to only a limited number of functionally relevant fusion genes.

Mechanisms for Structural Variation in the Human Genome

It has been known for several decades that genetic variation involving changes to chromosomal structure (i.e., structural variants) can contribute to disease; however this relationship has been brought into acute focus in recent years largely based on innovative new genomics approaches and technology. Structural variants (SVs) arise from improperly repaired DNA double-strand breaks (DSB). DSBs are a frequent occurrence in all cells and two major pathways are involved in their repair: homologous recombination and non-homologous end joining. Errors during these repair mechanisms can result in SVs that involve losses, gains and rearrangements ranging from a few nucleotides to entire chromosomal arms. Factors such as rearrangements, hotspots and induced DSBs are implicated in the formation of SVs. While de novo SVs are often associated with disease, some SVs are conserved within human subpopulations and may have had a meaningful influence on primate evolution. As the ability to sequence the whole human genome rapidly evolves, the diversity of SVs is illuminated, including very complex rearrangements involving multiple DSBs in a process recently designated as "chromothripsis". Elucidating mechanisms involved in the etiology of SVs informs disease pathogenesis as well as the dynamic function associated with the biology and evolution of human genomes.