Scientists have discovered numerous DNA repair systems already. ---They are constantly at work in the nucleus to prevent mutations. These include double-strand break repair (DSB), base excision repair (BER), nucleotide excision repair (NER), mismatch base repair (MMR), homologous recombination repair (HR), and non-homologous end-joining repair (NHEJ).spontaneous changes in DNA are temporary because they are immediately corrected by a set of processes that are collectively called DNA repair. Of the thousands of random changes created every day in the DNA of a human cell by heat, metabolic accidents, radiation of various sorts, and exposure to substances in the environment, only a few accumulate as mutations in the DNA sequence. We now know that fewer than one in 1000 accidental base changes in DNA results in a permanent mutation; the rest are eliminated with remarkable efficiency by DNA repair.
---The 53BP1 protein is a comparatively large protein that determines how cells will repair a particular type of DNA damage — the dangerous DNA double-strand break (DSB). This is when the two strands of DNA are both broken, leaving a free DNA end floating around in the cell’s nucleus. These DNA ends could inappropriately fuse, thus leading to the disruption of genetic information. Normally, cells with unrepaired DSBs self-destruct by apoptosis but, if not repaired, may begin the journey toward developing into disease, such as microcephaly with chorioretinopathy and some types of cancer.
As Chatterjee and Walker explained in 2017, “DNA repair and damage-bypass mechanisms faithfully protect the DNA by either removing or tolerating the damage to ensure an overall survival.”
In the new discovery, researchers Zhang et al. found that 53BP1 is involved in protein accumulation at condensed DNA regions in the
Human 53BP1 is primarily known as a key player in regulating DNA double strand break (DSB) repair choice; however, its involvement in other biological process is less well understood. Here, we report a previously uncharacterized function of 53BP1 at heterochromatin, where it undergoes liquid-liquid phase separation (LLPS) with the heterochromatin protein HP1α in a mutually dependent manner. Deletion of 53BP1 results in a reduction in heterochromatin centers and the de-repression of heterochromatic tandem repetitive DNA. We identify domains and residues of 53BP1 required for its LLPS, which overlap with, but are distinct from, those involved in DSB repair. Further, 53BP1 mutants deficient in DSB repair, but proficient in LLPS, rescue heterochromatin de-repression and protect cells from stress-induced DNA damage and senescence. Our study suggests that in addition to DSB repair modulation, 53BP1 contributes to the maintenance of heterochromatin integrity and genome stability through LLPS.
The result is 53BP1 stabilizes these proteins at these DNA regions, which is important for maintaining the structure, and therefore the proper function, of DNA. The authors call this “an unexpected, yet important, role of 53BP1 in maintaining heterochromatin structure and function, and consequently genome stability….” In summary, this discovery identifies a new and vital role of this protein, which works in partnership with systems of molecules that keep DNA in good working order.
Overall, we believe that our studies have revealed a previously uncharacterized layer of regulation of 53BP1 in genome stability maintenance, which is different from its canonical role in DSB repair. Nevertheless, together with recent publications, they introduced the LLPS concept to 53BP1, broadening our understanding about 53BP1’s biological function.