In short
When the body's natural DNA correction system fails, the consequences can range from increased cancer risk to accelerated aging. Mutations in genes responsible for DNA mismatch repair disrupt one of the cell's most essential quality control mechanisms.
What is DNA mismatch repair?
DNA mismatch repair (MMR) is a highly conserved biological pathway that plays a key role in maintaining the stability of our genetic material. This system acts as a quality control mechanism, recognizing and repairing errors that occur during DNA replication and recombination. The specificity of MMR is primarily for base-base mismatches and insertion/deletion mispairs that are generated when DNA copies itself.
During DNA replication, cells copy approximately 3 billion base pairs with remarkable accuracy. However, even with this precision, errors occur at a frequency of roughly 1 in 10 million to 10 billion base pairs per cell division. The mismatch repair pathway targets errors that escape the proofreading function of DNA polymerases, contributing an additional 50 to 1000-fold improvement to the overall fidelity of replication.
The repair process involves several key protein complexes. In human cells, the essential components include MutSα (a protein complex made of MSH2 and MSH6), MutSβ (made of MSH2 and MSH3), and MutLα (made of MLH1 and PMS2), along with several other proteins that participate in the actual repair work. Examples of mismatched bases that this system corrects include a G/T or A/C pairing, which are commonly due to chemical changes in bases during DNA replication.
How mutations affect the repair system
When genes encoding mismatch repair proteins carry mutations, the entire DNA quality control system becomes compromised. Loss of MMR function results in greatly increased rates of spontaneous mutation in organisms ranging from bacteria to humans. This condition is referred to as a mutator phenotype, where cells accumulate genetic errors at an accelerated rate.
The inactivation of MMR can occur through direct mutations in the repair genes or through epigenetic silencing, which means the genes are turned off by chemical modifications rather than changes to the DNA sequence itself. Either mechanism leads to the same result: a breakdown in the cell's ability to maintain genetic accuracy.
Different MMR protein complexes recognize different types of errors. MutSα and MutSβ both participate in recognizing mismatches, but they can be directed toward either repair of the mismatch or an alternative process called anti-recombination, depending on various factors including DNA structure and the relative amounts of repair proteins present.
Connection to cancer
Mutations in MMR genes cause hereditary nonpolyposis colorectal cancer, and loss of MMR function is associated with a significant fraction of sporadic cancers. Defects in MMR are associated with genome-wide instability, predisposition to certain types of cancer, resistance to certain chemotherapeutic agents, and abnormalities in reproduction and fertility in mammalian systems.
The connection between MMR deficiency and cancer development is straightforward: without proper DNA repair, cells accumulate mutations much more rapidly than normal. Over time, these mutations can affect genes that control cell growth, division, and death, eventually leading to cancer formation. The loss of MMR can occur through either inherited genetic defects or through changes acquired during a person's lifetime.
Microsatellite instability: A diagnostic marker
A hallmark of many MMR-deficient cells is instability at microsatellite regions, which are stretches of DNA consisting of repeated sequences of one or two nucleotides. During replication through these repetitive regions, the DNA copying machinery can slip, creating insertion or deletion errors. Normally, MMR corrects these errors, but when MMR is deficient, these mistakes accumulate.
This phenomenon, called microsatellite instability (MSI), is widely used as a diagnostic marker for loss of MMR activity in tumor cells. Most insertions and deletions involve A or T nucleotides and occur preferentially in stretches of repeated A or T bases. The likelihood of these errors in repetitive stretches is strongly related to the length of the repeat, which can cause extremely high localized mutation rates in specific regions.
Implications for immunotherapy
An important development in cancer treatment involves the relationship between MMR deficiency and response to immunotherapy. Patients with defective MMR (dMMR) tumors can benefit from immune checkpoint inhibitor therapy, particularly treatments targeting PD-1/PDL1.
The mechanism behind this benefit relates to the high mutation rate in MMR-deficient tumors. These mutations generate abnormal proteins called neoantigens that the immune system can recognize as foreign. Additionally, DNA from MMR-deficient cells can activate immune signaling pathways, leading to increased infiltration of immune cells into tumors.
However, approximately 50% of MMR-deficient tumors eventually do not respond to immune checkpoint inhibitor therapy. Researchers continue to investigate why some tumors become resistant and how to overcome this resistance.
Impact on mutation rates
The effect of MMR deficiency on mutation frequency is substantial. MMR deficiency greatly increases the frequency of both smaller insertions and deletions, as well as single-nucleotide changes throughout the genome. MMR deficiency not only increases mutation frequency but also changes the molecular spectrum of mutations, affecting which types of genetic errors occur most often.
Given the prominence of MMR in mutation avoidance and its ability to target a range of DNA lesions, this repair system has been under investigation in studies of aging mechanisms. The accumulation of mutations over time may contribute to age-related decline in cellular function and increase susceptibility to age-related diseases.
