A zebrafish model of dyskeratosis congenita reveals hematopoietic stem cell formation failure resulting from ribosomal protein-mediated p53 stabilization

Abstract

Dyskeratosis congenita (DC) is a bone marrow failure disorder characterized by shortened telomeres, defective stem cell maintenance, and highly heterogeneous phenotypes affecting predominantly tissues that require high rates of turnover. Here we present a mutant zebrafish line with decreased expression of nop10, one of the known H/ACA RNP complex genes with mutations linked to DC. We demonstrate that this nop10 loss results in 18S rRNA processing defects and collapse of the small ribosomal subunit, coupled to stabilization of the p53 tumor suppressor protein through small ribosomal proteins binding to Mdm2. These mutants also display a hematopoietic stem cell deficiency that is reversible on loss of p53 function. However, we detect no changes in telomere length in IntroductionDyskeratosis congenita (DC) is a congenital disease that in one comprehensive population-based study was found to represent up to 5% of all bone marrow failure disorders. 1 Patients with this disease typically present in the first decade of life with a classic triad of phenotypes, including nail dystrophy, oral leukopathies, and hyperpigmentation of the skin. 2 More than 90% of all DC patients will experience cytopenia before the age of 20. 3 Other phenotypes reminiscent of aging may develop later in life, including premature graying of the hair, alopecia, taurodant teeth, and osteoporosis. [4][5][6] Although the majority of DC patients die from bone marrow failure, an increased risk of malignancy also contributes to DC mortality. 7 The genotypes of DC vary as widely as the phenotypes and encompass genes that are, with one exception, involved in the H/ACA RNP complex and/or telomere maintenance. Dyskerin (DKC1) was the first H/ACA complex gene linked to DC, the X-linked form of which has been implicated in the more severe cases of DC. 8 DKC1 mutations have also been linked to HoyeraalHreidarsson syndrome, 9 which is now considered to be an extreme form of DC with an earlier onset and a shorter life expectancy. Mutations in 2 other H/ACA RNP complex genes, NHP2 5 and NOP10, 10 have also been linked to DC patients. Two subunits of the telomerase complex, telomerase reverse transcriptase (TERT) and the telomerase RNA component (TERC), together function in the de novo synthesis of telomere ends, 11 and mutations in each component have been described in DC patients, although in general the phenotypes of these patients are relatively milder than those linked to DKC1 mutations. [12][13][14] Other mutations in genes coding for proteins that either protect telomere ends or assist in the trafficking of telomerase to Cajal bodies, TINF2 and TCAB1, respectively, have also been identified. 15,16 One protein of unknown function, C16orf57, has also recently been linked to DC, and these are the only patients that do not display shortened telomere lengths. 17 Importantly, although most of the genotypes of DC identified to date have been single amino acid substitutions, it has recently been shown that a decrease in expression of DKC1, in lieu of any gene mu...

Poly(A)-specific ribonuclease deficiency impacts telomere biology and causes dyskeratosis congenita

“…Vertebrate models of DC (29,35,36) and studies in human DC (4,5,28) have reported increased levels of p53 both in the steady state and upon DNA damage. It is also known that p53 activity regulates hematopoietic stem cell quiescence and self-renewal (37).…”

Ribosome biogenesis dysfunction leads to p53-mediated apoptosis and goblet cell differentiation of mouse intestinal stem/progenitor cells

“…9,10 Studies on cellular and animal models suggest that unscheduled upregulation of p53 may account for many clinical symptoms associated with ribosomopathies. [11][12][13][14][15][16] There are now evidences that ribosome biogenesis dysfunction also triggers p53-independent mechanisms. [17][18][19] Because bone marrow defects is a frequent clinical manifestation of ribosomopathies, most studies focused on the hematopoietic tissue and less is known about the impact of ribosome biogenesis dysfunction in other organs.…”

RNA Processing and Ribosome Biogenesis in Bone Marrow Failure Disorders

“…It has always been debated as to whether DC is a disease related to ribosome biogenesis or loss of telomere maintenance. While much of the phenotype likely does result from lack of telomere maintenance, the data presented by Zhang et al and Pereboom et al, demonstrated that the associated anemia and hematopoietic failure results primarily from impaired rRNA processing and ribosome biogenesis and the subsequent stabilization of p53, common features also seen in DBA and SDS [23,24] .…”

“…All these proteins are involved in telomere maintenance, while several are also directly involved in small ribonuclear RNA processing, resulting in pseudouridylation of the snRNAs. It is still not clear as to whether alterations in the telomerase complex itself is the underlying cause of DC or whether telomere shortening in DC patients also has a major component linked to processing of RNAs required for the telomerase complex [23,24] . As might be expected tissues with the greatest proliferative/turn-over rates are the most affected by DC.…”

“…The H/ACA snoRNP protein components are also required for the correct processing and trafficking of TERC, the RNA component of the telomerase reverse transcriptase holoenzyme (TERT). Interestingly, in a zebrafish model, Pereboom, et al, demonstrate that NOP10 loss can give rise in the cytopenia phenotype in DC through the destabilization of the 40S ribosomal subunit as a result of 18S rRNA defects resulting in the cooperation of RPS7 with MDM2 and the stabilization of p53 in HSCs [24] . This hematopoietic failure could be rescued by the loss of functional p53.…”