Special Stem Cells With Single Sets of Chromosomes
Diana - Also in the news this week, researchers here in Cambridge have created mammalian stem cells that only contain a single set of chromosomes. Most mammalian cells are diploid - they contain two copies of each chromosome. This is a complication for cell biologists and geneticists, hoping to study the function of individual genes. So joining us now to discuss this work is Dr. Anton Wutz. Hello.
Anton - Good evening and thank you for having me.
Diana - You're very welcome. Good to have you here. Can you start off with why would haploid mammalian cells be useful?
Anton - As you just said, basically, normal animal cells are diploid. Meaning, they have two chromosomes - one from the father and one from the mother. So basically, for each gene, there are two copies present in a cell. The genome contains all the information that is needed for the organism to develop and scientists have already obtained the sequence of all of those genes. However, we have to still figure out how these genes interact and what their overall contribution to development is. In that sense, what has proven very fruitful is just to look at what happens if you lose the function of a particular gene and look for what effect it has on development. If you're now trying to mutate genes in a diploid cell, it's very hard to hit both copies of the very same gene and for this reason, it's very hard to determine what the loss of this gene has as a consequence for the cell. In haploid cells, you have only a single chromosome set and hence, if you introduce a particular mutation, automatically, gene function is ablated and you can study the resulting effect.
Diana - I see. So, rather than having pairs, just by having this one chromosome with the gene that you want to study, it cuts out that extra factor of uncertainty.
Anton - Yeah. Basically, the genome has sort of a backup copy and that is lacking in the haploid case.
Diana - I see. So how have you gone about creating these haploid cells?
(c) Courtesy: National Human Genome Research Institute
' alt='23 pairs of human chromosomes' >Anton - It has been long known that in mammals you can activate the egg cells or oocyte and trick it into thinking it is fertilised without actually supplying a paternal genome via the sperm which would be introduced during normal fertilisation. So, by chemical manipulation, you can activate an egg cell and it will divide, and form an embryo with just the maternal chromosome set. And we have taken these embryos now and removed a small cell clump from the blastocyst stage and brought these into culture conditions which have been highly optimised over recent years by a number of groups. This has allowed us to induce proliferation into these haploid embryonic cells in culture and allow us to maintain a permanently proliferative, so growing cell line, in culture.
Diana - Have you put them into anything living?
Anton - The cell type we cultivated is referred to as an embryonic stem cell and conventional diploid embryonic stem cells have the ability to form all cell types of the embryo - of the mouse embryo - so we were very interested in what actually is the potential of a haploid embryonic stem cell. So we have introduced our stem cells back into the blastocysts of mice and looked if they can contribute to development and to a large degree, they do. So they can contribute to multiple organs and form different cell types in the embryo. However, we noticed that when they enter development and differentiate into functional cell types, they diploidise. So the genome content becomes more normal.
Diana - They end up with pairs of chromosomes rather than the single ones. Why do you think they revert back to that state?
Anton - We're not particularly clear on this but one thought is that in mammals normally, in female mammals, one of the two X chromosomes becomes inactivated and that's to compensate for the dosage to the male genome equivalent which has only a single X-chromosome but also a Y-chromosome. So one idea is that these cells would not be balanced so the normal developmental program is optimised for one active X-chromosome and two sets of autosomes. In our haploid case, we have a single X-chromosome as opposed to a single set of autosomes. So the X-chromosome dosage is too high by a factor of two. And we think that by duplicating the maternal chromosome set, these cells can now inactivate one of the two X-chromosomes again and so have a more normal gene expression pattern for development.
Diana - I see. So you can actually get cells acting sort of fairly normally even within this haploid state, but could this actually shed light on another area of genetics, on epigenetics?
Anton - Indeed, the possibility with the haploid would be now to investigate different pathways. This can range from cell signalling and metabolic pathways, but also of course, the interest in my group is geared towards epigenetic pathways, gene regulatory pathways that act in development. And I think we can tweak those cells by deriving suitable reporter constructs into situations so that they can select for epigenetic mutations and study how these processes are regulated particularly in mammals.
Diana - So this could really open a whole new field up in genetics. That's fantastic! Well thanks, Anton. That's Anton Wutz from Cambridge University and that work was published in the journal Nature this week.