Synthetic Lethality
Educational note: Synthetic lethality is a cornerstone concept of modern targeted oncology, exploiting the tumor's own vulnerabilities against itself.
TL;DR
Synthetic lethality occurs when the loss of Gene A is tolerable for a cell, and the loss of Gene B is tolerable, but the simultaneous loss of both Gene A and Gene B causes the cell to die.
In cancer, if a tumor has naturally lost Gene A (due to a mutation), we can treat the patient with a drug that inhibits Gene B. The tumor cells die (because they lost both A and B), but the patient's normal cells survive (because they still have a functional Gene A).
1. The Classic Example: BRCA and PARP
The poster child for synthetic lethality in oncology is the interaction between BRCA mutations and PARP inhibitors (e.g., Olaparib).
- The Setup: DNA is constantly being damaged. Cells have two main repair crews for different types of damage.
- Crew 1: PARP repairs single-strand breaks.
- Crew 2: BRCA1/2 repairs double-strand breaks via Homologous Recombination (HR).
- In a normal cell: If you inhibit PARP with a drug, the cell gets single-strand breaks. When the cell divides, these turn into double-strand breaks. But the normal cell still has BRCA, so it fixes the double-strand breaks and survives.
- In a BRCA-mutated cancer cell: The tumor cell is missing the BRCA repair crew. It relies entirely on PARP to survive DNA damage. If you give a PARP inhibitor, the tumor cell gets double-strand breaks and has no way to fix them. The cancer cell undergoes apoptosis and dies.
This is why PARP inhibitors are highly effective in ovarian and breast cancers with BRCA1 or BRCA2 mutations.
2. Why is this so powerful?
- Targeting Tumor Suppressors: As discussed in Oncogenes and Tumor Suppressors, it is virtually impossible to use a drug to "restore" a lost tumor suppressor like BRCA or p53. Synthetic lethality allows us to target a loss-of-function mutation by attacking a different, druggable protein (like PARP).
- High Therapeutic Index: Because normal cells retain the wild-type tumor suppressor (Gene A), the drug (targeting Gene B) is significantly less toxic to healthy tissue compared to traditional chemotherapy.
3. The New Vanguard: PRMT5 and MTAP
While PARP/BRCA was the first major breakthrough, the next wave of synthetic lethality targets metabolic and epigenetic vulnerabilities.
- The MTAP Deletion: The gene CDKN2A is one of the most frequently deleted tumor suppressors in all of cancer (especially in pancreatic, lung, and glioblastoma). Right next to CDKN2A on the chromosome is a passenger gene called MTAP. When tumors delete CDKN2A, they almost always accidentally delete MTAP as well.
- The Synthetic Lethal Partner: Cells without MTAP accumulate a toxic metabolite that inhibits an enzyme called PRMT5. These tumor cells become exquisitely sensitive to PRMT5 inhibitors.
- The Result: Drugs targeting PRMT5 are currently in clinical trials specifically for patients whose tumors have MTAP deletions.
4. Finding New Pairs (CRISPR Screens)
How do bioinformaticians and biologists find new synthetic lethal pairs? They use CRISPR-Cas9 knockout screens.
- Take a cancer cell line known to have a specific mutation (Gene A).
- Use CRISPR to systematically knock out every other gene in the genome, one by one (Gene B).
- Observe which knockouts cause the cancer cells to die.
- If knocking out Gene B kills the mutant cell line, but does not kill a healthy cell line, you have discovered a potential synthetic lethal pair.