Dyskerin, required for telomerase RNA assembly, self-dimerizes, and mutations behind dyskeratosis congenita, a childhood disease of accelerated tissue aging, leave this dimer intact while directly disrupting dyskerin's contact with telomerase RNA
Dyskerin, required for telomerase RNA assembly, self-dimerizes, and mutations behind dyskeratosis congenita, a childhood disease of accelerated tissue aging, leave this dimer intact while directly disrupting dyskerin's contact with telomerase RNA
On September 16, a group from McGill University published in the journal RNA the first direct biochemical evidence that dyskerin self-dimerizes independently of RNA. The authors engineered a cell line with fully and reversibly depletable dyskerin and dissected precisely what four X-linked dyskeratosis congenita mutations disrupt.
Each cell division shortens the ends of chromosomes, the telomeres. When telomeres become critically short, the cell stops dividing: Leonard Hayflick described this limit in 1961. The enzyme telomerase can extend telomeres, and Elizabeth Blackburn, Carol Greider, and Jack Szostak received the 2009 Nobel Prize in Physiology or Medicine for elucidating this mechanism.
For telomerase to function, the cell must assemble its RNA component, hTR. The protein dyskerin is responsible for this: without it, hTR is degraded before it can become a functional molecule. When the dyskerin gene is mutated, children develop X-linked dyskeratosis congenita, one of the few nodes of aging with a human-proven link between deficient telomerase and tissue destruction: it devastates bone marrow, causes pulmonary fibrosis, damages skin, and raises cancer risk.
Detailed structural images of telomerase had already revealed two dyskerin molecules making contact at the very region where dyskeratosis mutations cluster, but direct proof of this contact was lacking. In 2019, the same laboratory showed that several of these mutations weaken binding to hTR, but in those experiments wild-type dyskerin remained alongside the mutant protein, and it was unclear whether the mutations disrupt the RNA contact directly or indirectly, by destroying the dyskerin-to-dyskerin interface.
The question could only be resolved by removing endogenous dyskerin completely and reversibly. Knockout is lethal, and RNA interference is either incomplete or fades over time. The authors inserted into the dyskerin gene a fragment of a bacterial enzyme that causes the protein to degrade in the absence of a small molecule in the medium, creating a switch into which exactly one protein variant could be substituted at a time.
In this cell line, dyskerin self-dimerized independently of RNA, and all four mutations left the dimer intact. The mutants did, however, bind hTR substantially worse, even though they localized correctly within the cell and held on to the other complex components just as firmly: the defect proved specific to the telomerase RNA contact.
The first attempt to capture this defect revealed nothing: a stable experimental version of the RNA gave binding too much time, and even mutant dyskerin managed to capture it as efficiently as wild type. First author Jian Qin explained:
We were surprised that the H/ACA domain of hTR bound equally well to wild-type dyskerin and to the disease-causing mutants.
The experiment was redesigned to give the mutant less time to retain the RNA before degradation, and the difference became apparent.
The study narrows the cause of dyskeratosis congenita to a single specific defect: the disrupted contact between dyskerin and telomerase RNA. For the related adult-onset diseases of the telomerase pathway, pulmonary fibrosis and aplastic anemia caused by the same genes, this provides a precise, testable mechanism in place of a structural conjecture. Efforts to repair such shortened telomeres are already underway: in one approach, the ZSCAN4 gene is transiently activated in patients' blood stem cells, and in the first small trial hematopoiesis improved in both participants.