How a crucial DNA repair protein works—and what it means for cancer treatment

Good news! Cancer is history (soon)!

"... Researchers have long struggled to understand how cancer cells hijack one of these proteins—called polymerase theta (Pol-theta)—for their own survival. But scientists ... have now captured the first detailed images of Pol-theta in action, revealing the molecular processes responsible for a range of cancers. 

The findings ... illuminate how Pol-theta undergoes a major structural rearrangement when it binds to broken DNA strands. By unveiling Pol-theta’s DNA-bound structure—its active state—the study provides a blueprint for designing more effective cancer drugs. ...

Cells normally use highly accurate mechanisms to fix these breaks, but some cancers—particularly those arising from BRCA1 or BRCA2 mutations, such as certain breast and ovarian cancers—lack this function. Instead, they depend on a more error-prone method, controlled by Pol-theta. ...

Prior research has shown that Pol-theta exists in two forms: a tetramer (four copies of the enzyme) and a dimer (two copies). But why or how Pol-theta changed between these forms was unknown.

Before this study, Pol-theta’s structure had only been captured in an inactive state, leaving a major knowledge gap regarding how the enzyme interacts with DNA. ...

Using cryo-electron microscopy and biochemical experiments, the team made a surprising discovery while capturing Pol-theta in the act of repairing DNA: Whenever Pol-theta bound to broken strands, it consistently switched from the tetrameric to a never-before-seen dimeric configuration.

Once in its active state, Pol-theta repairs DNA using a two-step process: First, the enzyme searches for small matching sequences called “microhomologies” on broken strands.

Once a matching sequence is found, Pol-theta holds the broken DNA strands together so that they can be stitched together—without needing extra energy. Most enzymes require an energy boost to function, but Pol-theta relies on the natural attraction between matching DNA sequences, allowing them to snap into place on their own. ..."

From the abstract:

"DNA double-strand breaks occur daily in all human cells and must be repaired with high fidelity to minimize genomic instability. Deficiencies in high-fidelity DNA repair by homologous recombination lead to dependence on DNA polymerase θ, which identifies DNA microhomologies in 3′ single-stranded DNA overhangs and anneals them to initiate error-prone double-strand break repair. The resulting genomic instability is associated with numerous cancers, thereby making this polymerase an attractive therapeutic target. However, despite the biomedical importance of polymerase θ, the molecular details of how it initiates DNA break repair remain unclear.

Here, we present cryo-electron microscopy structures of the polymerase θ helicase domain bound to microhomology-containing DNA, revealing DNA-induced rearrangements of the helicase that enable DNA repair. Our structures show that DNA-bound helicase dimers facilitate a microhomology search that positions 3′ single-stranded DNA ends in proximity to align complementary bases and anneal DNA microhomology. We characterize the molecular determinants that enable the helicase domain of polymerase θ to identify and pair DNA microhomologies to initiate mutagenic DNA repair, thereby providing insight into potentially targetable interactions for therapeutic interventions."

How a crucial DNA repair protein works—and what it means for cancer treatment | Scripps Research "New structural blueprint is key for better targeting cancer cells, particularly those with BRCA1 and 2 mutations."

Human polymerase θ helicase positions DNA microhomologies for double-strand break repair (no public access)

The Pol-theta enzyme (blue) joins two parts of a broken DNA strand (yellow). This process is mutagenic and can give rise to cancer.