Alignment Zone: new paper on chromosome missegregation

We have a new paper out. It’s about how chromosomes get shared during cell division.

The paper in a nutshell

In science-speak

Misaligned chromosomes outside the exclusion zone become ensheathed in multiple layers of endomembranes. This event consigns the chromosome to missegregation and generates a micronucleus.

In normal language

Cells can make mistakes when they divide. A mistake can mean that a chromosome becomes entangled in membranes. If the cell goes ahead and divides in that state, the chromosome turns into a very small version of the cell nucleus called a micronucleus. Cancer can start from cells with micronuclei.

What did we find?

When a cell divides, it makes a copy of every chromosome and then shares them equally between the two new cells. This is done by a process called mitosis. The sharing of chromosomes is done by a complicated machine in the cell called the mitotic spindle. Chromosome sharing is really important. If something goes wrong here it means that the two new cells are aneuploid (meaning they don’t have the right number of chromosomes).

Cancer cells are aneuploid and they also make lots of mistakes when sharing chromosomes. Understanding this how this happens is a major goal in cancer cell biology. It is also known that cancer can start from a cell that has formed a micronucleus.

Most scientists working on chromosome sharing have focussed, quite rightly, on the spindle. It had been known for many years that the spindle is in an area of the cell called the exclusion zone. This area has no membranes. This means that outside of the spindle is a tangled web of membranes. Not many scientists have examined these membranes during mitosis.

A human cell with endomembranes shown in green, chromosomes in red and the spindle in white. Taken from Ferrandiz et al. 2022

We wondered: what happens if a chromosome wanders off and gets lost in this web of membranes?

Homer here represents a chromosome getting lost in membranes.

What we found is that the chromosomes that move outside this zone become ensheathed in membranes. Several layers of them. They are not just lost in there but they are kind of trapped.

We thought that this didn’t look good… how can the chromosome be rescued by the spindle to complete normal division?

An ensheathed chromosome. Chromosome is red, membranes are purple.

The answer was that it can’t be rescued by the spindle. These ensheathed chromosomes don’t get shared properly to the two new cells. In fact they tend to form a micronucleus in one of the two new cells.

This part of the project was what we call correlative. We could observe ensheathing and correlate that with a bad outcome in cells. There is a saying “correlation is not causation” and we wanted to test if the ensheathing causes the bad outcome. To do this we came up with a new approach.

“Surgery” inside living cells

We invented a way to remove the membranes from chromosomes that were ensheathed. This is a bit like a surgery inside a living cell! With surgery, if a patient has a problem – say a blockage in an artery – the surgeon can remove the blockage and the blood can flow freely once more. Here we remove the membranes. If they are the cause of the problem, then the chromosome should be rescued by the spindle and shared normally. This is exactly what we found.

The details of the procedure are a bit technical. A brief description is that we can induce the membranes to stick to the cell’s plasma membrane (boundary of the cell). This works remarkably well and it has the potential to be used for many different things.

Removing membranes from ensheathed chromosomes. This diagram shows how we did it. Full details are in the paper by Ferrandiz et al. 2022

These experiments showed that chromosome ensheathing is a risk factor for aneuploidy and micronuclei formation. Full details are in the paper which is free to read.

Edit: Lisa Donker and Susana Godinho have written a very nice Spotlight article about our work.

The people

The work was led by ace researcher Nuria Ferrandiz in my group. She did the majority of the experiments and analysis. Laura Downie, a PhD student in the lab, contributed to the analysis of micronuclei formation. George Starling did some really early work on the project by segmenting 3D EM datasets.

We’ve been working on this topic since 2016. The project has seen some ups and downs. A maternity leave, a hard drive failure and a pandemic all slowed us down a little but we managed to post the work on bioRxiv in April 2021. The preprint was well received and picked up a few citations while it was undergoing peer review. It is now out in final form in J Cell Biol. Many thanks to our funder Cancer Research UK for supporting this work.

The code

The code can be found here. There is quite a lot of functionality in the code that was not even used in the paper in the end. I might write about that at some point!

The post title comes from “Alignment Zone” by The Oscillation taken from the first Psych Against Cancer compilation.

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