Impaired telomere integrity and rRNA biogenesis in PARN‐deficient patients and knock‐out models

Abstract

PARN, poly(A)‐specific ribonuclease, regulates the turnover of mRNAs and the maturation and stabilization of the h TR RNA component of telomerase. Biallelic PARN mutations were associated with Høyeraal–Hreidarsson (HH) syndrome, a rare telomere biology disorder that, because of its severity, is likely not exclusively due to h TR down‐regulation. Whether PARN deficiency was affecting the expression of telomere‐related genes was still unclear. Using cells from two unrelated HH individuals carrying novel PARN mutations and a human PARN knock‐out (KO) cell line with inducible PARN complementation, we found that PARN deficiency affects both telomere length and stability and down‐regulates the expression of TRF1 , TRF2 , TPP1 , RAP1 , and POT1 shelterin transcripts. Down‐regulation of dyskerin‐encoding DKC1 mRNA was also observed and found to result from p53 activation in PARN‐deficient cells. We further showed that PARN deficiency compromises ribosomal RNA biogenesis in patients' fibroblasts and cells from heterozygous Parn KO mice. Homozygous Parn KO however resulted in early embryonic lethality that was not overcome by p53 KO. Our results refine our knowledge on the pleiotropic cellular consequences of PARN deficiency.

From incomplete penetrance with normal telomere length to severe disease and telomere shortening in a family with monoallelic and biallelic PARN pathogenic variants

“…More recently, it was found that PARN directly trims rRNA precursors and that RNAi depletion of PARN causes their accumulation (Montellese et al, ). In addition, while this manuscript was in revision, Benyelles et al () reported accumulation of 18S pre‐rRNAs in samples derived from HH individuals, as we do here in our proband (Figure d–f), and in a cell line with biallelic PARN knockout. Interestingly, we observed that neither cells with a knockout of a single allele (Figure g) nor individuals heterozygous for pathogenic PARN variants (Figure d–f) had an accumulation of 18S pre‐rRNA, suggesting a dose‐dependent effect.…”

“…Nonetheless, telomeres shortened. It is possible that monoallelic loss of PARN causes telomere shortening through an RNA other than hTR in HCT116 cells, such as that encoding TPP1, as observed in PARN‐deficient HT1080 cells (Benyelles et al, ) or that increased hTR adenylation is sufficient to shorten telomeres.…”

“…Subsequently, patients with biallelic variants in PARN were identified in a cohort of patients with HH (Tummala et al, ). To date, greater than 50 cases with IPF, eight cases with HH, and three cases with DC, have been reported with PARN as the underlying etiology (Alder et al, ; Benyelles et al, ; Burris et al, ; Dhanraj et al, ; Kropski et al, ; Moon et al, ; Newton et al, ; Petrovski et al, ; Tummala et al, ). The phenomenon of monoallelic variants causing adult‐onset IPF and biallelic variants causing a pediatric TBD has also been observed for TERT and RTEL1 (Armanios et al, ; Ballew, Joseph, et al, ; Ballew, Yeager, et al, ; Deng et al, ; Gramatges, Qi, Sasa, Chen, & Bertuch, ; Kannengiesser et al, ; Le Guen et al, ; Marrone et al, ; Stuart et al, ; Tsakiri et al, ; Walne, Vulliamy, Kirwan, Plagnol, & Dokal, ).…”

CD8+ T‐cell senescence and skewed lymphocyte subsets in young Dyskeratosis Congenita patients with PARN and DKC1 mutations

“…This mutation is located in the β‐fold region of PARN (Figure 3A), predicted to lead to the loss of a hydrogen bond in the protein structure and is highly conserved in all 14 species (Figure 3B,F). The pathogenicity of the biallele mutation at this locus has been demonstrated in a Hoyeraal‐Hreidarsson patient reported by Benyelles 23 . Considering P1 also showed the manifestations of Hoyeraal‐Hreidarsson syndrome, we performed direct sequencing of the full PARN coding sequence in P1 and his parents.…”

Full length RTEL1 is required for the elongation of the single-stranded telomeric overhang by telomerase

“…While the investigated patient cells also show genome-wide damage, this damage is apparently tolerable when telomeres are sufficiently long and in itself does not cause a significant cell growth defects. Dyskerin, PARN and TCAB1/WRAP53β are involved in various pathways other than telomerase action, such as rRNA modification and biogenesis, mRNA stability, DDR and scaRNAs trafficking to the Cajal bodies ( 46–48 ). In the context of TBD, however, point mutations in these genes are all associated with the common phenotype of reduced telomerase action.…”