Archives for posts with tag: breakage-fusion-bridge cycle

The Leukaemia Foundation of Australia’s National MDS Day has just passed (14th July… but I was busy eating croissants so this post is a little late).

This time I thought I would tell you about a discovery that was made with the help of MDS.

Little by little, errors creep into the genes of healthy cells. Most aren’t important. But sometimes they mean the cell doesn’t work the way it’s supposed to. They might cause a cell to grow out of control and turn into cancer.

Myelodysplastic syndromes, or MDS, are a range of blood disorders caused by such errors in the genes. Some types are relatively mild, but about a third progress to become acute myeloid leukaemia (AML). Thanks to research on MDS we understand its causes a lot better than we did ten or fifteen years ago.

My lab recently published a paper which described gene changes that were remarkably similar in three cases of poor prognosis MDS and one of AML. Molecularly, the chromosomes had the same complicated structure, that could not have been predicted by the way they looked down the microscope. These features meant that we could work out the steps causing these genetic errors.

This is a Claymation that shows how Barbara McClintock’s classic breakage-fusion-bridge cycle causes gene abnormalities.

The video shows the telomeres (that cap and protect the ends of the chromosomes) falling off, making sticky chromosome ends which join together (see NOTE 2). This is the step that makes cancer-causing gene changes much more likely. It’s well accepted that this eroding away of the telomeres is a mechanism for creating cancerous gene changes. It’s been reproduced many, many times in the lab. The problem is that it’s not often been proven in actual cancers. But we did that.

Sometimes only part of the telomere erodes away – enough that it no longer protects the chromosomes from sticking together. But there’s enough telomere DNA left to be a molecular signature of the telomere.

dic 20-22

The arrow points to green dots in the middle of a chromosome. This is the left-over telomere signature that tells us that this abnormal chromosome was made by the joining together of sticky chromosome ends that had their telomeres eroded away. The other green dots are at the chromosome ends. The left and right photos show the same cell but in the right one the abnormal chromosome is identified by its red and blue label.

In our four cases, we tested the abnormal chromosomes to see if there was any of the telomere left where the two chromosomes had joined. Indeed this is what we found – the chromosomes had joined together because the chromosome ends had eroded away, leaving a small amount of non-functional telomere DNA. In these instances the ensuing breakage-fusion-bridge events caused a protective tumour suppressor gene to be lost, and, possibly, cancer-causing genes to multiply.

MDS and AML share many gene errors in common, so if we learn about one of them it can help us understand the other. This is often the case with cancer research in a broader sense – if we understand the basic mechanisms in one cancer it can help us understand the mechanisms at work in other cancers better. Telomere fusion could potentially play a role in any cancer, so our MDS research is relevant to cancer research in general.

NOTES

  1. The paper: The dicentric chromosome dic(20;22) is a recurrent abnormality in myelodysplastic syndromes and is a product of telomere fusion. Ruth MacKinnon, Hendrika Duivenvoorden, Lynda Campbell and Meaghan Wall, 2016. Cytogenetic and Genome Research 150(3-4):262-272
  2. The gene errors discussed here usually occur in the body cells rather than the reproductive cells, so they’re not inherited.
  3. For simplicity the Claymation shows telomere fusion in chromosomes that are dividing.  In fact it probably occurs when the DNA is unravelled in the interphase nucleus.

Barbara McClintock published a paper describing the breakage-fusion-bridge (BFB) cycle in 1939. Many of her ideas were well before their time. Like many such profound leaps in thinking, the BFB cycle took a long time to catch on. She wrote in 1973, “I stopped publishing detailed reports long ago when I realized, and acutely, the extent of disinterest and lack of confidence in the conclusions I was drawing …One must await the right time for conceptual change.” Her work was appreciated much later and she was awarded a Nobel Prize in 1983 for her discovery of “jumping genes“.

A few weeks back I introduced this post by describing normal chromosome division. This time we’ll look at the breakage-fusion-bridge cycle. This is one way chromosome division can go wrong. Very wrong, in the sense that it can cause the chromosomes to keep changing, and this can cause cancer.

A human chromosome with two centromeres is abnormal. Chromosomes with two centromeres are not unusual in cancer cells. In fact they’re probably a lot more common than we think, because in both research and diagnostic labs the centromeres are usually not looked at.

To recap, a normal chromosome has one centromere. Before the chromosome divides, the two identical halves (chromatids) are held together at the centromere. When the chromosome divides the centromere splits into two halves, the chromatids become the new chromosomes, and the centromeres take the two new chromosomes in different directions into the two new daughter cells.

So what happens if there are two centromeres? If they’re both aligned so that they head in the same direction it’s not a problem – together they take a complete new chromosome with them. The closer the centromeres are together the more likely this is.

Now follow the pictures and their captions. These describe chromosome division in an abnormal chromosome with two centromeres. Especially follow the yellow dots.

A twist between the two centromeres when the chromosomes align ready for chromosome division.

If the two centromeres on a chromosome go in the same direction there’s no problem. But if there’s a twist between the two centromeres when the chromosomes align ready for chromosome division….

....then, when the two halves of each centromere separate they head off in different directions.

….then, when the two halves of each centromere separate they go in opposite directions. We have a “bridge” spanning the gap between the two centromeres.

The bit of chromosome between them gets stretched and can break.

The bridge is stretched and can break.

The broken chromosomes in the new cell join together - the top daughter cell gets an extra copy of the yellow gene. The bottom cell loses this copy of yellow gene.

The broken chromosomes in the new cell join together – the top daughter cell gets an extra copy of the yellow gene. The bottom cell loses this copy of the yellow gene.

The new chromosome copies itself to make two equal halves.

The new chromosome copies itself to make two equal halves.

If this process repeats..

If this process repeats..

Fusion of the broken bits of chromosome in the top cell.

Fusion of the broken pieces creates a chromosome with four copies of the yellow gene.

After replication - the new chromosome with four copies of the yellow gene courtesy of the breakage-fusion-bridge cycle.

After replication.

If the yellow gene in the pictures is a cancer gene (“oncogene”) the cell with extra copies might grow and multiply faster than its neighbours. We call this natural selection – the cells that can grow faster than their neighbours become more common which means the genetic change causing that is undergoing “positive selection”. Yes, the cells in our body can evolve and we know this best as cancer.

All this change happens between the two centromeres where the bridge forms. So if we find a chromosome with this type of change on one side of the centromere only it’s a clue that this might have been caused by the breakage-fusion-bridge cycle.

These are modelling clay images from my breakage-fusion-bridge claymation. They’re a bit rough but I hope it helps you understand what happens. Many, perhaps most, images demonstrating the BFB cycle show a different version – where the abnormal chromosome is created by two chromatids of one chromosome breaking and joining together. Most examples don’t show the version I’ve presented – where two different chromosomes have joined together. Check this out on Google Images (search for breakage-fusion-bridge).

Here’s the answer to the quiz from the telomere post. The arrows point to the ring chromosomes. Being rings they have no ends, so no telomeres.

answer to ring chr

Further Reading:

B. McClintock 1939. The Behavior in Successive Nuclear Divisions of a Chromosome Broken at Meiosis. Proc Natl Acad Sci U S A. 1939 August; 25(8): 405–416.

M. Kinsella and V. Bafna 2012. Combinatorics of the Breakage-Fusion-Bridge Mechanism. J Comput Biol. 2012 June; 19(6): 662–678.

R. MacKinnon and L. Campbell 2011. The Role of Dicentric Chromosome Formation and Secondary Centromere Deletion in the Evolution of Myeloid Malignancy. Genetics Research InternationalVolume 2011 (2011), Article ID 643628.