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The p53 family comprises the tumor suppressor p53 and the structural homologs p63 and p73. How the three family members cooperate in tumor suppression remains unclear. Here, we report different but complementary functions of the individual members for regulating retinoblastoma protein (RB) function during myogenic differentiation. Whereas p53 transactivates the retinoblastoma gene, p63 and p73 induce the cyclin-dependent kinase inhibitor p57 to maintain RB in an active, hypophosphorylated state. DeltaNp73 inhibits these functions of the p53 family in differentiation control, prevents myogenic differentiation, and enables cooperating oncogenes to transform myoblasts to tumorigenicity. DeltaNp73 is frequently overexpressed in rhabdomyosarcoma and essential for tumor progression in vivo. These findings establish differentiation control as a key tumor suppressor activity of the p53 family.

p53 is known to prevent tumour formation by restricting the proliferation of damaged or oncogene-expressing cells. In contrast, how the p53 family member p73 suppresses tumour formation remains elusive. Using a step-wise transformation protocol for human cells, we show that, in premalignant stages, expression of the transactivationcompetent p73 isoform TAp73 is triggered in response to pRB pathway alterations. TAp73 expression at this stage of transformation results in increased sensitivity to chemotherapeutic drugs and oxidative stress and inhibits proliferation and survival at high cell density. Importantly, TAp73 triggers a transcriptional programme to prevent anchorage-independent growth, which is considered a crucial hallmark of fully transformed cells. An essential suppressor of anchorage-independent growth is KCNK1, which is directly transactivated by TAp73 and commonly downregulated in glioma, melanoma and ovarian cancer. Oncogenic Ras switches p73 expression from TAp73 to the oncogenic DNp73 isoform in a phosphatidyl-inositol 3-kinase-dependent manner. Our results implicate TAp73 as a barrier to anchorage-independent growth and indicate that downregulation of TAp73 is a key transforming activity of oncogenic Ras mutants.

Patients belonging to xeroderma pigmentosum (XP) complementation group C comprise one-third of all XP patients. Only four major reports compiled larger groups of XP-C patients from southern Europe (12 pts), North America (16 pts) and Africa (14 and 56 pts) as well as their genetic background (46 XPC mutations). We identified 16 XP-C patients from Germany. Interestingly, only five patients exhibited severe sun sensitivity. The mean age of XP diagnosis was 9.4 years, and the median age of the first skin cancer was 7 years. Neurological symptoms were absent in all but two patients. Primary fibroblasts from all 16 patients showed reduced post-UV cell survival (mean: 50% vs 93% in normal cells) and reduced reactivation of an UV-treated luciferase reporter gene (mean: 6.4% vs 30.7% in normal cells). XPC mRNA expression was also greatly reduced compared with normal cells (mean: 14.3%; range 8.3-25.7%) except in XP47MA (274.1%). All patients carried homozygous XPC mutations. Four mutations have been described previously: c.1747_1748delTG (found in 4/16), c.567 C>T (4/16), c.1839 C>T (1/16) and a complex insertion/deletion mutation in exon 9 (1/16). The novel frameshift mutations c.446_447delAG (2/16), c.1525insA (1/16) and c.2271delC (1/16) lead to truncated XPC proteins as does the novel nonsense mutation c.843C>T (1/16). XP47MA carries an interesting mutation (c.2538_2540delATC; p.Ile812del) resulting in an in-frame single amino acid deletion. This mutation results in a classical XP phenotype, a non-functional XPC protein, but elevated XPC mRNA expression. Our study indicates that extrinsic factors may contribute to XP-C symptom severity due to nonsense-mediated message decay.

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Only 16 XPG-defective patients with 20 different mutations have been described. The current hypothesis is that missense mutations impair repair (xeroderma pigmentosum (XP) symptoms), whereas truncating mutations impair both repair and transcription (XP and Cockayne syndrome (CS) symptoms). We identified three cell lines of XPG-defective patients (XP40GO, XP72MA, and XP165MA). Patients' fibroblasts showed a reduced post-UVC cell survival. The reduced repair capability, assessed by host cell reactivation, could be complemented by XPG cDNA. XPG mRNA expression of XP165MA, XP72MA, and XP40GO was 83%, 97%, and 82.5%, respectively, compared with normal fibroblasts. XP165MA was homozygous for a p.G805R mutation; XP72MA and XP40GO were both compound heterozygous (p.W814S and p.E727X, and p.L778P and p.Q150X, respectively). Allele-specific complementation analysis of these five mutations revealed that p.L778P and p.W814S retained considerable residual repair activity. In line with the severe XP/CS phenotypes of XP72MA and XP165MA, even the missense mutations failed to interact with the transcription factor IIH subunits XPD and to some extent cdk7 in coimmunoprecipitation assays. Immunofluorescence techniques revealed that the mutations destabilized early recruitment of XP proteins to localized photodamage and delayed their redistribution in vivo. Thus, we identified three XPG missense mutations in the I-region of XPG that impaired repair and transcription and resulted in severe XP/CS.

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