We don’t usually think of cancer as a disease related to amyloid or to protein aggregation. In cancer, the control systems that dictate the cell life cycle are lost and cells begin to proliferate out of control. A key element in the control system is the “tumor suppressor” protein, p53. p53 has been described as “the guardian of the genome” because it protects genes from mutation. However, the gene encoding p53 can be mutated itself, leading to formation of a dysfunctional protein, loss of control, and cancer.
An interesting recent discovery is that a sub-group of the mutations lead to protein aggregation, similar to what happens in amyloidoses, such as Alzheimer’s or Parkinson’s diseases. This finding is highly important because p53 acts as a homotetramer (a protein complex made of four identical subunits) and to be functional, each one of the four subunits has to have the correct structure. If just one is mutated, the complex falls apart. Because our genome has two copies of each gene, if a structure-altering mutation occurs in one of the copies and that mutation leads to a dysfunctional protein, 50% of the protein produced will be dysfunctional. But when four intact copies are needed for the protein to function and one copy of the gene is mutated, only 1/16, or 6.25%, of the tetramers will be normal and functional.
The problem with amyloid formation is that the normal protein tends to co-aggregate with the bad one, reducing the percentage of available normal protein even more. And if that is not bad enough, protein “cousins” of p53, called p63 and p73, also co-aggregate with the mutant protein, reducing the cells ability to control its processes even more. Moreover, the aggregates can be transmitted to other cells and induce abnormal aggregation in them, a possible mechanism for the propagation of cancer.
In addition to the loss of control, the rogue protein aggregates themselves are cytotoxic, similarly to protein aggregates in other amyloidoses. This is where the molecular tweezer, CLR01, might help. CLR01 is known to alter the formation of the toxic aggregates and facilitate their degradation by the body’s clearance mechanisms. Can it do that with mutant p53? Facilitate clearance of the mutant protein and possibly release the good protein? To begin to answer these questions, the team of Dr. Danny Segal at Tel Aviv University examined the effect of CLR01 on two prevalent mutant forms of p53 in a series of biophysical and biochemical tests. They found that CLR01 stabilized intermediate-size aggregates of the mutant proteins. Thus, the transition from small oligomers to the intermediate-size aggregates was accelerated, but further aggregation was inhibited when the mutant proteins interacted with CLR01.
These observations led to an important question: are the intermediate aggregates still toxic to cells? When the researchers added the aggregates of mutant p53 itself, without CLR01, to cultured cells, the cells died, but when they added the mixture of mutant p53 with CLR01, the molecular tweezer protected the cells and they survived. These findings suggest that CLR01 may be helpful in cases of cancer caused by p53 mutations because it may help prevent both the formation of toxic protein aggregates and the co-aggregation of healthy protein with the mutated form.
The study was published in the American Chemical Society journal Biochemistry.