dyskerin-dimerization-telomerase-rna-binding

Dyskerin, the protein required for telomerase RNA assembly, forms a homodimer on its own. The mutations that cause dyskeratosis congenita, a childhood disease of accelerated tissue aging, leave this homodimer intact and instead directly disrupt dyskerin's binding to telomerase RNA.

On September 16, a team from McGill University published in the journal RNA the first direct biochemical evidence that dyskerin homodimerizes independently of RNA. The authors engineered a cell line with fully and reversibly depletable dyskerin and dissected the precise effects of four X-linked dyskeratosis congenita mutations.

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 step: without it, hTR degrades before it can become a functional molecule. When the dyskerin gene is mutated, children develop X-linked dyskeratosis congenita, one of the few proven links in human aging between deficient telomerase and tissue destruction: the disease destroys bone marrow, causes pulmonary fibrosis, damages the skin, and raises cancer risk.

Detailed structural images of telomerase had already revealed two dyskerin molecules making contact precisely where dyskeratosis mutations cluster most frequently, but direct proof of this contact was lacking. In 2019, the same laboratory showed that several of these mutations weaken binding to hTR. In those experiments, however, normal dyskerin remained alongside the mutant protein, and it was unclear whether the mutations disrupted the RNA interaction directly or indirectly, by destabilizing the contact between the two dyskerin molecules themselves.

Answering this question required removing background dyskerin completely and reversibly. A 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. This created a switch: endogenous dyskerin could be eliminated entirely, and exactly one variant of the protein could be reintroduced in its place.

In this cell line, dyskerin dimerized with itself independently of RNA, and all four mutations left this dimer intact. Their binding to hTR, however, was notably weaker, even though the mutant proteins localized correctly in the cell and held on to the other components of the complex just as tightly. The defect proved specific to the contact with telomerase RNA.

The first attempt to catch this defect showed nothing: a stable experimental version of the RNA gave the binding reaction too much time, and even mutant dyskerin captured it no worse than wild type. Jian Qin, the first author, 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 so that the mutant had less time to retain the RNA before it degraded, and the difference became apparent.

This work narrows the cause of dyskeratosis congenita to a single specific defect: the disrupted contact between dyskerin and telomerase RNA. For related telomerase pathway diseases in adults, pulmonary fibrosis and aplastic anemia caused by mutations in the same genes, this provides a precise, testable mechanism in place of a structural hypothesis. Efforts to repair such critically short telomeres are already underway: patients receive transient activation of the ZSCAN4 gene in hematopoietic stem cells, and in a first small trial, hematopoiesis improved in both participants.