Chromosome instability syndromes.

Fanconi anaemia (FA), ataxia telangiectasia (A-T), Nijmegen breakage syndrome (NBS) and Bloom syndrome (BS) are clinically distinct, chromosome instability (or breakage) disorders. Each disorder has its own pattern of chromosomal damage, with cells from these patients being hypersensitive to particular genotoxic drugs, indicating that the underlying defect in each case is likely to be different. In addition, each syndrome shows a predisposition to cancer. Study of the molecular and genetic basis of these disorders has revealed mechanisms of recognition and repair of DNA double-strand breaks, DNA interstrand crosslinks and DNA damage during DNA replication. Specialist clinics for each disorder have provided the concentration of expertise needed to tackle their characteristic clinical problems and improve outcomes. Although some treatments of the consequences of a disorder may be possible, for example, haematopoietic stem cell transplantation in FA and NBS, future early intervention to prevent complications of disease will depend on a greater understanding of the roles of the affected DNA repair pathways in development. An important realization has been the predisposition to cancer in carriers of some of these gene mutations.

Three new BLM gene mutations associated with Bloom syndrome.

Bloom's syndrome (BS) is a rare autosomal recessive disease predisposing patients to all types of cancers affecting the general population. BS cells display a high level of genetic instability, including a 10-fold increase in the rate of sister chromatid exchanges, currently the only objective criterion for BS diagnosis. We have developed a method for screening the BLM gene for mutations based on direct genomic DNA sequencing. A questionnaire based on clinical information, cytogenetic features, and family history was addressed to physicians prescribing BS genetic screening, with the aim of confirming or guiding diagnosis. We report here four BLM gene mutations, three of which have not been described before. Three of the mutations are frameshift mutations, and the fourth is a nonsense mutation. All these mutations introduce a stop codon, and may therefore be considered to have deleterious biological effect. This approach should make it possible to identify new mutations and to correlate them with clinical information.

Clinical features of Bloom syndrome and function of the causative gene, BLM helicase.

Bloom syndrome is a rare autosomal recessive genetic disorder characterized by growth deficiency, unusual facies, sun-sensitive telangiectatic erythema, immunodeficiency and predisposition to cancer. The causative gene for Bloom syndrome is BLM, which encodes the BLM RecQ helicase homolog protein. The first part of this review describes a long-term follow-up study of two Bloom syndrome siblings. Subsequently, the focus is placed on the functional domains of BLM. Laboratory diagnosis of Bloom syndrome by detecting mutations in BLM is laborious and impractical, unless there are common mutations in a population. Immunoblot and immunohistochemical analyses for the detection of the BLM protein using a polyclonal BLM antibody, which are useful approaches for clinical diagnosis of Bloom syndrome, are also described. In addition, a useful adjunct for the diagnosis of Bloom syndrome in terms of the BLM function is investigated, since disease cells must have the defective BLM helicase function. This review also discusses the nuclear localization signal of BLM, the proteins that interact with BLM and tumors originating from Bloom syndrome.

Bloom syndrome cells undergo p53-dependent apoptosis and delayed assembly of BRCA1 and NBS1 repair complexes at stalled replication forks.

Bloom syndrome (BS) is a hereditary disorder characterized by pre- and postnatal growth retardation, genomic instability, and cancer. BLM, the gene defective in BS, encodes a DNA helicase thought to participate in genomic maintenance. We show that BS human fibroblasts undergo extensive apoptosis after DNA damage specifically when DNA replication forks are stalled. Damage during S, but not G1, caused BLM to rapidly form foci with gammaH2AX at replication forks that develop DNA breaks. These BLM foci recruited BRCA1 and NBS1. Damaged BS cells formed BRCA1/NBS1 foci with markedly delayed kinetics. Helicase-defective BLM showed dominant-negative activity with respect to apoptosis, but not BRCA1/NBS1 recruitment, suggesting catalytic and structural roles for BLM. Strikingly, inactivation of p53 prevented the death of damaged BS cells and delayed recruitment of BRCA1/NBS1. These findings suggest that BLM is an early responder to damaged replication forks. Moreover, p53 eliminates cells that rapidly assemble BRCA1/NBS1 without BLM, suggesting that BLM is essential for timely BRCA1/NBS1 function.

Human diseases deficient in RecQ helicases.

RecQ helicases are conserved from bacteria to man. Mutations in three of the human RecQ family members give rise to genetic disorders characterized by genomic instability and a predisposition to cancer. RecQ helicases are therefore caretakers of the genome, and although they do not directly regulate tumorigenesis, they influence stability and the rate of accumulation of genetic alterations, which in turn, result in tumorigenesis. Maintenance of genome stability by RecQ helicases likely involves their participation in DNA replication, recombination, and repair pathways.

Caretaker tumour suppressor genes that defend genome integrity.

Cancers arise as a result of genetic changes that impact upon cell proliferation through promoting cell division and/or inhibiting cell death. Tumour suppressor (TS) genes are the targets for many of these genetic changes. In general, both alleles of TS genes must be disrupted to observe a phenotypic effect. Broadly speaking, there are two types of TS gene: 'gatekeepers' and 'caretakers'. In contrast to gatekeepers, caretaker genes do not directly regulate proliferation, but act to prevent genomic instability. Thus, mutation of caretaker genes leads to accelerated conversion of a normal cell to a neoplastic cell. Many caretaker genes are required for the maintenance of genome integrity. This review focuses on those caretaker genes that play a role, directly or indirectly, in the repair of DNA strand breaks by the homologous recombination pathway, and that are associated with cancer-prone clinical syndromes, in particular ataxia telangiectasia, hereditary breast cancer, Bloom's syndrome and Werner's syndrome.

Diseases associated with photosensitivity.

Photosensitive disorders may be classified as those entirely caused by solar exposure and the photoaggravated disorders. Those in the former category include polymorphic light eruption, juvenile spring eruption, actinic prurigo, hydroa vacciniforme, solar urticaria, also chronic actinic dermatitis. Genodermatoses whose expression mainly depends on UV or light exposure include the DNA repair deficient disorders, some disorders of cornification, the Smith-Lemli-Opitz syndrome and porphyria. Examples of photoaggravated diseases include lupus erythematosus, erythema multiforme, atopic eczema, psoriasis, viral exanthemata, pemphigus, dermatitis herpetiformis and rosacea. Drugs and chemicals may interact with UV to induce photosensitivity. In many of these diseases the action spectrum is known or may be determined by phototesting. Recognition of the reaction patterns associated with the photodermatoses greatly assists clinical classification of the photodermatoses.

Growth deficiency and malnutrition in Bloom syndrome.

OBJECTIVES: To describe the growth and nutritional status of a pediatric population with Bloom syndrome. STUDY DESIGN: Longitudinal growth data from 148 patients in the Bloom's Syndrome Registry (85 male, 63 female) were compiled retrospectively from physician and parent records to develop graphed statistics of weight-for-age, height-for-age, fronto-occipital circumference-for-age, and weight-for-height for both sexes with comparisons with the normal population. RESULTS: Term birth measurements confirm that the growth deficiency of Bloom syndrome has prenatal onset. Stunting persists throughout life, and an adolescent growth spurt is not apparent from the smoothed data. Growth continues by at least 1 cm/yr until age 21 years for both sexes. More than half of children with Bloom syndrome are significantly wasted until age 8 years, which is not related to early death or underlying malignancy. The mean body mass index for adults with Bloom syndrome after age 25 years is low normal (n = 22, mean = 20.2 kg/m2). CONCLUSIONS: Children with Bloom syndrome have significant growth retardation and wasting.

Repair of base-base mismatches in simian and human cells.

Mismatched heteroduplexes arise as intermediates of several dissimilar genetic processes. The outcome of these genetic events will therefore be influenced by the efficiency and specificity of mismatch repair. We have studied the correction of base-base mispairs in simian and human fibroblasts by transfecting the cells with derivatives of SV40 DNA, each harboring a single mispair in a defined orientation. Analysis of plaques revealed that correction efficiencies for homomispairs followed the pattern G.G greater than C.C greater than or equal to A.A greater than T.T. Repair bias was influenced by flanking sequences. Correction efficiences for heteromispairs followed the pattern of G.T greater than A.C greater than C.T greater than A.G and repair favored the retention of G + C by a substantial margin. This repair specificity could lead to a gene conversion bias favoring the accumulation of G + C in sequences subject to high levels of recombination or unequal exchange.

Sensitivity of Bloom syndrome fibroblasts to mitomycin C.

Lymphocytes and fibroblasts from people with Bloom syndrome, an autosomal recessive disorder associated with a predisposition to a wide variety of cancers, are known to be hypersensitive to ethylating agents as measured by sister chromatid exchange induction. Recently, hypersensitivity to cell killing by mitomycin C has also been reported in Bloom syndrome fibroblasts from three donors. We report here results which confirm the hypersensitivity of Bloom syndrome fibroblasts as measured by cell killing but show that they have a normal sensitivity to mitomycin C as measured by sister chromatid exchange induction. These results are discussed in terms of their relevance to the diversity of response of Bloom syndrome cells to mutagens, and the nature of the primary defect in Bloom syndrome.

Analyses of bromodeoxyuridine-associated sister chromatid exchanges (SCEs) in Bloom syndrome based on cell fusion: single and twin SCEs in endoreduplication.

When Bloom syndrome (BS) cells labeled with bromodeoxyuridine (BrdUrd) for one round of DNA replication were fused with nonlabeled normal cells, the hybrid cells had a normal level of sister chromatid exchange (SCE) at the first mitosis after fusion. However, when normal cells treated with mitomycin C (MC) were fused with nontreated normal cells, the MC-induced SCE was not affected by fusion with normal cells. Single and twin SCEs were analyzed in the Colcemid-induced endoreduplicated normal and BS lymphoid B cells from diplochromosomes. In normal cells, the same number of SCEs occurs in each of the two cell cycles; the SCE ratio of single (6.30 SCEs per cell) to twin (2.92 SCEs per cell) was 2:1 on the endoreduplicated-cell basis, showing 1:1 on the diploid-cell basis. In BS cells, the SCE ratio of single (144.8 SCEs per cell) to twin (5.9 SCEs per cell) was 25:1 on the endoreduplicated-cell basis and was 12:1 on the diploid-cell basis. These studies strongly suggest that most of the BS SCEs occur during the second cell cycle when BrdUrd-containing DNA is used as template for replication and that the normal level of BS SCE observed at the first mitosis of the hybrid cells is the result of SCE inhibition resulting from the fusion with normal cells.