Increased Chromosome Breakage Research Articles

Inactivating TDP2 missense mutation in siblings with congenital abnormalities reminiscent of fanconi anemia

Mutations in TDP2, encoding tyrosyl-DNA phosphodiesterase 2, have been associated with a syndromal form of autosomal recessive spinocerebellar ataxia, type 23 (SCAR23). This is a very rare and progressive neurodegenerative disorder described in only nine patients to date, and caused by splice site or nonsense mutations that result in greatly reduced or absent TDP2 protein. TDP2 is required for the rapid repair of DNA double-strand breaks induced by abortive DNA topoisomerase II (TOP2) activity, important for genetic stability in post-mitotic cells such as neurons. Here, we describe a sibship that is homozygous for the first TDP2 missense mutation (p.Glu152Lys) and which presents with clinical features overlapping both SCAR23 and Fanconi anemia (FA). We show that in contrast to previously reported SCAR23 patients, fibroblasts derived from the current patient retain significant levels of TDP2 protein. However, this protein is catalytically inactive, resulting in reduced rates of repair of TOP2-induced DNA double-strand breaks and cellular hypersensitivity to the TOP2 poison, etoposide. The TDP2-mutated patient-derived fibroblasts do not display increased chromosome breakage following treatment with DNA crosslinking agents, but both TDP2-mutated and FA cells exhibit increased chromosome breakage in response to etoposide. This suggests that the FA pathway is required in response to TOP2-induced DNA lesions, providing a possible explanation for the clinical overlap between FA and the current TDP2-mutated patients. When reviewing the relatively small number of patients with SCAR23 that have been reported, it is clear that the phenotype of such patients can extend beyond neurological features, indicating that the TDP2 protein influences not only neural homeostasis but also other tissues as well.

ERCC1 mutations impede DNA damage repair and cause liver and kidney dysfunction in patients.

ERCC1-XPF is a multifunctional endonuclease involved in nucleotide excision repair (NER), interstrand cross-link (ICL) repair, and DNA double-strand break (DSB) repair. Only two patients with bi-allelic ERCC1 mutations have been reported, both of whom had features of Cockayne syndrome and died in infancy. Here, we describe two siblings with bi-allelic ERCC1 mutations in their teenage years. Genomic sequencing identified a deletion and a missense variant (R156W) within ERCC1 that disrupts a salt bridge below the XPA-binding pocket. Patient-derived fibroblasts and knock-in epithelial cells carrying the R156W substitution show dramatically reduced protein levels of ERCC1 and XPF. Moreover, mutant ERCC1 weakly interacts with NER and ICL repair proteins, resulting in diminished recruitment to DNA damage. Consequently, patient cells show strongly reduced NER activity and increased chromosome breakage induced by DNA cross-linkers, while DSB repair was relatively normal. We report a new case of ERCC1 deficiency that severely affects NER and considerably impacts ICL repair, which together result in a unique phenotype combining short stature, photosensitivity, and progressive liver and kidney dysfunction.

Rad54/Rad54B deficiency is associated to increased chromosome breakage in mouse spermatocytes.

Rad54 protein is a key player of the homologous recombination pathway, required for deposition and stabilisation of Rad51 foci at double strand breaks. Its role at the meiotic prophase, when double strand breaks are physiologically introduced to allow recombination, is well described. However, the hypothesis that Rad54 deficiency affect chromosome integrity of germ cells in unirradiated and irradiated animals has not been tested yet. In this study, the occurrence of spontaneous and X-ray-induced chromosome aberrations was assessed by analysis of spermatocyte MI spreads or by application of micronucleus assay in early spermatids isolated from wild type and Rad54/Rad54B knockout (KO) mice. In Rad54/Rad54B KO mice, the spontaneous chromosome aberration frequency detected at MI was >10-fold higher than in wild type animals. In addition, after exposure to 1 Gy X-rays at the radiosensitive stage of diplotene, an enhanced response to radiation was observed in Rad54-deficient animals, corresponding to a 2-3 sensitivity factor in comparison to wild type mice. Also the spontaneous frequency of micronucleated round spermatids was on the average 10-fold higher in KO than in wild type mice, indicating that Rad54/Rad54B KO spermatocytes carrying chromosome aberrations are able to pass through the meiotic divisions and to continue the spermatogenesis process. Our results provide the first evidence of the role of Rad54/Rad54B in the maintenance of a stable karyotype during male meiosis, and suggest that Rad54/Rad54B deficiency may impact on the DNA integrity of developing mouse gametes.

Acute toxicity and genotoxicity of silver nanoparticle in rats.

ObjectiveThe potential risk of a nanoparticle as a medical application has raised wide concerns, and this study aims to investigate silver nanoparticle (AgNP)-induced acute toxicities, genotoxicities, target organs and the underlying mechanisms.MethodsSprague-Dawley rats were randomly divided into 4 groups (n = 4 each group), and AgNP (containing Ag nanoparticles and released Ag+, 5 mg/kg), Ag+ (released from the same dose of AgNP, 0.0003 mg/kg), 5% sucrose solution (vechicle control) and cyclophophamide (positive control, 40 mg/kg) were administrated intravenously for 24 h respectively. Clinical signs and body weight of rats were recorded, and the tissues were subsequently collected for biochemical examination, Ag+ distribution detection, histopathological examination and genotoxicity assays.ResultsThe rank of Ag detected in organs from highest to lowest is lung>spleen>liver>kidney>thymus>heart. Administration of AgNP induced a marked increase of ALT, BUN, TBil and Cre. Histopathological examination results showed that AgNP induced more extensive organ damages in liver, kidneys, thymus, and spleen. Bone marrow micronucleus assay found no statistical significance among groups (p > 0.05), but the number of aberration cells and multiple aberration cells were predominately increased from rats dosed with Ag+ and AgNP (p < 0.01), and more polyploidy cells were generated in the AgNP group (4.3%) compared with control.ConclusionOur results indicated that the AgNP accumulated in the immune system organs, and mild irritation was observed in the thymus and spleen of animals treated with AgNP, but not with Ag+. The liver and kidneys could be the most affected organs by an acute i.v. dose of AgNP, and significantly increased chromosome breakage and polyploidy cell rates also implied the potential genotoxicity of AgNP. However, particle-specific toxicities and potential carcinogenic effect remain to be further confirmed in a chronic toxicity study.

Dissecting the Roles of Divergent and Convergent Transcription in Chromosome Instability.

The interplay of transcription, topological tension, and chromosome breakage is a subject of intense interest, but, with so many facets to the problem, it is difficult to test. Here, we vary the orientation of promoters relative to one another in a yeast system that permits sensitive detection of chromosome breaks. Interestingly, convergent transcription that would direct RNA polymerases into one another does not increase chromosome breakage. In contrast, divergent transcription that would create underwound and potentially single-stranded DNA does cause a marked increase in chromosome breakage. Furthermore, we examine the role that topoisomerases are playing in preventing genome instability at these promoters and find that Top2 is required to prevent instability at converging promoters.

Exploring Cancer Development in Adulthood: Cholinesterase Depression and Genotoxic Effect From Chronic Exposure to Organophosphate Pesticides Among Rural Farm Children

ABSTRACT Children are the vulnerable group in the agricultural community due to their early exposure to pesticides through the dynamic interplay between genetic predisposition, environment, and host-related factors. This study aims to identify the possible association between the depression in blood cholinesterase level and genotoxic effect among farm children. The results of micronuclei assay and comet assay showed that the reduced blood cholinesterase level from organophosphate pesticide exposure is significantly associated with an increase in chromosome breakage and DNA strand breaks. These genotoxicity end points suggest that farm children's cells experience early DNA damage that may lead to uncontrolled cell proliferation during their adulthood. Thus, farm children who grow up near pesticide-treated farmland have a higher probability of developing cancer than children with minimal or zero exposure to pesticides.

Telomere Attrition and Chromosomal Instability During Long-Term Cultivation of Hematopoietic Stem Cells,

Abstract 4001By means of a large RNA interference screen, Ebert et al. identified RPS14 as the target of del(5q), since haploinsufficiency of RPS14 in human hematopoietic stem cells (HSC) induces an erythroid differentiation defect, a characteristic feature of myelodysplastic syndromes (MDS) with isolated deletion in 5q (Nature451, 335–339 (2008)). However, to the best of our knowledge, these cells have not been observed for more than two weeks in culture. We have shown that telomere shortening and chromosomal instability play a crucial role, particularly during progression of MDS with del(5q) into acute myeloid leukemia (AML) (K Lange et al, Genes Chromosomes Cancer49, 260–269 (2010) and G Göhring et al, Leukemia, 2011 (in press)). Because of the limited proliferation of primary MDS cells in culture and because there is no appropriate MDS cell line, a good human cell culture model is urgently needed to investigate the relationship of telomere attrition and chromosomal instability during the progression of MDS with del(5q).Therefore, it was our aim was to create a long-term culture (LTC) model to observe the effects of RPS14 haploinsufficiency on differentiation capacity, DNA repair, telomere shortening and chromosomal instability in human HSCs.CD34+ cells were isolated from umbilical cord blood via magnetic cell separation. Knockdown of RPS14 to a level of 40 – 50% was performed via lentiviral transduction of shRNA vectors. As controls, in addition to untransduced mock cells, CD34+ cells were transduced with scrambled shRNA. LTC was performed on murine feeder layers (M2-10B4).In short-term culture, as expected, knockdown of RPS14 led to an erythroid differentiation defect, decreased proliferative activity and an increased level of apoptosis. Cultivation on murine feeder layers enabled expansion of CD34+ cells for more than 6 weeks.Generally, within 6 weeks RPS14 -deficient CD34+ cells showed a poorer proliferation than the control cells with a 214-fold versus a 6080-fold multiplication, respectively. Median numbers of γH2AX foci, indicators of DNA double-strand breaks, did not differ between RPS14-deficient and control cells with a median number of 9.4 and 8.2 foci/cell, respectively. Induction of γH2AC foci via Mitomycin C, a DNA cross-linking agent inducing double-strand breaks, did not reveal an altered DNA repair capacity with 14.3 and 11.6 foci/cell.Colony-forming assays showed that, even though RPS14 haploinsufficiency immediately led to an erythroid differentiation defect, this differentiation capacity decreased even further within the weeks of follow-up. This however, although with a delay of about 4 weeks, could also be observed in the control cells (Figure 1). [Display omitted] Telomere length decreased in RPS14- deficient cells as well as in control cells. Initially, telomeres showed a median length of about 12.5 kb, decreasing to a median length of 10.3 kb (range 8.4 – 11.4 kb) and 5.8 kb (range 5.7 – 8.9 kb) after 2 and 4 weeks, respectively, which afterwards elongated to a level of 8.7 kb (range 8.5 –8.9 kb) after 6 weeks. The mechanism underlying this primary shortening and subsequent re-elongation might be due to an up-regulation of telomerase or of the alternative lengthening of telomeres (ALT) mechanism. Cytogenetic investigations demonstrated an increase in chromosomal breakage in all cultures, pointing towards induction during LTC rather than due to RPS14 deficiency. Remarkably, chromosomal breakage and chromosome aberrations in single cells within the first 4 weeks of cultivation of hematopoietic stem cells transduced with scrambled shRNA was followed by clonal dominance of monosomy 7 after 6 weeks. This may be an effect caused by insertional mutagenesis. LM-PCR is currently under way to obtain insights into the affected insertion site(s). Monosomy 7 is one of the most frequent chromosome aberrations in MDS and frequently occurs as an additional aberration in MDS/AML with inversion 3q generating an MDS-EVI1 fusion transcript. Thus, it will be interesting to see whether monosomy 7 in our LTC model cooperates with EVI1 activation as in sporadic MDS and AML.In conclusion, a LTC model on feeder layer cells seems to be an appropriate system to analyze hematopoietic stem cells and MDS in culture. However, cell culture artefacts inducing telomere shortening and chromosomal instability or insertional mutagenesis have to be taken into account and regular cytogenetic analyses should always be performed. Disclosures:No relevant conflicts of interest to declare.

X‐linked VACTERL with hydrocephalus syndrome: Further delineation of the phenotype caused by FANCB mutations

X-linked VACTERL-hydrocephalus syndrome (X-linked VACTERL-H) is a rare disorder caused by mutations in the gene FANCB which underlies Fanconi Anemia (FA) complementation group B. Cells from affected males have increased chromosome breakage on exposure to DNA cross-linking agents. Only five FANCB mutations found in six affected males, including an affected uncle and nephew, have been reported. We have identified FANCB mutations in a further four affected families. The VACTERL-H phenotype segregates as an X-linked recessive trait in three of these. Each mutation is predicted to truncate the FANCB open reading frame and results in highly skewed X-inactivation in unaffected carrier females. Phenotypic data were available on six affected males. Comparison of the clinical findings in our patients with published clinical data (total 12 patients) shows that ventriculomegaly, bilateral absent thumbs and radii, vertebral defects, renal agenesis, and growth retardation are the major phenotypic signs in affected males. Less frequent are brain, pituitary, ear and eye malformations, gastrointestinal atresias (esophageal, duodenal and anal), tracheoesophageal fistula, lung segmentation defects, and small genitalia. Three of six of our patients survived the perinatal period. One boy lived up to 2 years 10 months but developed aplastic anemia and died of renal failure. These data show that loss-of-function FANCB mutations result in a recognizable, multiple malformation phenotype in hemizygous males for which we propose clinical criteria to aid diagnosis.

Fanconi anemia‐like presentation in an infant with constitutional deletion of 21q including the RUNX1 gene

We describe a newborn female with a de novo interstitial deletion of chromosome 21q21.1-22.12 including the RUNX1 gene who had developmental delay, multiple congenital anomalies, tetralogy of Fallot, anemia, and chronic thromobocytopenia requiring frequent platelet transfusions from birth. Because of her physical and hematologic abnormalities, she was tested for Fanconi anemia (FA). Lymphocytes and fibroblasts from this patient demonstrated increased chromosome breakage with exposure to the clastogen mitomycin C, but not, in contrast to most FA patients, to diepoxybutane. Further testing by Western analysis and complementation testing did not show a defect in the function of known Fanconi proteins. Her constitutional deletion was later found to span 13.2 Mb by chromosome microarray analysis, encompassing the RUNX1 gene that has been implicated in thrombocytopenia and predisposition to acute myelogenous leukemia (AML) when in the haploinsufficient state. We compare her phenotype to other individuals with similar 21q deletions and thrombocytopenia, as well as those with FA. We suggest that deletion of RUNX1 or another critical gene within the deleted region may result in chromosomal instability similar to that seen in FA.

An AT-Rich Sequence in Human Common Fragile Site FRA16D Causes Fork Stalling and Chromosome Breakage in S. cerevisiae

Common fragile sites are regions of human chromosomes prone to breakage. Fragile site FRA16D spans the WWOX/FOR tumor suppressor gene and has been linked to cancer-causing deletions and translocations. Using a genetic assay in yeast, we found that a short AT-rich region (Flex1) within FRA16D increases chromosome fragility, whereas three other sequences within FRA16D do not. To our knowledge, this is the first identification of a sequence element within a common fragile site that increases chromosome fragility. The fragility of Flex1 was exacerbated by the absence of Rad52 or the presence of hydroxyurea. Flex1 contains a polymorphic AT repeat predicted to form a DNA structure, and two-dimensional gel analysis showed accumulation of stalled replication forks at the Flex1 sequence that was dependent on AT length. Our data suggest that the FRA16D Flex1 sequence causes increased chromosome breakage by forming secondary structures that stall replication fork progression.

Chromosomal Instability at Common Fragile Sites in Seckel Syndrome

Seckel syndrome (SCKL) is a rare, genetically heterogeneous disorder, with dysmorphic facial appearance, growth retardation, microcephaly, mental retardation, variable chromosomal instability, and hematological disorders. To date, three loci have been linked to this syndrome, and recently, the gene encoding ataxia-telangiectasia and Rad3-related protein (ATR) was identified as the gene mutated at the SCKL1 locus. The ATR mutation affects splicing efficiency, resulting in low levels of ATR in affected individuals. Elsewhere, we reported increased instability at common chromosomal fragile sites in cells lacking the replication checkpoint gene ATR. Here, we tested whether cells from patients carrying the SCKL1 mutation would show increased chromosome breakage following replication stress. We found that, compared with controls, there is greater chromosomal instability, particularly at fragile sites, in SCKL1-affected patient cells after treatment with aphidicolin, an inhibitor of DNA polymerase alpha and other polymerases. The difference in chromosomal instability between control and patient cells increases at higher levels of aphidicolin treatment, suggesting that the low level of ATR present in these patients is not sufficient to respond appropriately to replication stress. This is the first human genetic syndrome associated with increased chromosome instability at fragile sites following replication stress, and these findings may be related to the phenotypic findings in patients with SCKL1.

The 4N Cell Cycle Delay in Fanconi Anemia Reflects Growth Arrest in Late S Phase

Fanconi anemia (FA) is a human genetic disorder characterized by hypersensitivity to DNA crosslinking agents. Its cellular phenotypes include increased chromosome breakage and a marked cell-cycle delay with 4N DNA content after introduction of interstrand DNA crosslinks (ICL). To further understand the nature of this delay previously described as a G2/M arrest, we introduced ICL specifically during G2 and monitored the cells for passage into mitosis. Our results showed that, even at the highest doses, postreplication ICL produced neither G2/M arrest nor chromosome breakage in FA-A or FA-C cells. This suggests that, similar to wild-type cells, DNA replication is required to trigger both responses. Therefore, the 4N cell DNA content observed in FA cells after ICL treatment also represents incomplete DNA replication and arrest in late S phase. FA fibroblasts from complementation groups A and C were able to recover from the ICL-induced cell-cycle arrest, but took ∼3 times longer than controls. These results indicate that the FA pathway is required for the efficient resolution of ICL-induced S-phase arrest.

Isochromosome (7)(q10) in Shwachman Syndrome Without MDS/AML and Role of Chromosome 7 Anomalies in Myeloproliferative Disorders

Shwachman syndrome (SS) is an autosomal recessive disorder in which bone marrow dysfunction is observed, with development of myelodysplastic syndromes (MDS) and acute myeloid leukemias (AML) in up to one third of the cases. Inconclusive data are available as to increased chromosome breakage in SS, while chromosome 7 anomalies, and often an isochromosome (7)(q10), are frequent in cases with MDS/AML. We report on the consistent presence of an i(7)(q10) in the bone marrow and blood lymphocytes in one of two sisters affected with SS without any clinical or cytological signs of MDS/AML. Thus, this patient was either a case of constitutional mosaicism for the i(7)(q10), or this had to be acquired in a nondysplastic and non-neoplastic marrow clone. DNA polymorphism analysis demonstrated the paternal origin of the i(7q). We postulate that the SS mutation acts as a mutator gene, and causes karyotype instability; abnormal clones would thus arise in the marrow, and chromosome 7 anomalies, i(7q) in particular, will in turn lead to MDS/AML. If this interpretation is correct, it would be also an indication to consider chromosome 7 anomalies in general, out of SS, as primary changes in MDS/AML pathogenesis.

Evaluation of micronuclei and chromosomal breakage in the 1cen-q12 region by the butadiene metabolites epoxybutene and diepoxybutane in cultured human lymphocytes.

1,3-Butadiene is a widely used industrial chemical and common environmental pollutant that has been associated with increased risks of leukemias and lymphomas. Butadiene and its metabolites, 1, 2-epoxybutene (EB) and diepoxybutane (DEB), have been shown to be genotoxic in a wide variety of test organisms. The objective of this research was to evaluate techniques for the rapid detection of chromosomal alterations occurring in humans exposed to butadiene. We have used a multicolored fluorescence in situ hybridization (FISH) method and the CREST-modified micronucleus assay to detect chromosomal breakage induced by EB (10-300 microM) and DEB (0.5-10 microM) in cultured human lymphocytes. A significant dose-related increase in the formation of micronuclei was seen in lymphocytes treated with DEB at concentrations as low as 2.5 microM, but not with EB over the dose range tested. Over 80% of the micronuclei induced by DEB were CREST-negative, indicating their origin from chromosomal breakage. Multicolor FISH using two adjacent chromosome-specific probes showed a significant increase in chromosomal breakage in the 1cen-q12 region induced by DEB at concentrations as low as 2.5 microM, but not by EB. Since DEB is likely to be one of the metabolites contributing to the genotoxic effects of butadiene, the sensitivity of the tandem FISH approach to detect breakage induced by diepoxybutane indicates that this technique may be useful for monitoring chromosomal alterations in butadiene-exposed workers.

Ashkenazi Jewish population frequency of the Bloom syndrome gene 2281 delta 6ins7 mutation.

Bloom syndrome is an autosomal recessive disorder characterized clinically by small size, sun-sensitive facial erythema, and immunodeficiency, and cytogenetically by increased chromosome breakage and sister chromatid exchange. Genomic instability renders Bloom syndrome patients at elevated risk for multiple cancers. Bloom syndrome occurs most commonly in the Ashkenazi Jewish population due to an apparent founder effect. The BLM gene on chromosome 15q26.1 was identified to encode a RecQ DNA helicase. Multiple mutations were identified, with Ashkenazi Jewish Bloom syndrome patients almost exclusively homozygous for a complex frameshift mutation (6-bp deletion/7-bp insertion at BLM nucleotide 2,281). This molecular genetic study seeks to verify the Ashkenazi Jewish carrier frequency of the BLM 2281 delta 6ins7 allele using semiautomated allele-specific oligonucleotide (ASO) analysis. Anonymized DNA samples from 1,016 Ashkenazi Jewish individuals and 307 non-Jewish individuals were screened. Ten Ashkenazi heterozygote carriers for the 2281 delta 6ins7 mutation were identified, giving a carrier frequency estimate of 0.98%, or approximately 1 carrier out of 102 individuals in the Ashkenazi Jewish population. These results are consistent with previous estimates, and combining our findings with the published molecular data collectively yields an Ashkenazi Jewish carrier frequency of approximately 1 in 104. Given its high population frequency and detection rate among Ashkenazi Jewish patients, the blmAsh mutation constitutes an appropriate addition to screening panels for Ashkenazi Jewish disease testing.

Cytogenetic abnormalities in Langerhans cell histiocytosis.

We present the cytogenetic investigations of five histiocytic tumour lesions from children. In four cases there was a confirmed diagnosis of Langerhans cell histiocytosis (LCH) and one case of histiocytosis that did not fulfil all the criteria for true LCH. All five cases showed cytogenetic abnormalities, including the first report of an abnormal clone in LCH. The clone showed a t(7;12)(q11.2;p13) translocation and was detected in only a small percentage of cells. This case and a further three also contained non-clonal abnormalities and an increase in chromosome breakage. The fifth case, the only one in which no acquired abnormalities were seen, had a constitutional paracentric inversion of chromosome 13q.

Genomic instability associated with limb defects

We report a child in whom we observed markedly increased genome-wide spontaneous chromosomal breakage in both leucocytes and fibroblasts associated with severe growth retardation, radial aplasia, leucopenia, mild hydrocephalus and an unusual trichodystrophy. Exposure to DNA cross-linking agents diepoxybutane, mitomycin-C and mustine hydrochloride in this case did not result in the increased chromosome breakage seen in Fanconi anaemia. It may be that this child has a defect in postreplicative DNA repair interacting with the protein components deficient in FA.

Increased chromosome radiosensitivity during pregnancy

It was necessary to consider the risks of exposure of pregnant women, not only in relation to the child, but also in relation to their own hypersensitivity. We have demonstrated that pregnancy increases radiosensitivity of chromosome in the mouse at the end of gestation. This is of importance since it may have implications on radioprotection of pregnant women and give experimental guidelines to the problems of hypersensitivity to drugs and cancer aggravation during pregnancy. Blood obtained from women at various times of pregnancy was exposed to ionizing radiations. By comparison to non-pregnant women, an increase in chromosome breakage was observed in metaphases from lymphocytes, after short-term culture in the presence of the serum of the same donor. Immediately after delivery, this increase in radiosensitivity disappeared. In a prospective study, serial analyses showed a very strong correlation between the amount of pregnancy hormones, progesterone in particular, and the increase in radiosensitivity. Pregnant women may have an increased sensitivity to ionizing radiation during the second half of their pregnancy. This study provides the first evidence in human that radiosensitivity may vary in relation to physiological conditions.

Clastogenic factors in plasma of HIV-1 infected patients

The objective of this study was to investigate the clastogenic activity of plasma ultrafiltrates from HIV-I infected patients. Clastogenic factors are chromosome-damaging agents with low molecular weight (<10,000 daltons) which cause chromosome aberrations, sister chromatid exchanges, DNA strand breakage, and gene mutation. They have first been described in the plasma of irradiated persons, but they are also found in hereditary breakage syndromes and chronic inflammatory diseases with autoimmune reactions. Their formation and their clastogenic effects are modulated by superoxide anion radicals. We analyzed a total of 22 HIV-1 positive patients in comparison to 20 reference plasma samples from healthy HIV negative blood donors of similar age. The plasma ultrafiltrates (filter cutoff 10,000 daltons) from patients induced a statistically significant increase in chromosomal breakage in the cytogenetic test system (20.5 ± 6.8 aberrations per 100 cells), while no increase was observed in test cultures exposed to plasma ultrafiltrates from healthy blood donors (6.3 ± 2.9 aberrations per 100 cells). The breakage values were slightly, but not significantly, lower in the 10 patients with more than 200 T-helper cells/ml (18 ± 4 aberrations per 100 cells), than in the 12 patients with less than 200 T-helper cells/ml (22.3 ± 7.9 aberrations per 100 cells). HIV patients with high clastogenic activity (induction of more than 20 aberrations per 100 cells, range 20 to 39) showed higher plasma levels for malondialdehyde than those with lower clastogenic activity (less than 20 aberrations per 100 cells, range 12 to 18). However, the difference was statistically not significant. Another lipid peroxidation product, 4-hydroxynonenal, was increased equally in both groups. There were no significant differences in water- and lipid-soluble plasma antioxidants between the low- and high-breakage group. In agreement with previous findings, the clastogenic effects of plasma ultrafiltrates in the test cultures were reduced by the antioxidant enzyme superoxide dismutase. The presence of clastogenic factors in the plasma of HIV patients is further evidence for a prooxidant state in these persons. Since clastogenic factor formation appears to occur at an early stage of the disease, it may be significant for virus release or activation, because of the superoxide anion stimulating effects of clastogenic factors. From a practical standpoint, clastogenic factors may be useful for evaluation of promising drugs.

Localisation of the Fanconi anaemia complementation group A gene to chromosome 16q24.3.

Fanconi anaemia (FA) is an autosomal recessive disorder associated with diverse developmental abnormalities, bone-marrow failure and predisposition to cancer. FA cells show increased chromosome breakage and hypersensitivity to DNA cross-linking agents such as diepoxybutane and mitomycin C. Somatic-cell hybridisation analysis of FA cell lines has demonstrated the existence of at least five complementation groups (FA-A to FA-E), the most common of which is FA-A. This genetic heterogeneity has been a major obstacle to the positional cloning of FA genes by classical linkage analysis. The FAC gene was cloned by functional complementation, and localised to chromosome 9q22.3 (ref. 2), but this approach has thus far failed to yield the genes for the other complementation groups. We have established a panel of families classified as FA-A by complementation analysis, and used them to search for the FAA gene by linkage analysis. We excluded the previous assignment by linkage of an FA gene to chromosome 20q, and obtained conclusive evidence for linkage of FAA to microsatellite markers on chromosome 16q24.3. Strong evidence of allelic association with the disease was detected with the marker D16S303 in the Afrikaner population of South Africa, indicating the presence of a founder effect.