July 02, 2018
But in fact everyone’s genome CAN be thought of as a book, sort of like a giant cookbook. Within your genome are many “recipes” for gene products — DNA sequences that each give instructions on how, when, and where to make a protein or RNA molecule. The recipes of the genome are used to “cook up” a person!
When a parent cell divides and creates another cell, it makes a copy of its genome “cookbook” and passes it down to the new cell. It’s extremely important to make sure that there aren’t any errors in the newly copied text, as mistakes in recipes for crucial enzymes can have disastrous results for the cell. Indeed, our cells (as well as yeast cells, and in fact, all organisms) have enzymes that are dedicated to scrupulously inspecting the new copies of our genome cookbooks that are made during cell division, and then helping to correct any errors.
A certain class of these enzymes are known as DNA mismatch repair (MMR) proteins. These enzymes carefully proofread the newly made copy of the genome, and if something doesn’t match the original version, they will help fix the error.
You might think that having more of these MMR proteins would be a good thing, because there would be more thorough checking and repairing of the new copy of the genome. But the situation isn’t so simple, especially when it comes to human cancer.
Previous studies show that some cancers and cancer predisposition syndromes have less of the MMR proteins. This makes sense because cancers almost always arise due to genome mutations, and it is known that if a cell has less of the error-checking MMR proteins, there are more genome mutations. However, other studies report a puzzling discrepancy: in some cancers, there are in fact MORE of the MMR proteins rather than less!
In their recent GENETICS study, Chakraborty and coworkers decided to investigate this discrepancy further. They analyzed data from two databases of human cancer genome information, The Cancer Genome Atlas (TCGA) and the cBioPortal for Cancer Genomics, specifically looking at how much the genes encoding MMR proteins were turned on or off in cancer.
They observed that many types of human cancer cells make more of two particular MMR proteins: MSH2 and MSH6. It turns out (as it often does!) that our good buddy Saccharomyces cerevisiae has MMR proteins very similar to the human ones, also encoded by genes called MSH2 and MSH6. So Chakraborty and coworkers made yeast cells that overexpress the yeast genes MSH2 and MSH6 and used the awesome power of yeast genetics (and genomics) (#APOYG!) to investigate whether this might lead to cellular and genome changes like those seen in human cancers.
Using various yeast genetic assays to measure rates of genome alterations such as homologous recombination, mutations, and loss of large chromosome regions, Chakraborty et al. observed increased rates for all of these measures of genome instability in cells when both MSH2 and MSH6 were overexpressed, but not for overexpression of either one alone (or of other MMR proteins). So even though there are more of the “good guy” MMR proteins in these cells, this actually ends up making the genome of the cell MORE likely to get damaging mutations of the same types seen in cancer cells.
So why is too much of a good thing such a bad thing in this case? The authors hypothesize that both Msh2p and Msh6p act together as a joined pair to go to the spot where the chromosomal DNA is actively being copied (“replicated”). When they are both overexpressed, too many of the Msh2p-Msh6p pairs can go to the replication spot and actually interfere with the copying process.
It’s as if you were a medieval scribe carefully copying an illuminated manuscript of a genome cookbook, and instead of one supervisor occasionally checking your work, there are a bunch of people constantly looking over your shoulder and maybe even bumping into your arm, causing you to make mistakes in your writing. They may even make you drop the book you’re copying, scattering pages so that you might leave some out or put them back in the wrong order, seriously messing up your work!
Here’s hoping our cells don’t overdo it with their MMR proteins, so that they can be careful with their cookbook copying job and do it “write”!
by Barbara Dunn, Ph.D. and Kevin MacPherson, M.S.
Categories: Research Spotlight