yeast buds before dividing
Credit: Carolyn Larabell
Research conducted in yeast suggests ribosomopathies are caused by a sequence of mistakes at the molecular level.
First, a genetic mutation prompts the production of defective ribosomes.
Then, a quality-control system eliminates most of these faulty ribosomes. This leaves few available for cells to produce required proteins, which causes anemia and bone marrow failure.
Next, a second mutation suppresses the quality-control system, making more ribosomes available to cells. However, these ribosomes are defective and cause changes in gene expression patterns that can result in cancer.
Jonathan Dinman, PhD, of the University of Maryland, and his colleagues described this chain of events in Proceedings of the National Academy of Sciences.
The researchers set out to investigate the structural, biochemical, and other defects in ribosomes that may lead to cancer. They selected budding yeast as their model system, as the assembly of its ribosomes shares many characteristics with human cells.
The team used the rpL10-R98S (uL16-R98S) mutant yeast model of the most commonly identified ribosomal mutation in T-cell acute lymphoblastic leukemia (T-ALL). They showed that the rpl10-R98S mutation causes a late-stage 60S subunit maturation failure that targets mutant ribosomes for degradation (the quality-control system).
When the researchers grew the mutant yeast cells on a petri dish, the cells grew very slowly. The team suggested that, because of the cells’ quality-control system, the majority of defective ribosomes carrying the T-ALL mutation do not “pass inspection.”
This severely limits the supply of ribosomes available to produce proteins, only providing enough ribosomes for cells to barely survive. This supply-and-demand problem hits rapidly dividing cells like blood cells particularly hard, and can therefore cause anemia and bone marrow failure in humans.
The bone marrow cells are subjected to selective pressure, an evolutionary process that favors the reproduction of things that resolve problems limiting their ability to thrive. In this case, cells would be favored that could circumvent the rpl10-R98S mutation.
After a few weeks, a group of fast-growing cells appeared on the petri dish containing the rpl10-R98S mutant yeast cells. The researchers sequenced the genomes of these cells and found a mutation in a second gene, NMD3, which suppresses the growth and ribosome biogenesis defects of rpl10-R98S cells.
So the mutation, NMD3-Y379D, increased the total number of ribosomes available to the cells, enabling cells with the mutation to make more protein, grow quickly, and take over the population. However, the available ribosomes were still defective.
NMD3-Y379D did not suppress the structural, biochemical, and translational fidelity defects of rpL10-R98S ribosomes. And the translational defects affected telomere maintenance. The mutant cells exhibited shortened telomeres, which have been linked to cancer.
The researchers proposed 2 different, but not mutually exclusive, explanations for these effects. The rpL10-R98S ribosomes could be directly changing patterns of gene expression and promoting T-ALL, and/or NMD3-Y379D could be driving T-ALL.
“Our yeast work has established a new paradigm that we are now translating to humans,” Dr Dinman said. “Once we determine which ribosomal mutations suppress the quality-control system in humans, we may be able to identify a potential drug target.”
