BLM helicase unwinds lagging strand substrates to assemble the ALT telomere damage response

The Bloom syndrome (BLM) helicase is critical for alternative lengthening of telomeres (ALT), a homology-directed repair (HDR)-mediated telomere maintenance mechanism that is prevalent in cancers of mesenchymal origin. The DNA substrates that BLM engages to direct telomere recombination during ALT remain unknown. Here, we determine that BLM helicase acts on lagging strand telomere intermediates that occur specifically in ALT-positive cells to assemble a replication-associated DNA damage response. Loss of ATRX was permissive for BLM localization to ALT telomeres in S and G2, commensurate with the appearance of telomere C-strand-specific single-stranded DNA (ssDNA). DNA2 nuclease deficiency increased 5'-flap formation in a BLM-dependent manner, while telomere C-strand, but not G-strand, nicks promoted ALT. These findings define the seminal events in the ALT DNA damage response, linking aberrant telomeric lagging strand DNA replication with a BLM-directed HDR mechanism that sustains telomere length in a subset of human cancers.

Distinct pathways of homologous recombination controlled by the SWS1-SWSAP1-SPIDR complex

Homology-directed repair (HDR), a critical DNA repair pathway in mammalian cells, is complex, leading to multiple outcomes with different impacts on genomic integrity. However, the factors that control these different outcomes are often not well understood. Here we show that SWS1-SWSAP1-SPIDR controls distinct types of HDR. Despite their requirement for stable assembly of RAD51 recombinase at DNA damage sites, these proteins are not essential for intra-chromosomal HDR, providing insight into why patients and mice with mutations are viable. However, SWS1-SWSAP1-SPIDR is critical for inter-homolog HDR, the first mitotic factor identified specifically for this function. Furthermore, SWS1-SWSAP1-SPIDR drives the high level of sister-chromatid exchange, promotes long-range loss of heterozygosity often involved with cancer initiation, and impels the poor growth of BLM helicase-deficient cells. The relevance of these genetic interactions is evident as SWSAP1 loss prolongs Blm-mutant embryo survival, suggesting a possible druggable target for the treatment of Bloom syndrome.

Bloom syndrome and the underlying causes of genetic instability

Autosomal hereditary recessive diseases characterized by genetic instability are often associated with cancer predisposition. Bloom syndrome (BS), a rare genetic disorder, with <300 cases reported worldwide, combines both. Indeed, patients with Bloom's syndrome are 150 to 300 times more likely to develop cancers than normal individuals. The wide spectrum of cancers developed by BS patients suggests that early initial events occur in BS cells which may also be involved in the initiation of carcinogenesis in the general population and these may be common to several cancers. BS is caused by mutations of both copies of the BLM gene, encoding the RecQ BLM helicase. This review discusses the different aspects of BS and the different cellular functions of BLM in genome surveillance and maintenance through its major roles during DNA replication, repair, and transcription. BLM's activities are essential for the stabilization of centromeric, telomeric and ribosomal DNA sequences, and the regulation of innate immunity. One of the key objectives of this work is to establish a link between BLM functions and the main clinical phenotypes observed in BS patients, as well as to shed new light on the correlation between the genetic instability and diseases such as immunodeficiency and cancer. The different potential implications of the BLM helicase in the tumorigenic process and the use of BLM as new potential target in the field of cancer treatment are also debated.

The RecQ helicase Sgs1 drives ATP-dependent disruption of Rad51 filaments

DNA helicases of the RecQ family are conserved among the three domains of life and play essential roles in genome maintenance. Mutations in several human RecQ helicases lead to diseases that are marked by cancer predisposition. The Saccharomyces cerevisiae RecQ helicase Sgs1 is orthologous to human BLM, defects in which cause the cancer-prone Bloom's Syndrome. Here, we use single-molecule imaging to provide a quantitative mechanistic understanding of Sgs1 activities on single stranded DNA (ssDNA), which is a central intermediate in all aspects of DNA metabolism. We show that Sgs1 acts upon ssDNA bound by either replication protein A (RPA) or the recombinase Rad51. Surprisingly, we find that Sgs1 utilizes a novel motor mechanism for disrupting ssDNA intermediates bound by the recombinase protein Rad51. The ability of Sgs1 to disrupt Rad51-ssDNA filaments may explain some of the defects engendered by RECQ helicase deficiencies in human cells.

Loss of RMI2 Increases Genome Instability and Causes a Bloom-Like Syndrome

Bloom syndrome is a recessive human genetic disorder with features of genome instability, growth deficiency and predisposition to cancer. The only known causative gene is the BLM helicase that is a member of a protein complex along with topoisomerase III alpha, RMI1 and 2, which maintains replication fork stability and dissolves double Holliday junctions to prevent genome instability. Here we report the identification of a second gene, RMI2, that is deleted in affected siblings with Bloom-like features. Cells from homozygous individuals exhibit elevated rates of sister chromatid exchange, anaphase DNA bridges and micronuclei. Similar genome and chromosome instability phenotypes are observed in independently derived RMI2 knockout cells. In both patient and knockout cell lines reduced localisation of BLM to ultra fine DNA bridges and FANCD2 at foci linking bridges are observed. Overall, loss of RMI2 produces a partially active BLM complex with mild features of Bloom syndrome.

Cells contain specific protein complexes that are needed to correct errors during the replication and segregation of DNA. Impairment in the activity of these proteins can be detrimental to the viability of the cell and organism development. Bloom syndrome is an example of a genome instability disorder where cells cannot efficiently untangle DNA after replication. The only gene that is known to cause Bloom syndrome is the BLM helicase. In this article, we describe two affected individuals with Bloom-like features with a homozygous deletion of the RMI2 gene. The RMI2 protein has previously been shown to form a complex with BLM, topoisomerase III alpha and RMI1. Deletion of RMI2 in patient and unrelated cell lines show hyper-recombination and chromosome entanglements during cell division. Furthermore, we show that the BLM and FANCD2 proteins are diminished in the binding of DNA bridges that need to be dissolved during the late stages of cell division. Therefore, loss of RMI2 produces a milder Bloom phenotype and impairs the full activity of the BLM complex.

A case of Bloom syndrome with uncommon clinical manifestations confirmed on genetic testing

Bloom syndrome, a rare autosomal-recessive disorder, characteristically presents with photosensitivity, telangiectatic facial erythema, and growth deficiency. We present a case of Bloom syndrome with uncommon clinical manifestations including alopecia areata, eyebrow hair loss, flat nose, reticular pigmentation, and short sharpened distal phalanges with fingernails that were wider than they were long. We detected the Bloom syndrome gene, BLM, which is one of the members of the RecQ family of DNA helicases, and found changes in 2 heterozygous nucleotide sites in the patient as well as her father and mother.

Pyrimidine Pool Disequilibrium Induced by a Cytidine Deaminase Deficiency Inhibits PARP-1 Activity, Leading to the Under Replication of DNA

Genome stability is jeopardized by imbalances of the dNTP pool; such imbalances affect the rate of fork progression. For example, cytidine deaminase (CDA) deficiency leads to an excess of dCTP, slowing the replication fork. We describe here a novel mechanism by which pyrimidine pool disequilibrium compromises the completion of replication and chromosome segregation: the intracellular accumulation of dCTP inhibits PARP-1 activity. CDA deficiency results in incomplete DNA replication when cells enter mitosis, leading to the formation of ultrafine anaphase bridges between sister-chromatids at "difficult-to-replicate" sites such as centromeres and fragile sites. Using molecular combing, electron microscopy and a sensitive assay involving cell imaging to quantify steady-state PAR levels, we found that DNA replication was unsuccessful due to the partial inhibition of basal PARP-1 activity, rather than slower fork speed. The stimulation of PARP-1 activity in CDA-deficient cells restores replication and, thus, chromosome segregation. Moreover, increasing intracellular dCTP levels generates under-replication-induced sister-chromatid bridges as efficiently as PARP-1 knockdown. These results have direct implications for Bloom syndrome (BS), a rare genetic disease combining susceptibility to cancer and genomic instability. BS results from mutation of the BLM gene, encoding BLM, a RecQ 3'-5' DNA helicase, a deficiency of which leads to CDA downregulation. BS cells thus have a CDA defect, resulting in a high frequency of ultrafine anaphase bridges due entirely to dCTP-dependent PARP-1 inhibition and independent of BLM status. Our study describes previously unknown pathological consequences of the distortion of dNTP pools and reveals an unexpected role for PARP-1 in preventing DNA under-replication and chromosome segregation defects.

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.

Aberrant chromosome morphology in human cells defective for Holliday junction resolution

In somatic cells, Holliday junctions can be formed between sister chromatids during the recombinational repair of DNA breaks or after replication fork demise. A variety of processes act upon Holliday junctions to remove them from DNA, in events that are critical for proper chromosome segregation. In human cells, the BLM protein, inactivated in individuals with Bloom's syndrome, acts in combination with topoisomerase IIIα, RMI1 and RMI2 (BTR complex) to promote the dissolution of double Holliday junctions. Cells defective for BLM exhibit elevated levels of sister chromatid exchanges (SCEs) and patients with Bloom's syndrome develop a broad spectrum of early-onset cancers caused by chromosome instability. MUS81-EME1 (refs 4-7), SLX1-SLX4 (refs 8-11) and GEN1 (refs 12, 13) also process Holliday junctions but, in contrast to the BTR complex, do so by endonucleolytic cleavage. Here we deplete these nucleases from Bloom's syndrome cells to analyse human cells compromised for the known Holliday junction dissolution/resolution pathways. We show that depletion of MUS81 and GEN1, or SLX4 and GEN1, from Bloom's syndrome cells results in severe chromosome abnormalities, such that sister chromatids remain interlinked in a side-by-side arrangement and the chromosomes are elongated and segmented. Our results indicate that normally replicating human cells require Holliday junction processing activities to prevent sister chromatid entanglements and thereby ensure accurate chromosome condensation. This phenotype was not apparent when both MUS81 and SLX4 were depleted from Bloom's syndrome cells, suggesting that GEN1 can compensate for their absence. Additionally, we show that depletion of MUS81 or SLX4 reduces the high frequency of SCEs in Bloom's syndrome cells, indicating that MUS81 and SLX4 promote SCE formation, in events that may ultimately drive the chromosome instabilities that underpin early-onset cancers associated with Bloom's syndrome.

Lens opacities in Bloom syndrome: case report and review of the literature

Bloom syndrome is an autosomal recessive disorder characterized by proportionate short stature, photosensitivity, immunodeficiency, hypogonadism and a tendency to develop various malignancies. The greatly increased frequency of sister chromatid exchanges (reciprocal exchange of homologous segments between the two sister chromatids of a chromosome) is regarded as pathognomonic for BS. We describe an 18-year old girl who presented with short stature. She was diagnosed with BS based on an extremely increased frequency of sister chromatid exchanges. Ophthalmological examination revealed mild lens opacities bilaterally, which, to our knowledge, has not been previously reported to be associated with BS.

Endogenous γ-H2AX-ATM-Chk2 Checkpoint Activation in Bloom's Syndrome Helicase–Deficient Cells Is Related to DNA Replication Arrested Forks

The Bloom syndrome helicase (BLM) is critical for genomic stability. A defect in BLM activity results in the cancer-predisposing Bloom syndrome (BS). Here, we report that BLM-deficient cell lines and primary fibroblasts display an endogenously activated DNA double-strand break checkpoint response with prominent levels of phosphorylated histone H2AX (gamma-H2AX), Chk2 (p(T68)Chk2), and ATM (p(S1981)ATM) colocalizing in nuclear foci. Interestingly, the mitotic fraction of gamma-H2AX foci did not seem to be higher in BLM-deficient cells, indicating that these lesions form transiently during interphase. Pulse labeling with iododeoxyuridine and immunofluorescence microscopy showed the colocalization of gamma-H2AX, ATM, and Chk2 together with replication foci. Those foci costained for Rad51, indicating homologous recombination at these replication sites. We therefore analyzed replication in BS cells using a single molecule approach on combed DNA fibers. In addition to a higher frequency of replication fork barriers, BS cells displayed a reduced average fork velocity and global reduction of interorigin distances indicative of an elevated frequency of origin firing. Because BS is one of the most penetrant cancer-predisposing hereditary diseases, it is likely that the lack of BLM engages the cells in a situation similar to precancerous tissues with replication stress. To our knowledge, this is the first report of high ATM-Chk2 kinase activation and its linkage to replication defects in a BS model.

Role of the BLM helicase in replication fork management

Genomic DNA is particularly vulnerable to mutation during S-phase when the two strands of parental duplex DNA are separated during the process of semi-conservative DNA replication. Lesions that are normally repaired efficiently in the context of double stranded DNA can cause replication forks to stall or, more dangerously, collapse. Cells from Bloom's syndrome patients, that lack the RecQ helicase BLM, show defects in the response to replicative stress and contain a multitude of chromosomal aberrations, which primarily arise through excessive levels of homologous recombination. Here, recent findings are reviewed that further our understanding of the role that BLM plays in the management of damaged replication forks.

BLM is required for faithful chromosome segregation and its localization defines a class of ultrafine anaphase bridges

Mutations in BLM cause Bloom's syndrome, a disorder associated with cancer predisposition and chromosomal instability. We investigated whether BLM plays a role in ensuring the faithful chromosome segregation in human cells. We show that BLM-defective cells display a higher frequency of anaphase bridges and lagging chromatin than do isogenic corrected derivatives that eptopically express the BLM protein. In normal cells undergoing mitosis, BLM protein localizes to anaphase bridges, where it colocalizes with its cellular partners, topoisomerase IIIalpha and hRMI1 (BLAP75). Using BLM staining as a marker, we have identified a class of ultrafine DNA bridges in anaphase that are surprisingly prevalent in the anaphase population of normal human cells. These so-called BLM-DNA bridges, which also stain for the PICH protein, frequently link centromeric loci, and are present at an elevated frequency in cells lacking BLM. On the basis of these results, we propose that sister-chromatid disjunction is often incomplete in human cells even after the onset of anaphase. We present a model for the action of BLM in ensuring complete sister chromatid decatenation in anaphase.

False-positive quadruple screen test for trisomy 18 in a patient with a fetus with Bloom's syndrome

In the literature, conflicting reports on the significance of false-positive maternal serum multiple marker testing for trisomy 18 are encountered; however, the biology of this finding is discussed infrequently. We present such a case in association with Bloom's syndrome in the fetus. The fetus had intrauterine growth restriction, seen early in the second trimester, oligohydramnios, and was delivered at 34 weeks of gestation for impending fetal compromise. We propose that the adverse outcome of the pregnancy with false-positive serum analyte testing for trisomy 18 might result from a small-sized placenta and perhaps pathology at receptor level.

Molecular pathogenesis of osteosarcoma

Osteosarcoma is a devastating but rare disease, whose study has illuminated both the basic biology and clinical management of cancer over the past 30 years. These contributions have included insight into the roles of key cancer genes such as the retinoblastoma tumor suppressor gene and TP53, the identification of familial cancer syndromes implicating DNA helicases, and dramatic improvements in survival by the use of adjuvant chemotherapy. This review provides a synoptic overview of our current understanding of the molecular causes of osteosarcoma, and suggests future directions for study.

Mechanism of homologous recombination: mediators and helicases take on regulatory functions

Homologous recombination (HR) is an important mechanism for the repair of damaged chromosomes, for preventing the demise of damaged replication forks, and for several other aspects of chromosome maintenance. As such, HR is indispensable for genome integrity, but it must be regulated to avoid deleterious events. Mutations in the tumour-suppressor protein BRCA2, which has a mediator function in HR, lead to cancer formation. DNA helicases, such as Bloom's syndrome protein (BLM), regulate HR at several levels, in attenuating unwanted HR events and in determining the outcome of HR. Defects in BLM are also associated with the cancer phenotype. The past several years have witnessed dramatic advances in our understanding of the mechanism and regulation of HR.

Bloom syndrome in an Indian child

A girl presented with severely stunted growth, photosensitivity, and a characteristic facies. Cytogenetic studies were suggestive of Bloom syndrome. This disorder has not been previously documented in the literature in an Indian child. Minor variations in characteristics in this patient have been highlighted. Cytogenetically, she was found to be a low sister chromatid exchange mosaicism of Bloom syndrome.

Bloom syndrome: multiple retinopathies in a chromosome breakage disorder

AIM

To describe multiple retinal abnormalities in a patient with Bloom syndrome, including early macular drusen, diabetic retinopathy, and the onset of leukaemic retinopathy.

METHODS

Clinical data were collected over 1 year of follow up, and ocular abnormalities in Bloom syndrome were reviewed from the literature.

RESULTS

A 39 year old man with a rare autosomal recessive "chromosome breakage" syndrome was followed. A variety of ocular findings have been reported in Bloom syndrome; this patient had hard drusen in both maculae, non-proliferative diabetic retinopathy, and haemorrhagic retinopathy as a herald of acute lymphocytic leukaemia.

CONCLUSIONS

Bloom syndrome is a rare disorder of genomic instability, in which a variety of ocular abnormalities have been found. Described here are multiple retinal manifestations arising from characteristic systemic associations of diabetes mellitus and leukaemia, as well as macular hard drusen.

Possible anti-recombinogenic role of Bloom's syndrome helicase in double-strand break processing

Bloom's syndrome (BS) which associates genetic instability and predisposition to cancer is caused by mutations in the BLM gene encoding a RecQ family 3'-5' DNA helicase. It has been proposed that the generation of genetic instability in BS cells could result from an aberrant non-homologous DNA end joining (NHEJ), one of the two main DNA double-strand break (DSB) repair pathways in mammalian cells, the second major pathway being homologous recombination (HR). Using cell extracts, we report first that Ku70/80 and the catalytic subunit of the DNA-dependent protein kinase (DNA-PKcs), key factors of the end-joining machinery, and BLM are located in close proximity on DNA and that BLM binds to DNA only in the absence of ATP. In the presence of ATP, BLM is phosphorylated and dissociates from DNA in a strictly DNA-PKcs-dependent manner. We also show that BS cells display, in vivo, an accurate joining of DSBs, reflecting thus a functional NHEJ pathway. In sharp contrast, a 5-fold increase of the HR-mediated DNA DSB repair in BS cells was observed. These results support a model in which NHEJ activation mediates BLM dissociation from DNA, whereas, under conditions where HR is favored, e.g. at the replication fork, BLM exhibits an anti-recombinogenic role.

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.

Bloom syndrome in sibs: first reports of hepatocellular carcinoma and Wilms tumor with documented anaplasia and nephrogenic rests

The triad of small body size, immunodeficiency, and sun-sensitive facial erythema characterizes the phenotype Bloom syndrome (BS), a rare autosomal recessive disorder with a striking predisposition to multiple types of cancers that arise earlier than expected in the general population. Here we report two sibs with BS. The older, a 15-year-old-girl, developed a hepatocellular carcinoma, a neoplasm not yet reported in association with BS. Her younger brother developed an anaplastic Wilms tumor (WT) associated with nephrogenic rests at the age of 31/2 years, and this was followed by a myelodysplastic syndrome. Complex cytogenetic abnormalities were identified in all three neoplasms. These examples expand the spectrum of malignancies occurring in BS to include liver cell neoplasms, and confirm the association of nephrogenic rests with WT, even in the setting of BS.

Successful pregnancy in a woman with Bloom syndrome

Bloom syndrome is a rare autosomal recessive disorder notable for increased chromosome fragility and an increased rate of somatic mutation. The clinical manifestations include small stature, a characteristic dermatologic lesion, and an excess incidence of malignancy. Fertility is generally reduced. A 19-year-old white woman with Bloom syndrome was successfully treated for preterm labor at 32 weeks' gestation, and ultimately delivered a healthy male infant at 35 weeks' gestation. Reports of pregnancy in women with Bloom syndrome are few. Despite reduced fertility, conception can occur, and women with Bloom syndrome should receive appropriate reproductive counseling to prevent unintended pregnancies and increased surveillance for preterm birth.

Numerous colonic adenomas in an individual with Bloom's syndrome

Bloom's syndrome (BS) is a rare recessive disorder caused by germline mutation of the BLM gene. Individuals with BS manifest growth retardation, immunodeficiency, and a predisposition to cancer. In this report, we describe an individual with BS and multiple colonic adenomas reminiscent of familial adenomatous polyposis coli (FAP). Molecular studies revealed APC mutations in 4 of 6 adenomas, including 2 adenomas with the identical APC mutation and microsatellite instability in 1 of 6 adenomas. These results demonstrate similar pathways to colorectal neoplasia in BS as in the normal population and suggest that individuals with BS may be particularly susceptible to colorectal neoplasia.

Immunohistochemical expression and pathogenesis of BLM in the human brain and visceral organs

Bloom syndrome (BS) involves the clinical features of telangiectatic erythema, immunodeficiency, and an increased risk for cancer. In order to clarify the pathogenetic significance of the responsible gene, BLM, which encodes a protein possessing homology to Escherichia coli RecQ helicase, the immunohistochemistry of BLM was examined in human brains and visceral organs from fetuses to adults and an adult with BS, using anti-BLM antibodies. Purkinje cells exhibited positive BLM immunoreactivity from 21 gestational weeks (GW), which transiently increased at approximately 40 GW. Neurons of the pontine tegmentum were immunolabeled from the early fetal period. In visceral organs, positive BLM immunoreactivity was observed in the Hassal corpuscles in the thymus from 24 GW, in beta-cells in the Langerhans islets of the pancreas from 36 GW, and in sperm cells and sperms of the testes from 11 years of age. But in a patient with BS, it was negative in the pancreas and testis tissues examined. The characteristic effect of BLM on specific cells in different periods suggests that the BLM gene product is closely related to neuronal development as well as immune, insulin secretory and sperm functions, which appear in different periods, and disorders of which are major symptoms of BS.

Bloom helicase is involved in DNA surveillance in early S phase in vertebrate cells

Bloom syndrome (BS) is a recessive human genetic disorder characterized by short stature, immunodeficiency and an elevated risk of malignancy. The gene mutated in BS, BLM, encodes a RecQ-type DNA helicase. BS cells have mutator phenotypes such as hyper-recombination, chromosome instability and an increased frequency of sister chromatid exchange (SCE). To define the primary role of BLM, we generated BLM(-/-) mutants of the chicken B-cell line DT40. In addition to characteristics of BLM(-/-) cells reported previously by the other group, they are hypersensitive to genotoxic agents such as etoposide, bleomycin and 4-nitroquinoline-1-oxide and irradiation with the short wave length of UV (UVC) light, whereas they exhibit normal sensitivity to X-ray irradiation and hydroxyurea. UVC irradiation to BLM(-/-) cells during G(1) to early S phase caused chromosomal instability such as chromatid breaks and chromosomal quadriradials, leading to eventual cell death. These results suggest that BLM is involved in surveillance of base abnormalities in genomic DNA that may be encountered by replication forks in early S phase. Such surveillance would maintain genomic stability in vertebrate cells, resulting in the prevention of cellular tumorigenesis.

Back mutation can produce phenotype reversion in Bloom syndrome somatic cells

A unique and constant feature of Bloom syndrome (BS) cells is an excessive rate of sister-chromatid exchange (SCE). However, in approximately 20% of persons with typical BS, mosaicism is observed in which a proportion of lymphocytes (usually a small one) exhibits a low-SCE rate. Persons with such mosaicism predominantly are genetic compounds for mutation at BLM, and the low-SCE lymphocytes are the progeny of a precursor cell in which intragenic recombination between the two sites of BLM mutation had generated a normal allele. Very exceptionally, however, persons with BS who exhibit mosaicism are homozygous for the causative mutation. In two such exceptional homozygous persons studied here, back mutation has been demonstrated: one person constitutionally was homozygous for the mutation 1544insA and the other for the mutation 2702G-->A. Revertant (low-SCE) lymphoblastoid cells in each person were heterozygous for their mutations, i.e., a normal allele was now present. The normal alleles must have arisen by back mutation in a precursor cell, in one person by the deletion of an A base and, in the other, the nucleotide substitution of a G base for an A base. Thus, back mutation now becomes, together with intragenic recombination, an important genetic mechanism to consider when explaining examples of a reversion of somatic cells to "normal" in persons with a genetically determined abnormal phenotype.

ATM-dependent phosphorylation and accumulation of endogenous BLM protein in response to ionizing radiation

Bloom's syndrome (BS), a rare genetic disease, arises through mutations in both alleles of the BLM gene which encodes a 3'-5' DNA helicase identified as a member of the RecQ family. BS patients exhibit a high predisposition to development of all types of cancer affecting the general population and BLM-deficient cells display a strong genetic instability. We recently showed that BLM protein expression is regulated during the cell cycle, accumulating to high levels in S phase, persisting in G2/M and sharply declining in G1, suggesting a possible implication of BLM in a replication (S phase) and/or post-replication (G2 phase) process. Here we show that, in response to ionizing radiation, BLM-deficient cells exhibit a normal p53 response as well as an intact G1/S cell cycle checkpoint, which indicates that ATM and p53 pathways are functional in BS cells. We also show that the BLM defect is associated with a partial escape of cells from the gamma-irradiation-induced G2/M cell cycle checkpoint. Finally, we present data demonstrating that, in response to ionizing radiation, BLM protein is phosphorylated and accumulates through an ATM-dependent pathway. Altogether, our data indicate that BLM participates in the cellular response to ionizing radiation by acting as an ATM kinase downstream effector.

Cancer predisposition caused by elevated mitotic recombination in Bloom mice

Bloom syndrome is a disorder associated with genomic instability that causes affected people to be prone to cancer. Bloom cell lines show increased sister chromatid exchange, yet are proficient in the repair of various DNA lesions. The underlying cause of this disease are mutations in a gene encoding a RECQ DNA helicase. Using embryonic stem cell technology, we have generated viable Bloom mice that are prone to a wide variety of cancers. Cell lines from these mice show elevations in the rates of mitotic recombination. We demonstrate that the increased rate of loss of heterozygosity (LOH) resulting from mitotic recombination in vivo constitutes the underlying mechanism causing tumour susceptibility in these mice.

Nuclear structure in normal and Bloom syndrome cells

Bloom syndrome (BS) is a rare cancer-predisposing disorder in which the cells of affected persons have a high frequency of somatic mutation and genomic instability. BLM, the protein altered in BS, is a RecQ DNA helicase. This report shows that BLM is found in the nucleus of normal human cells in the nuclear domain 10 or promyelocytic leukemia nuclear bodies. These structures are punctate depots of proteins disrupted upon viral infection and in certain human malignancies. BLM is found primarily in nuclear domain 10 except during S phase when it colocalizes with the Werner syndrome gene product, WRN, in the nucleolus. BLM colocalizes with a select subset of telomeres in normal cells and with large telomeric clusters seen in simian virus 40-transformed normal fibroblasts. During S phase, BS cells expel micronuclei containing sites of DNA synthesis. BLM is likely to be part of a DNA surveillance mechanism operating during S phase.

p53 splice acceptor site mutation and increased HsRAD51 protein expression in Bloom's syndrome GM1492 fibroblasts

GM1492 human diploid skin fibroblasts derived from a patient with Bloom's syndrome (BS), lack detectable p53 mRNA and protein as shown by Northern and Western blotting, and express an increased RecA-like activity. Here we demonstrate that the p53 gene is grossly intact in GM1492 cells according to Southern blotting. DNA sequencing did not reveal any mutations in the promoter region of p53. A highly sensitive RT-PCR produced a p53 cDNA fragment that was shorter than expected. DNA sequence analysis of p53 cDNA showed that exon 6 was missing, explaining the shorter PCR product. Furthermore, sequencing of genomic DNA revealed a base substitution at the nucleotide preceding the AG splice acceptor site of intron 5. The omission of exon 6 creates a frameshift at the junction of exons 5 and 7, and a premature stop codon in exon 7. The aberrant transcript is predicted to encode a truncated p53 protein containing 189 amino acid residues. Moreover, Western blotting demonstrated elevated HsRAD51 protein levels in GM1492 cells. The lack of sufficient levels of wild-type p53 and increased levels of HsRad51 protein may contribute to the elevated RecA-like activity in the GM1492 fibroblasts.

The establishment of telomerase-immortalized cell lines representing human chromosome instability syndromes

The limited life span of normal human cells represents a substantial obstacle for biochemical analysis, genetic manipulation and genetic screens. To overcome this technical barrier, immortal human cell lines are often derived from tumors or produced by transformation with viral oncogenes such as SV40 large T antigen. Cell lines produced by these approaches are invariably transformed, genomically unstable and display cellular properties that differ from their normal counterpart. It was recently shown that the ectopic expression of hTERT, encoding the catalytic subunit of human telomerase, can extend the life span of normal human cells without causing cellular transformation and genomic instability. In the present study, we have used hTERT to extend the life span of normal human skin fibroblasts derived from patients afflicted with syndromes of genomic instability and/or premature aging. Our results show that hTERT efficiently extends the life span without altering the characteristic phenotypic properties of the cells. Thus, the ectopic expression of telomerase represents a major improvement over the use of viral oncogenes for the establishment of human cell lines.

Myelodysplastic syndrome associated with monosomy 7 in a child with Bloom syndrome

Bloom syndrome is a genomic instability syndrome associated with predisposition to development of various types of malignancy. In this report, we described a 7-year-old boy with Bloom syndrome (BS) and myelodysplastic syndrome (MDS) associated with monosomy 7 and loss of the Y chromosome. To our knowledge, this was the first case with BS showing monosomy 7 and MDS during the early childhood period.

Mucinous carcinoma of the colon in a 16-year-old Turkish boy with Bloom syndrome: cytogenetic, histopathologic, TP53 gene and protein expression studies

A 17-year-old Turkish boy with Bloom syndrome (BS) developed mucinous carcinoma of the transverse colon. He was followed from 2 to 17 years of age. Increased sister chromatid exchanges (SCE) were observed, and he was diagnosed with BS at the age of 7. Sun-sensitive skin lesions were examined by skin biopsy, and histopathological studies of these lesions were done. During the follow-up period, an intraabdominal mass at the transverse colon was found, and mucinous carcinoma of colon was diagnosed at the age of 16. We examined TP53 protein expression from paraffin-embedded colon tissue of the patient with an immunohistochemical method. Polymerase chain reaction products of exons 4-9 of the TP53 gene were examined by SSCP. No evidence of overexpression of TP53 protein or mutations of the TP53 gene was observed. The patient in this report is the first case with a mucinous carcinoma of colon diagnosed at an early age in the Bloom Syndrome Registry. Based on our results, carcinoma of the colon in BS patient may occur earlier than 35 years of age and the TP53 gene may not be directly related to carcinoma in Bloom syndrome.

Sclerosing hyaline necrosis of the liver in Bloom syndrome

Bloom syndrome is a rare autosomal recessive disorder characterized by normally proportioned but strikingly small body size, a characteristic facies and photosensitive facial skin lesion, immunodeficiency, and a marked predisposition to development of a variety of cancers. We describe here, we believe for the first time, pronounced sclerosing hyaline necrosis with Mallory bodies in the liver of a patient with Bloom syndrome. Mallory bodies are cytoplasmic eosinophilic inclusions, which are more common in visibly damaged, swollen hepatocytes in various liver diseases but are never found in normal liver. The possible pathogenesis of this finding in Bloom syndrome is discussed.

Two siblings with Bloom's syndrome exhibit different clinical features: possible effect of sex

Bloom's syndrome is a rare autosomal recessive disease. All patients with Bloom's syndrome have prenatally onset growth retardation and an increased tendency to develop various types of cancer. Features other than these are not constant and may not be present in some of the patients. Reason for the phenotypic heterogeneity is not clear. Different mutations in the same locus may explain the heterogeneous phenotypes in different ethnic groups. Here we present a seven-year-old boy and his four-year-old sister, both with Bloom's syndrome, who exhibit different clinical features with respect to sun-sensitive skin lesions. The sister has severe facial sun-sensitive skin lesions whereas her brother has none. It is expected that two siblings who are supposed to have the same mutation should also have similar clinical features. Possible role of environmental effects and sex are discussed.

Differential expression of p53, p21waf1/cip1 and hdm2 dependent on DNA damage in Bloom's syndrome fibroblasts

The Bloom's syndrome gene, BLM, encodes a protein which bears homology to the RecQ helicases. It is believed to be involved in DNA replication and has been implicated in the maintenance of genomic stability. To investigate whether BLM was involved in cellular responses to DNA damage Bloom's syndrome fibroblasts were treated with either UV or ionizing radiation and the levels of p53 and two of its down stream effectors, p21waf1/cip1 and hdm2, were determined by western blot analysis. Following 20 J/m2 UVC-radiation we observed that the maximal accumulation of p21waf1/cip1 and hdm2 proteins preceded that of p53 in both a normal diploid fibroblast cell strain (GM0038) and in two Bloom's syndrome cell strains. Furthermore, the Bloom's syndrome cells demonstrated a delayed and prolonged accumulation of all three proteins and a delayed recovery of the protein levels back to pre-damage levels compared with the normal cell strain. Conversely, normal and Bloom's syndrome cell response following 2.5 Gy of ionizing radiation was quite similar for p21waf1/cip1 and hdm2, but differed significantly for p53. Maximum accumulation of p53 occurred within 2 h of damage and preceded that of p21waf1/cip1 and hdm2. These results suggest that the BLM protein may play a role in the detection of certain types of DNA damage and in the cellular response to that damage.

Histopathologic and ultrastructural study of lupus-like skin lesions in a patient with Bloom syndrome

The histopathology of the lupus-like skin lesions associated with Bloom syndrome has been sporadically described. Skin biopsies from a 2-year-old boy with the classical features of Bloom syndrome, including lupus-like skin lesions, demonstrated marked interface changes with basal liquefaction degeneration, a moderate superficial mononuclear infiltrate, pigmentary incontinence, and capillary dilation in the papillary dermis. Immunophenotyping of the dermal infiltrate revealed predominance of T-cells. Basement membrane thickening on periodic acid-Schiff examination was not seen. Direct immunofluorescence failed to demonstrate deposits of immunoglobulin other than nonspecific IgM deposition along the basement membrane zone of lesional skin. Ultrastructurally, the most striking findings were disintegration of basal cell cytoplasm and tubuloreticular inclusions in vascular endothelia. Taken together, the histologic and ultrastructural features of lupus-like lesions associated with Bloom syndrome mimic those of cutaneous lupus erythematosus, with the exception of paucity of immune deposits at the dermoepidermal junction.

Correction of the Bloom syndrome cellular phenotypes

Bloom syndrome (BLM) is a genetic disorder associated with predisposition to cancer and chromosome instability. However, the most readily recognized clinical feature of the syndrome is growth retardation. Introduction of the previously cloned BLM gene into BLM cells yielded correction of the chromosome instability and slow growth phenotypes. Additionally, asynchronous cultures of complemented clones revealed a lower percentage of cells in S-phase than uncomplemented BLM cells. These results support the notion that BLM is a defect in which short stature, chromosome instability and cancer predisposition are all associated with an error in DNA replication.

Microsatellite instability in B-cell lymphoma originating from Bloom syndrome

Bloom syndrome (BS) is a rare autosomal recessive genetic disorder characterized by lupus-like erythematous telangiectasias of the face, sun sensitivity, stunted growth infertility and immunodeficiency. In addition, BS patients are highly predisposed to cancers. Although recently the causative gene of BS (BLM) was identified as a DNA helicase homologue, the function of BLM in DNA replication has not been elucidated. In this study, p53 mutation and microsatellite instability in B-cell lymphomas originating from 2 sibling BS patients were investigated. In the originally developed tumor of both patients, no p53 mutation was detected. In one patient, however, after treatment by ionizing radiation the B-cell lymphoma recurred, showing a 9-bp deletion in exon 7. In lymphoma cells and an EB-virus-transformed cell line from BS lymphocytes of this patient, microsatellite instability was also detected from the reduced length of microsatellite DNA markers, although in the other patient microsatellite instability was not detected. Thus, 2 B-cell lymphomas, despite having the same BLM mutation, showed different phenotypes in terms of p53 mutation and microsatellite instability.

GM1492 human diploid skin fibroblasts lack the p53-dependent G1 cell-cycle checkpoint

Recently, we reported that GM1492 human diploid skin fibroblasts derived from a Bloom's patient upon UV-C irradiation fail to increase p53 to a detectable level and nevertheless accumulate in the G1-phase of the cell-cycle. Here we show that in GM1492 cells other types of DNA-damaging agents also fail to induce p53 as well as WAF1, a p53-regulated gene product involved in G1 cell-cycle arrest. Furthermore, the p53-dependent G1 cell-cycle checkpoint is indeed defective in these cells: However, induction of GADD45 mRNA still occurs in GM1492 after irradiation with UV-C. Since GADD45 is known to inhibit the entry into S, these data suggest that the observed accumulation of GM1492 cells in G1 after UV-C irradiation occurs at the G1/S boundary and is due to an inhibition of initiation of DNA-replication.

Diagnostic relevance of abortion-associated human embryonic antigen expressed on the cell surface of tumour promoter-treated Bloom syndrome cells

We detected stable expression of human embryonic antigen associated with spontaneous abortion (HEAA) on the cell surface of a tumour promoter-treated B lymphoblastoid cell line (BS-SHY) originating from Bloom syndrome. We used indirect immunofluorescence and diluted serum from 44 patients who had recurrent spontaneous abortions. With the use of the panning procedure, we separated characteristic cells expressing strong HEAA. The BS-SHY-HEAA cells separated here would be useful for measuring serum antibody (against HEAA) produced by patients with recurrent abortions. It was also noted that aborters who received husbands' leukocyte immunization have lost this antibody, and have delivered successful pregnancies at term. Using HEAA proteins, we conducted Western blotting analysis for the amino acid sequencing (mol. wt 77 kDa). Amino acid sequencing data indicated that HEAA had 87.5% homology to the immunoglobulin (Ig) VHIII region in the framework. Recently, the protective value of high dose i.v. administration of immunoglobulin in the treatment of recurrent spontaneous abortions has been reported to be similar to that of leukocyte immunization. Therefore, the BS-SHY-HEAA cells appear to provide a valuable tool for rapid serological diagnosis and for evaluating the efficacy of immunotherapy with husbands' leukocytes in patients with recurrent spontaneous abortions.

DNA mutation induced in the sequence upstream of the secreted MYU C-terminal coding sequence by ultraviolet irradiation in the cell line of Bloom's syndrome

Selective IgM deficiency is commonly found in Bloom's syndrome (BS). We reported that membrane-bound mu (micron(s)) mRNA was well transcribed but secreted mu (microseconds) mRNA was not, although there was no mutation or deletion in the sequence including the microseconds C-terminal coding sequence in the patients with BS. Furthermore, we have shown previously, preferential damage to IgM production by ultraviolet (UV) irradiation of the cells of the patient. In the study described here, mutation in the sequence which is upstream of the 5' end of the microseconds C-terminal coding sequence was induced by UV irradiation in the lymphoblastoid cell line (LCL) of BS patient. These results suggest that abnormal repair of DNA damage is present in this LCL, and that preferential damage to IgM production by UV irradiation in this LCL may be due to the abnormal repair of DNA damage.

Bloom's syndrome

Bloom's syndrome is a rare autosomal recessively transmitted disorder, the main clinical feature of which is small body size. A sun-sensitive, erythematous facial skin lesion, an excess of well-demarcated hyper- and hypopigmented skin lesions located anywhere on the body, and increased numbers of bacterial infections due to immunodeficiency are accompanying features of diagnostic value. In Bloom's syndrome, the complications are formidable: cancer, chronic lung disease, and diabetes. Cancers of the types and sites seen in the general population arise frequently and unusually early. Bloom's syndrome cells are hypermutable, and excessive numbers of somatic mutations are responsible for many of the clinical features. The clinical diagnosis is confirmed cytogenetically by demonstrating a characteristic chromosome instability.

Bloom's syndrome cells GM1492 lack detectable p53 protein but exhibit normal G1 cell-cycle arrest after UV irradiation

The tumor suppressor gene p53 is thought to be a key factor in the onset of G1 cell-cycle arrest following DNA damage. However, here we describe cells derived from a patient with Bloom's syndrome, lacking any detectable p53 protein, that still shows a functional G1 cell-cycle checkpoint after irradiation with UV-C. Comparison with cells from other Bloom's patients showed that the absence of p53 protein is not a specific characteristic of Bloom's syndrome.

Response of radiation-sensitive human cells to defined DNA breaks

We have investigated the response of four human cell lines, representing a range of sensitivities to ionizing radiation, to enzymes which induce defined DNA double-strand breaks (dsbs). Cell lines were derived from a normal individual, from the cancer-prone disorders ataxia-telangiectasia (AT) and Bloom's syndrome (BS), and from an immunodeficient individual (46BR). The molecular defects in AT and BS are unknown, while 46BR is known to be DNA ligase I deficient. We assayed the clonogenic survival of the cell lines following in vivo scission of the DNA by the restriction endonucleases PvuII and BanI. These two enzymes differ in their action; PvuII gives rise to dsbs with blunt termini, while BanI generates staggered ends with a 4 bp overhang. We found a correlation between the sensitivity of the cell lines to X-rays and to the blunt-end cutter PvuII, but not to the cohesive-end cutter BanI.

Characteristic mucinous ovarian cancer antigen is expressed in malignantly transformed Bloom's syndrome cells

Using a double-antibody panning procedure, we separated a unique cancer antigen cell line (BS-SHI-4M OVC-MU) expressing a mucinous ovarian cancer (OVC) antigen from a malignantly transformed Bloom's syndrome cell line. In order to gain information concerning a mucinous OVC antigen, we tested this unique cell line in the reaction to sera from patients with various OVCs, Krukenberg (KR) tumor, and signet ring cell cancer of the stomach under immunofluorescence and Western blotting protocols and determined the mucinous OVC antigen band at M(r) 84,000. We also undertook an immune electron microscopic study to gain information concerning the antigen-antibody reaction [BS-SHI-4M OVC-MU cells-sera from patients with mucinous OVC and KR tumor] and concerning the antigenic determinant of the membrane using preembedding methods. Occasional protein A-gold particles were observed along the cell membrane of BS-SHI-4M OVC-MU cells, when treated with sera from mucinous OVC and KR tumor patients, but no labeling was observed in the cell membrane when treated with sera from normal patients and those with other cancers. Results of the immune electron microscopic study strongly support the data from the antigen-antibody reaction obtained by immunofluorescence and Western blotting analyses. The BS-SHI-4M OVC-MU cells separated here would be useful for serodiagnosis of mucinous OVC and KR tumors and for follow-up of patients after therapy.

Non-endemic Burkitt's lymphoma in a patient with Bloom's syndrome

Bloom's syndrome is an autosomal recessive disorder characterized by intrauterine growth retardation, typical physical signs, immunodeficiency and an increased risk of developing neoplasms at a young age, compared to the general population. Factors possibly involved in the pathogenesis of non-endemic Burkitt's lymphoma in a five year old girl with Bloom's syndrome are discussed. These include immunodeficiency, upregulated c-myc expression and an Epstein-Barr viral infection.