Dyskeratosis congenita and the DNA damage response

Abstract

Dyskeratosis congenita (DC) is a heterogeneous bone marrow failure disorder with known mutations in components of telomerase and telomere shelterin. Recent work in a mouse model with a dyskerin mutation has implicated an increased DNA damage response as part of the cellular pathology, while mouse models with Terc and Tert mutations displayed a normal response. To clarify how these contradictory results might apply to DC pathology in humans, we studied the cellular phenotype in primary cells from DC patients of several genetic subtypes, focussing on T lymphocytes to remain close to the haematopoietic system. We observed novel cell cycle abnormalities in conjunction with impaired growth and an increase in apoptosis. Using flow cytometry and confocal microscopy we examined induction of the DNA damage proteins γ-H2AX and 53BP1 and the cell cycle protein TP53 (p53). We found an increase in damage foci at telomeres in lymphocytes and an increase in the basal level of DNA damage in fibroblasts, but crucially no increased response to DNA damaging agents in either cell type. As the response to induced DNA damage was normal and levels of global DNA damage were inconsistent between cell types, DNA damage may contribute differently to the pathology in different tissues.

Defective telomere elongation and hematopoiesis from telomerase-mutant aplastic anemia iPSCs

“…Telomerase mutations are genetic risk factors for apparently acquired aplastic anemia (6), idiopathic pulmonary fibrosis (7,8), liver disorders (9)(10)(11), and acute myeloid leukemia (12,13). The clinical phenotype of individuals within families harboring a given mutation is variable, extending from asymptomatic carriers through mild laboratory findings to severe aplastic anemia (clinically indistinguishable from acquired aplastic anemia and without the accompanying physical examination features of dyskeratosis congenita).…”

Marked overlap of four genetic syndromes with dyskeratosis congenita confounds clinical diagnosis

“…Pathologically, DC is characterized by selective exhaustion of highly proliferative cells 1 that have critically short telomeres and exhibit an abnormal DNA damage response. 2 The Dyskeratosis Congenita Registry (DCR, London, UK) is a collection of patients that have a clinical diagnosis of DC or an overlapping phenotype as defined by Dokal et al 3 The diagnostic inclusion criteria are:…”

“…(2) One mucocutaneous feature plus BM failure and two other somatic features of DC. (3) Aplastic anaemia (AA), myelodysplastic syndrome (MDS) or idiopathic pulmonary fibrosis (IPF) associated with a pathogenic telomerase variant.…”

The Pathways of Double-Strand Break Repair

“…With its molecular weight of about 470 kDa, DNA-PKcs is by far the largest enzyme found to operate in D-NHEJ (Hill & Lee, 2010;Kirwan et al, 2011;Weterings & Chen, 2007). The enormous size of DNA-PKcs accommodates many important domains that may be involved in the regulation of its enzymatic activity and the interaction with other proteins (Fig.…”

“…DNA-PKcs autophosphorylation appears to be important for DSB repair as DNA-PKcs mutated at key phosphorylation sites (T2609 and S2056 at ABCDE and PQR clusters respectively) is impaired in its function in D-NHEJ (Cui et al, 2005;Meek et al, 2007). Elegant experiments demonstrate that DNA-PKcs autophosphorylation facilitates structural shifts, which allow other D-NHEJ end processing or ligation factors (polynucleotide kinase phosphatase, PNKP, terminal deoxynucleotidyl transferase, TDT, DNA polymerases  and , LigIV/XRCC4/XLF complex) to be recruited to DNA ends (Kirwan et al, 2011). After end processing, two locally available DNA ends are joined through the coordinated action of the LigIV/XRCC4/XLF and the DNA-PK complexes and if the two sealed DNA ends originate from one DSB the integrity of the DNA molecule is restored (Ahnesorg et al, 2006;Wu et al, 2007;Yano et al, 2009) (Fig.…”