Double Trouble at Chromosome Ends
Published:15 May2024 Source:Rockefeller University
Half a century ago, scientists Jim Watson and Alexey Olovnikov independently realized that there was a problem with how our DNA gets copied. A quirk of linear DNA replication dictated that telomeres that protect the ends of chromosomes should have been growing shorter with each round of replication, a phenomenon known as the end-replication problem. But a solution was forthcoming: Liz Blackburn and Carol Greider discovered telomerase, an enzyme that adds the telomeric repeats to the ends of chromosomes. Now, new research published in Nature suggests that there are two end-replication problems, not one. Further, telomerase is only part of the solution -- cells also use the CST-Polα-primase complex, which has been extensively studied in de Lange's laboratory.
Since the description of the DNA double helix, it is known that DNA has two complementary strands running in opposite directions -- one from 5' to 3'; the other from 3' to 5'. When DNA is replicated, the two strands are separated by the replication machinery, also called the replisome. The process is fairly direct until the ends of the chromosomes. The end-replication problem arises because leading strand synthesis fails to reproduce the last part of the telomere, leaving a blunt leading-end telomere without it characteristic and crucial 3' overhang. Telomerase solves this problem by adding single-stranded TTAGGG repeats to the telomere end. However, Takai's work suggested that the end-replication problem was twice as serious as previously thought, impacting both strands of DNA. The results of Yeeles' in vitro replication experiments were very clear. The replisome does not generate Okazaki fragments on the 3' overhang; it actually stops lagging-strand synthesis long before the leading strand reaches the 5' end. This second end-replication problem means that both strands of DNA will shorten with each division. Telomerase was only preventing this from happening at the leading strand and Hiro's data suggested that CST-Polα-primase fixed the second end-replication problem, that of the lagging strand.
Takai spent the next four years designing new assays to confirm Yeeles' findings in vivo. He was able to measure how much DNA is lost due to the lagging-strand end-replication problem, revealing how many CCCAAT repeats need to be added by CST-Polα-primase to keep telomeres intact. The results change our understanding of telomere biology -- requiring revision of the textbooks. But the findings may also have clinical implications. Individuals who inherit mutations in CST-Polα-primase suffer from telomere disorders, such as Coats plus syndrome, which is characterized by an eye disorder and abnormalities in the brain, bones, and GI tract. Through a better understanding of how we maintain our telomeres, strides could one day be made in addressing these devastating disorders.