Cancer Initiation: Checkpoint Loss and Paul Nurse's "Wee Mutants"

Thursday, January 28, 2016

Checkpoint Loss and Paul Nurse's "Wee Mutants"

In previous posts we have referred to "Checkpoint Loss" as a presumed underlying cause of loss of control of the cell cycle. Here, we discuss further the historical science behind the concept of checkpoints and their loss. The first "popular" discussion of checkpoints comes as part of Paul Nurse's Nobel Lecture of 2001.        In it, he refers to the "wee mutants" as those members of his fission yeast culture which divided before they reached full size. He reasoned that that there must be some cellular mechanism that kept cells from progressing through the cell cycle before their "Synthesis" or "S" phase was complete. As such, cells which had a defective mechanism, or checkpoint, would appear smaller under the microscope. These cells could be isolated and geno-typed to find the specific genetic, or DNA based mutation that was responsible.
  Based upon isolating the "Wee Mutants" and development of a genetic toolkit for working with fission yeast, Paul Nurse and his associates decoded the mechanism of cell cycle regulation in Eukaryots.
   Possibly Foremost in this system are the "Cyclins" or those protein products that accumulate as each stage of the cell cycle progresses. When the level of  a specific cyclin in the cell reaches a threshold level, it activates an enzyme known as a cyclin dependent kinase ( CDK ) .  A kinase is an enzyme which phosphorylates, or adds a phosphate group, and these CDKs act to phosphorylate a substrate known as E2F. When E2F has been phosphorylated, it can no longer bind to retinoblastoma (RB, pRB ), and as such it is free to translate to the nucleus where it is a transcription factor, and acts to move the cell through the cell cycle.
  We already have a great cancer research flag flown up here.  The gene retinoblastoma ( pRB ) has already been predicted and described as associated with cancer. The prefix "retinoblast" is a type of precurser cell in the retina of the eye.  A particular type  of cancer then has been characterized as " cancer arisen from the precursor cells of the retina" or "retinoblastoma". Susceptibility to retinoblastoma had been studied as a genetic condition, and lead to the formation of the "two hit" hypothesis of Alfred Knudson.
  The "two hit" hypothesis would have worked like this. When a patient suffers a mutation in the second allele of rb, it fails to function. As such, it can no longer bind to E2F and prevent progression through a checkpoint to the next stage of the cell cycle. This was the foundation for cancer causation for most of modern medical history.
  Nevertheless, with the development of more sophisticated medical technology, it was discovered that in fact, more often than not, the gene sequence of rb is in good shape. What has happened is that its expression has been suppressed by promoter methylation. This promoter methylation is a foundation of what is now called  epigenetics.
   A missing link in the logic here is that fission yeast are single cell biota. As such, they do not suffer cancer. Cancer is a disease of organisms which have some degree of differentiation. This differentiation is increasingly being as a function of DNA methylation. As such, we have proposed a methylation maintenance model to describe the molecular/genetic organization of higher animals.
  As such, our cancer model is currently more sophisticated than Nurses "wee mutant" mode. When a checkpoint is lost, dividing cells fail to complete the duplication of DNA methylation patterns upon division, or mitosis.  This leads to a clinically observable condition known as global hypomethylation which is observed on the molecular level as a particular cancer progresses. As such, not only does DNA sequence mutate as cancer progresses, regulation of expression disintegrates as well.
  On an cellular level, this happens quite often. There are multiple levels of defense. The first is epigenetic suppression of telomerase. A somatic cell can divide a certain number of times until its telomeres expire and all descendants become senescent. 
  The next level of defense involves triggering the immune system. We refer to this as the rattlesnake hypothesis.  In brief, this states that a cell contains a number of "danger flags" known as wingspans antigens. When these epigenetically suppressed genes become expressed, their sole purpose is to trigger the immune system to attack the cell. In the presence of a healthy immune system, this mechanism is sufficient to protect against cancer.
  In summary, although much of what has been known as cancer research has been swept to the side by the advance in molecular analytics, the original concept of the checkpoint, and associated "wee mutant" seems to be one that will stay as a foundation of future cancer models.

No comments:

Post a Comment