By Gesa Junge, PhD
Each of our cells contains about two meters of DNA which needs to be stored in cells that are often less than 100uM in diameter, and to make this possible, the DNA is tightly packed into chromosomes. As the human cell prepares to divide, the 23 pairs of chromosomes neatly line up and attach to the spindle apparatus via their middle point, the centrosome. The spindle apparatus is part of the cell’s scaffolding and it pulls the chromosomes to opposite ends of the cell as the cell divides, so that every new daughter cell ends up with exactly one copy of each chromosome. This is important; cells with more or less than one copy of a chromosome are called aneuploid cells, and aneuploidy can lead to genetic disorders such as Down Syndrome (three copies of chromosome 21).
In some cancer cells, chromosomes with two centromeres (dicentric chromosomes) can be detected, which can happen when the ends of two chromosomes fuse in a process called telomere crisis. Telomeres are a sort of buffer zone at the ends of the chromosome which consist of repeats of non-coding DNA sequences, meaning there are no genes located here. As one of the DNA strands is not replicated continuously but in fragments, the telomeres get shorter over the lifespan of a cell, and short telomeres can trigger cell cycle arrest before the chromosomes get so short that genetic information is lost. But occasionally, and especially in cancer cells, chromosome ends fuse and a chromosome becomes dicentric. Then it can attach to the spindle apparatus in two points and may end up being pulled apart as the two daughter cells separate, sort of like a rope tied to two cars that drive in opposite directions. This string of chromosome is referred to as a chromatin bridge.
Researchers at Rockefeller University are studying these chromatin bridges and what their relevance is for the health of the cell. A paper by John Maciejowski and colleagues found that the chromatin bridges actually stay intact for quite a long time. Chromosomes are pretty stable, and so the chromatin bridges lasted for an average of about 9 hours (3-20h) before snapping and quickly being pulled back into the original cell (see video). Also, the nucleus of the cell was often heart-shaped as opposed to the usual round shape, which suggests that the chromatin bridge physically pulls on the membrane surrounding the nucleus, the nuclear envelope. Indeed, proteins that make up the nuclear envelope (e.g. LAP2) were seen on the chromatin bridge, suggesting that they take part of the nuclear envelope with them as they divide. Also, cells with chromatin bridges had temporary disruptions to their nuclear envelope at some point after the bridge was resolved, more so than cells without chromatin bridges.
The chromatin bridges also stained positive for replication protein A (RPA), which binds single stranded DNA. DNA usually exists as two complementary strands bound together, and the two strands really only separate to allow for DNA to be copied or transcribed to make protein. Single-stranded DNA is very quickly bound by RPA, which stabilises it so it does not loop back on itself and get tangled up in secondary structures. The Rockefeller study showed that a nuclease, a DNA-eating enzyme, called TREX1 is responsible for generating the single-stranded DNA on chromatin bridges. And this TREX1 enzyme seems to be really important in resolving the chromatin bridges: cells that do not have TREX1 resolve their chromatin bridges later than cells that do have TREX1.
So how are chromatin bridges important for cells, the tissue and the organism (i.e. us)? The authors of this study suggest that chromatin bridges can lead to a phenomenon called chromothripsis. In chromothripsis, a region of a chromosome is shattered and then put back together in a fairly random order and with some genes facing the wrong direction. Think of a new, neatly color-sorted box of crayons that falls on the floor, and then someone hastily shoves all the crayons back in the box with no consideration for color coordination or orientation. Chromothripsis occurs in several types of cancers, but it is still not really clear how often, in what context and exactly how the genes on a chromosome end up in such a mess.
According to this study, chromothripsis may be a consequence of telomere crisis, and chromatin bridges could be part of the mechanism: A chromosome fuses ends with another chromosome and develops two centromeres. The dicentric chromosome attaches to two opposite spindles and is pulled apart during cell division, generating a chromatin bridge which is attacked by TREX that turns it into single-stranded DNA, the bridge snaps and in the process the DNA scatters, and returns to the parent cell where it is haphazardly reassembled, leaving a chromothripsis region.
The exact mechanisms of this still need to be studied and the paper mentions a few important discussion points. For example, all the experiments were performed in cell culture, and the picture may look very different in a tumor in a human being. And what exactly causes the bridge to break? Also, there are probably more than one potentially mechanism linking telomere crisis to chromothripsis. But it is a very interesting study that shines some light on the somewhat bizarre phenomenon of chromothripsis, and the importance of telomere crisis.
Reference: Maciejowski et al, Cell. 2015 Dec 17; 163(7): 1641–1654.