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Mutational inactivation of genes controlling the DNA-damage response contributes to cancer susceptibility within families and within the general population as well as to sporadic tumorigenesis. Claspin (CLSPN) encodes a recently recognized mediator protein essential for the ATR and CHK1-dependent checkpoint elicited by replicative stress or the presence of ssDNA. Here, we describe a study to determine whether mutational disruption of CLSPN contributes to cancer susceptibility and sporadic tumorigenesis. We resequenced CLSPN from the germline of selected cancer families with a history of breast cancer (n = 25) or a multicancer phenotype (n = 46) as well as from a panel of sporadic cancer cell lines (n = 52) derived from a variety of tumor types. Eight nonsynonymous variants, including a recurrent mutation, were identified from the germline of two cancer-prone individuals and five cancer cell lines of breast, ovarian, and hematopoietic origin. None of the variants was present within population controls. In contrast, mutations were rare within genes encoding the CLSPN-interacting protein ATR and its binding partner ATRIP. One variant of CLSPN, encoding the I783S missense mutation, was defective in its ability to mediate CHK1 phosphorylation following DNA damage and was unable to rescue sensitivity to replicative stress in CLSPN-depleted cells. Taken together, these observations raise the possibility that CLSPN may encode a component of the DNA-damage response pathway that is targeted by mutations in human cancers, suggesting the need for larger population-based studies to investigate whether CLSPN variants contribute to cancer susceptibility.

The CHEK2-1100delC mutation is recurrent in the population and is a moderate risk factor for breast cancer. To identify additional CHEK2 mutations potentially contributing to breast cancer susceptibility, we sequenced 248 cases with early-onset disease; functionally characterized new variants and conducted a population-based case-control analysis to evaluate their contribution to breast cancer risk. We identified 1 additional null mutation and 5 missense variants in the germline of cancer patients. In vitro, the CHEK2-H143Y variant resulted in gross protein destabilization, while others had variable suppression of in vitro kinase activity using BRCA1 as a substrate. The germline CHEK2-1100delC mutation was present among 8/1,646 (0.5%) sporadic, 2/400 (0.5%) early-onset and 3/302 (1%) familial breast cancer cases, but undetectable amongst 2,105 multiethnic controls, including 633 from the US. CHEK2-positive breast cancer families also carried a deleterious BRCA1 mutation. 1100delC appears to be the only recurrent CHEK2 mutation associated with a potentially significant contribution to breast cancer risk in the general population. Another recurrent mutation with attenuated in vitro function, CHEK2-P85L, is not associated with increased breast cancer susceptibility, but exhibits a striking difference in frequency across populations with different ancestral histories. These observations illustrate the importance of genotyping ethnically diverse groups when assessing the impact of low-penetrance susceptibility alleles on population risk. Our findings highlight the notion that clinical testing for rare missense mutations within CHEK2 may have limited value in predicting breast cancer risk, but that testing for the 1100delC variant may be valuable in phenotypically- and geographically-selected populations., ((c) 2007 Wiley-Liss, Inc.)

Noonan syndrome, the most common single-gene cause of congenital heart disease, is characterized by short stature, characteristic facies, learning problems and leukemia predisposition. Gain-of-function mutations in PTPN11, encoding the tyrosine phosphatase SHP2, cause approximately 50% of Noonan syndrome cases. SHP2 is required for RAS-ERK MAP kinase (MAPK) cascade activation, and Noonan syndrome mutants enhance ERK activation ex vivo and in mice. KRAS mutations account for <5% of cases of Noonan syndrome, but the gene(s) responsible for the remainder are unknown. We identified missense mutations in SOS1, which encodes an essential RAS guanine nucleotide-exchange factor (RAS-GEF), in approximately 20% of cases of Noonan syndrome without PTPN11 mutation. The prevalence of specific cardiac defects differs in SOS1 mutation-associated Noonan syndrome. Noonan syndrome-associated SOS1 mutations are hypermorphs encoding products that enhance RAS and ERK activation. Our results identify SOS1 mutants as a major cause of Noonan syndrome, representing the first example of activating GEF mutations associated with human disease and providing new insights into RAS-GEF regulation.

The number of cells in an organism is determined by regulating both cell proliferation and cell death. Relatively few mechanisms have been identified that can modulate both of these processes. In a screen for Drosophila mutations that result in tissue overgrowth, we identified salvador (sav), a gene that promotes both cell cycle exit and cell death. Elevated Cyclin E and DIAP1 levels are found in mutant cells, resulting in delayed cell cycle exit and impaired apoptosis. Salvador contains two WW domains and binds to the Warts (or LATS) protein kinase. The human ortholog of salvador (hWW45) is mutated in three cancer cell lines. Thus, salvador restricts cell numbers in vivo by functioning as a dual regulator of cell proliferation and apoptosis.

Li Fraumeni Syndrome (LFS) is a multicancer phenotype, most commonly associated with germ-line mutations in TP53. In a kindred with LFS without an inherited TP53 mutation, we have previously reported a truncating mutation (1100delC) in CHK2, encoding a kinase that phosphorylates p53 on Ser(20). Here, we describe a CHK2 missense mutation (R145W) in another LFS family. This mutation destabilizes the encoded protein, reducing its half-life from >120 min to 30 min. This effect is abrogated by treatment of cells with a proteosome inhibitor, suggesting that CHK2(R145W) is targeted through this degradation pathway. Both 1100delC and R145W germ-line mutations in CHK2 are associated with loss of the wild-type allele in the corresponding tumor specimens, and neither tumor harbors a somatic TP53 mutation. Our observations support the functional significance of CHK2 mutations in rare cases of LFS and suggest that such mutations may substitute for inactivation of TP53.