TFIIH: at the crossroads of cancer and ageing

It is widely although not uniformly accepted that ageing is caused by time-dependent accumulation of damage. A side effect of ageing is an increased risk of developing a disease, such as cancer. Yet, the primary molecular target of damage accumulation has remained obscure. A clue has emerged from the notion that human inborn premature ageing syndromes are often associated with mutations in genes involved in DNA metabolism such as nucleotide excision repair (NER). In this thesis, mechanisms of pathological and normal ageing are addressed by genocopying naturally occurring human NER mutants with accelerated ageing and/or cancer phenotype in mouse model systems. Pathological conditions associated with mutations in the XPD and XPB genes, encoding the helicase components of the multifunctional TFIIH complex, range from a dramatic 1000 times elevated cancer predisposition (Xeroderma pigmentosum) to the severe neurodevelopmental premature ageing disorders trichothiodystrophy (TTD) and XP combined with Cockayne syndrome (XPCS). The aim of this thesis is to understand the mechanisms of accelerated ageing and to uncover variables determining the immense clinical heterogeneity ofNER disorders. Chapter 1 introduces the theory of ageing, summarizes clinical consequences ofNER mutations and describes the basic DNA repair mechanims involved. Chapter 2 presents experiments revealing TTD as a progeroid syndrome and defines the crucial role of DNA damage and repair in the rate ofTTD-related ageing. Chapter 3 reveals interallelic complementation between differentially compromised XPD molecules in mice and suggests a new variable in genotype-phenotype relationships within human recessive disease. Chapter 4 shows that CS and TTD share a common root cause in defective DNA repair. Chapter 5 uncovers the widely pleiotropic effects of combining different NER defects with the latent Xp!JXPCS mutation, ranging from moderately enhanced ageing to immediate postnatal lethality. Chapter 6 describes how interallelic complementation can be used for the reconstitution of CS pathology in mice and outlines some potentials for therapy. Chapter 7 reviews all major features of the NER mouse models generated in this thesis and elsewhere and sets forward a model for NER- associated disease