The impact of comparative genomic hybridization/single-nucleotide polymorphism microarray in risk stratification of pediatric acute lymphoblastic leukemia.

BACKGROUND: The objective of this study is to assess the concordance and added value of combined comparative genomic hybridization plus single-nucleotide polymorphism microarray (CGH/SNP) analyses in pediatric acute lymphoblastic leukemia (ALL) risk stratification compared to conventional cytogenetic methods. PROCEDURE: This is a retrospective study that included patients aged 1-18 years diagnosed with de novo ALL at Sainte-Justine Hospital between 2016 and 2021. Results from conventional cytogenetic and molecular analyses were collected and compared to those of CGH/SNP. RESULTS: A total of 135 ALL patients were included. Sample failures or non-diagnostic analyses occurred in 17.8% cases with G-banding karyotypes versus 1.5% cases with CGH/SNP. The mean turnaround time for results was significantly faster for CGH/SNP than karyotype with 5.8 versus 10.7 days, respectively. The comparison of ploidy assessment by CGH/SNP and G-banding karyotype showed strong concordance (r = .82, p < .001, r2 = .68). Furthermore, G-banding karyotype did not detect additional clinically relevant aberrations that were missed by the combined analysis of CGH/SNP and fluorescence in situ hybridization. The most common gene alterations detected by CGH/SNP were deletions involving CDKN2A (35.8%), ETV6 (31.3%), CDKN2B (28.4%), PAX5 (20.1%), IKZF1 (12.7%), and copy-neutral loss of heterozygosity (CN-LOH) of 9p (9.0%). Among these, only ETV6 deletion was found to have a significant prognostic impact with superior event-free survival in both univariate and multivariate analyses (adjusted hazard ratio 0.08, 95% confidence interval: 0.01-0.50, p = .02). CONCLUSION: CGH/SNP provided faster, reliable, and highly concordant results than those obtained by conventional cytogenetics. CGH/SNP identified recurrent gene deletions in pediatric ALL, of which ETV6 deletion conferred a favorable prognosis.

Genome-wide analysis of gene dosage in 24,092 individuals estimates that 10,000 genes modulate cognitive ability.

Genomic copy number variants (CNVs) are routinely identified and reported back to patients with neuropsychiatric disorders, but their quantitative effects on essential traits such as cognitive ability are poorly documented. We have recently shown that the effect size of deletions on cognitive ability can be statistically predicted using measures of intolerance to haploinsufficiency. However, the effect sizes of duplications remain unknown. It is also unknown if the effect of multigenic CNVs are driven by a few genes intolerant to haploinsufficiency or distributed across tolerant genes as well. Here, we identified all CNVs > 50 kilobases in 24,092 individuals from unselected and autism cohorts with assessments of general intelligence. Statistical models used measures of intolerance to haploinsufficiency of genes included in CNVs to predict their effect size on intelligence. Intolerant genes decrease general intelligence by 0.8 and 2.6 points of intelligence quotient when duplicated or deleted, respectively. Effect sizes showed no heterogeneity across cohorts. Validation analyses demonstrated that models could predict CNV effect sizes with 78% accuracy. Data on the inheritance of 27,766 CNVs showed that deletions and duplications with the same effect size on intelligence occur de novo at the same frequency. We estimated that around 10,000 intolerant and tolerant genes negatively affect intelligence when deleted, and less than 2% have large effect sizes. Genes encompassed in CNVs were not enriched in any GOterms but gene regulation and brain expression were GOterms overrepresented in the intolerant subgroup. Such pervasive effects on cognition may be related to emergent properties of the genome not restricted to a limited number of biological pathways.

Pervasive and cooperative deadenylation of 3'UTRs by embryonic microRNA families.

To understand how miRNA-mediated silencing impacts on embryonic mRNAs, we conducted a functional survey of abundant maternal and zygotic miRNA families in the C. elegans embryo. We show that the miR-35-42 and the miR-51-56 miRNA families define maternal and zygotic miRNA-induced silencing complexes (miRISCs), respectively, that share a large number of components. Using a cell-free C. elegans embryonic extract, we demonstrate that the miRISC directs the rapid deadenylation of reporter mRNAs with natural 3'UTRs. The deadenylated targets are translationally suppressed and remarkably stable. Sampling of the predicted miR-35-42 targets reveals that roughly half are deadenylated in a miRNA-dependent manner, but with each target displaying a distinct efficiency and pattern of deadenylation. Finally, we demonstrate that functional cooperation between distinct miRISCs within 3'UTRs is required to potentiate deadenylation. With this report, we reveal the extensive and direct impact of miRNA-mediated deadenylation on embryonic mRNAs.

Mammalian miRNA RISC recruits CAF1 and PABP to affect PABP-dependent deadenylation.

MicroRNAs (miRNAs) inhibit mRNA expression in general by base pairing to the 3'UTR of target mRNAs and consequently inhibiting translation and/or initiating poly(A) tail deadenylation and mRNA destabilization. Here we examine the mechanism and kinetics of miRNA-mediated deadenylation in mouse Krebs-2 ascites extract. We demonstrate that miRNA-mediated mRNA deadenylation occurs subsequent to initial translational inhibition, indicating a two-step mechanism of miRNA action, which serves to consolidate repression. We show that a let-7 miRNA-loaded RNA-induced silencing complex (miRISC) interacts with the poly(A)-binding protein (PABP) and the CAF1 and CCR4 deadenylases. In addition, we demonstrate that miRNA-mediated deadenylation is dependent upon CAF1 activity and PABP, which serves as a bona fide miRNA coactivator. Importantly, we present evidence that GW182, a core component of the miRISC, directly interacts with PABP via its C-terminal region and that this interaction is required for miRNA-mediated deadenylation.

Diagnostic utility of molecular and cytogenetic analysis in lipoblastoma: a study of two cases and review of the literature.

AIMS: Lipoblastoma is a benign neoplasm of embryonic white fat tissue that results from the proliferation of primitive adipocytes, in which histological features can be ambiguous. In order to discriminate between lipoblastoma and other lipogenic and lipomatous tumours, we studied chromosomal alterations and protein expression in two cases of lipoblastoma in infants. METHODS AND RESULTS: Standard cytogenetic analysis, fluorescence in-situ hybridization, array comparative genomic hybridization and Western blotting allowed us to demonstrate the presence of chromosome abnormalities involving the 8q11-13 region containing the pleomorphic adenoma gene 1 (PLAG1), which are classically reported in lipoblastoma, and aberrant expression of PLAG1. CONCLUSIONS: This report illustrates two different tumorigenic pathways implicating PLAG1 in lipoblastoma: amplification through multiple copies of a small marker chromosome derived from chromosome 8, and a paracentric inversion of the long arm of chromosome 8. Both these anomalies induced aberrant expression of PLAG1, emphasizing the role of PLAG1 in tumorigenesis. The aberrant expression of PLAG1 protein has been hypothesized, but this is the first report to demonstrate its occurrence in lipoblastoma.

Mutations in TMEM231 cause Joubert syndrome in French Canadians.

BACKGROUND: Joubert syndrome (JBTS) is a predominantly autosomal recessive disorder characterised by a distinctive midhindbrain malformation, oculomotor apraxia, breathing abnormalities and developmental delay. JBTS is genetically heterogeneous, involving genes required for formation and function of non-motile cilia. Here we investigate the genetic basis of JBTS in 12 French-Canadian (FC) individuals. METHODS AND RESULTS: Exome sequencing in all subjects showed that six of them carried rare compound heterozygous mutations in CC2D2A or C5ORF42, known JBTS genes. In addition, three individuals (two families) were compound heterozygous for the same rare mutations in TMEM231(c.12T>A[p.Tyr4*]; c.625G>A[p.Asp209Asn]). All three subjects showed a severe neurological phenotype and variable presence of polydactyly, retinopathy and renal cysts. These mutations were not detected among 385 FC controls. TMEM231 has been previously shown to localise to the ciliary transition zone, and to interact with several JBTS gene products in a complex involved in the formation of the diffusion barrier between the cilia and plasma membrane. siRNA knockdown of TMEM231 was also shown to affect barrier integrity, resulting in a reduction of cilia formation and ciliary localisation of signalling receptors. CONCLUSIONS: Our data suggest that mutations in TMEM231 cause JBTS, reinforcing the relationship between this condition and the disruption of the barrier at the ciliary transition zone.

The helicase protein DHX29 promotes translation initiation, cell proliferation, and tumorigenesis.

Translational control plays an important role in cell growth and tumorigenesis. Cap-dependent translation initiation of mammalian mRNAs with structured 5'UTRs requires the DExH-box protein, DHX29, in vitro. Here we show that DHX29 is important for translation in vivo. Down-regulation of DHX29 leads to impaired translation, resulting in disassembly of polysomes and accumulation of mRNA-free 80S monomers. DHX29 depletion also impedes cancer cell growth in culture and in xenografts. Thus, DHX29 is a bona fide translation initiation factor that potentially can be exploited as a target to inhibit cancer cell growth.

Measuring and Estimating the Effect Sizes of Copy Number Variants on General Intelligence in Community-Based Samples.

Importance;: Copy number variants (CNVs) classified as pathogenic are identified in 10% to 15% of patients referred for neurodevelopmental disorders. However, their effect sizes on cognitive traits measured as a continuum remain mostly unknown because most of them are too rare to be studied individually using association studies. Objective: To measure and estimate the effect sizes of recurrent and nonrecurrent CNVs on IQ. Design, Setting, and Participants: This study identified all CNVs that were 50 kilobases (kb) or larger in 2 general population cohorts (the IMAGEN project and the Saguenay Youth Study) with measures of IQ. Linear regressions, including functional annotations of genes included in CNVs, were used to identify features to explain their association with IQ. Validation was performed using intraclass correlation that compared IQ estimated by the model with empirical data. Main Outcomes and Measures: Performance IQ (PIQ), verbal IQ (VIQ), and frequency of de novo CNV events. Results: The study included 2090 European adolescents from the IMAGEN study and 1983 children and parents from the Saguenay Youth Study. Of these, genotyping was performed on 1804 individuals from IMAGEN and 977 adolescents, 445 mothers, and 448 fathers (484 families) from the Saguenay Youth Study. We observed 4928 autosomal CNVs larger than 50 kb across both cohorts. For rare deletions, size, number of genes, and exons affect IQ, and each deleted gene is associated with a mean (SE) decrease in PIQ of 0.67 (0.19) points (P = 6 × 10-4); this is not so for rare duplications and frequent CNVs. Among 10 functional annotations, haploinsufficiency scores best explain the association of any deletions with PIQ with a mean (SE) decrease of 2.74 (0.68) points per unit of the probability of being loss-of-function intolerant (P = 8 × 10-5). Results are consistent across cohorts and unaffected by sensitivity analyses removing pathogenic CNVs. There is a 0.75 concordance (95% CI, 0.39-0.91) between the effect size on IQ estimated by our model and IQ loss calculated in previous studies of 15 recurrent CNVs. There is a close association between effect size on IQ and the frequency at which deletions occur de novo (odds ratio, 0.86; 95% CI, 0.84-0.87; P = 2.7 × 10-88). There is a 0.76 concordance (95% CI, 0.41-0.91) between de novo frequency estimated by the model and calculated using data from the DECIPHER database. Conclusions and Relevance: Models trained on nonpathogenic deletions in the general population reliably estimate the effect size of pathogenic deletions and suggest omnigenic associations of haploinsufficiency with IQ. This represents a new framework to study variants too rare to perform individual association studies and can help estimate the cognitive effect of undocumented deletions in the neurodevelopmental clinic.

Indexing Effects of Copy Number Variation on Genes Involved in Developmental Delay.

A challenge in clinical genomics is to predict whether copy number variation (CNV) affecting a gene or multiple genes will manifest as disease. Increasing recognition of gene dosage effects in neurodevelopmental disorders prompted us to develop a computational approach based on critical-exon (highly expressed in brain, highly conserved) examination for potential etiologic effects. Using a large CNV dataset, our updated analyses revealed significant (P < 1.64 × 10(-15)) enrichment of critical-exons within rare CNVs in cases compared to controls. Separately, we used a weighted gene co-expression network analysis (WGCNA) to construct an unbiased protein module from prenatal and adult tissues and found it significantly enriched for critical exons in prenatal (P < 1.15 × 10(-50), OR = 2.11) and adult (P < 6.03 × 10(-18), OR = 1.55) tissues. WGCNA yielded 1,206 proteins for which we prioritized the corresponding genes as likely to have a role in neurodevelopmental disorders. We compared the gene lists obtained from critical-exon and WGCNA analysis and found 438 candidate genes associated with CNVs annotated as pathogenic, or as variants of uncertain significance (VOUS), from among 10,619 developmental delay cases. We identified genes containing CNVs previously considered to be VOUS to be new candidate genes for neurodevelopmental disorders (GIT1, MVB12B and PPP1R9A) demonstrating the utility of this strategy to index the clinical effects of CNVs.

Polymorphisms in genes encoding drugs and xenobiotic metabolizing enzymes, DNA repair enzymes, and response to treatment of childhood acute lymphoblastic leukemia.

PURPOSE: The most common childhood malignancy, acute lymphoblastic leukemia (ALL), remains the leading cause of cancer-related death in children because of resistant cases in which underlying predisposing factors are poorly understood. The interindividual variation in the activity of xenobiotic metabolizing enzymes that modify individual somatic mutation burden in the context of environmental exposure was shown to modify susceptibility to childhood ALL. Variable DNA repair capacity may further modulate induced DNA lesions. Similarly, differential capacity of ALL patients to process carcinogens and chemotherapeutic drugs could both modify an individual's risk of recurrent malignancy and response to therapy. EXPERIMENTAL DESIGN: We investigated the relationship between the risk of relapse in ALL patients and functional polymorphisms in genes encoding carcinogen-metabolizing enzymes, including CYP1A1, CYP2D6, CYP2E1, MPO, GSTM1, GSTT1, GSTP1, NAT1, NAT2, NQO1, as well as DNA-repair enzymes hMLH1, hMSH3, XRCC1, XPF, and APE. Our study included 320 children with ALL, of which 68 relapsed or died because of this disease within 5 years of follow-up. RESULTS: Among children of the latter group, we found that carriers of CYP1A1*2A and NQO1*2 variants had worse disease prognosis according to Kaplan-Meier (P = 0.003) and Cox regression (P

Expression of hepatitis B virus X oncoprotein inhibits transcription-coupled nucleotide excision repair in human cells.

The hepatitis B virus X protein (HBx) is implicated in liver cancer development, and this presumably involves its ability to bind and functionally inactivate the p53 tumour suppressor. For example expression of HBx in cultured cells has been shown to inhibit global nucleotide excision repair, a p53-dependent subpathway of nucleotide excision repair (NER) which eliminates helix-distorting DNA adducts, e.g., UV-induced cyclobutane pyrimidine dimers (CPDs), from the genome overall. However it remains undetermined whether HBx also interferes with transcription-coupled NER (TCNER), another NER subpathway which removes DNA adducts uniquely from the transcribed strand (TS) of active genes. To address this, we employed the model human lymphoblastoid strain TK6 and its isogenic p53-null counterpart NH32, in conjunction with derivatives of these strains constitutively expressing HBx (TK6-HBx and NH32-HBx). Relative to TK6, following exposure to either UVB (290-320 nm) or UVC (254 nm), TK6-HBx, NH32 and NH32-HBx manifested significantly reduced apoptotic capacity to varying degrees, although no striking differences in clonogenic survival between the four strains were observed. As previously documented in our laboratory [Proc. Natl. Acad. Sci. 100 (2003) 7219-7224], ligation-mediated PCR analysis revealed NH32 to be deficient compared with TK6 in CPD removal along the TS strand of the chromosomal c-jun locus following UVB exposure, but to be proficient in this respect following UVC exposure, i.e., the requirement for p53 in TCNER exhibits wavelength dependence in human cells. Remarkably however, in contrast to the situation for NH32, TK6-HBx and NH32-HBx manifested defective repair along the TS of c-jun after irradiation with either UVB or UVC. The data demonstrate that HBx expression can reduce the efficiency of TCNER in addition to GNER in human cells via p53-independent as well as p53-dependent pathways.

Analyses of bulky DNA adduct levels in human breast tissue and genetic polymorphisms of cytochromes P450 (CYPs), myeloperoxidase (MPO), quinone oxidoreductase (NQO1), and glutathione S-transferases (GSTs).

Environmental carcinogens are converted into DNA-reactive metabolites by phase I and phase II enzymes that are involved in the activation and detoxification of xenobiotics. Several of these enzymes display genetic polymorphisms that alter their activity leading to individual variation in DNA damage levels and thus cancer susceptibility. We investigated the relationship between DNA adduct levels and genetic polymorphisms in key enzymes of chemical carcinogenesis: CYP1A1, CYP1A2, GSTT1, GSTM1, GSTP1, NQO1 and MPO. Levels of DNA adducts were determined in human breast tissue using the 32P-postlabeling method. A significantly higher adduct level was observed for individuals with the A-463 variant in the MPO gene (P=0.008), providing the first observation of an association between a predicted reduced MPO gene transcription and a higher level of DNA adducts. Furthermore, levels of DNA adducts were about 45% higher in individuals with either GSTP1*B or GSTP1*C variants compared to those homozygous for the wild-type allele. When the MPO and GSTP1 were examined together, individuals with these combined variant genotypes had significantly higher adduct levels than all other genotype combinations (P=0.003).

SNP arrays: comparing diagnostic yields for four platforms in children with developmental delay.

BACKGROUND: Molecular karyotyping is now the first-tier genetic test for patients affected with unexplained intellectual disability (ID) and/or multiple congenital anomalies (MCA), since it identifies a pathogenic copy number variation (CNV) in 10-14% of them. High-resolution microarrays combining molecular karyotyping and single nucleotide polymorphism (SNP) genotyping were recently introduced to the market. In addition to identifying CNVs, these platforms detect loss of heterozygosity (LOH), which can indicate the presence of a homozygous mutation or uniparental disomy. Since these abnormalities can be associated with ID and/or MCA, their detection is of particular interest for patients whose phenotype remains unexplained. However, the diagnostic yield obtained with these platforms is not confirmed, and the real clinical value of LOH detection has not been established. METHODS: We selected 21 children affected with ID, with or without congenital malformations, for whom standard genetic analyses failed to provide a diagnosis. We performed high-resolution SNP array analysis with four platforms (Affymetrix Genome-Wide Human SNP Array 6.0, Affymetrix Cytogenetics Whole-Genome 2.7 M array, Illumina HumanOmni1-Quad BeadChip, and Illumina HumanCytoSNP-12 DNA Analysis BeadChip) on whole-blood samples obtained from children and their parents to detect pathogenic CNVs and LOHs, and compared the results with those obtained on a moderate resolution array-based comparative genomic hybridization platform (NimbleGen CGX-12 Cytogenetics Array), already used in the clinical setting. RESULTS: We identified a total of four pathogenic CNVs in three patients, and all arrays successfully detected them. With the SNP arrays, we also identified a LOH containing a gene associated with a recessive disorder consistent with the patient's phenotype (i.e., an informative LOH) in four children (including two siblings). A homozygous mutation within the informative LOH was found in three of these patients. Therefore, we were able to increase the diagnostic yield from 14.3% to 28.6% as a result of the information provided by LOHs. CONCLUSIONS: This study shows the clinical usefulness of SNP arrays in children with ID, since they successfully detect pathogenic CNVs, identify informative LOHs that can lead to the diagnosis of a recessive disorder. It also highlights some challenges associated with the use of SNP arrays in a clinical laboratory.

CHD2 haploinsufficiency is associated with developmental delay, intellectual disability, epilepsy and neurobehavioural problems.

BACKGROUND: The chromodomain helicase DNA binding domain (CHD) proteins modulate gene expression via their ability to remodel chromatin structure and influence histone acetylation. Recent studies have shown that CHD2 protein plays a critical role in embryonic development, tumor suppression and survival. Like other genes encoding members of the CHD family, pathogenic mutations in the CHD2 gene are expected to be implicated in human disease. In fact, there is emerging evidence suggesting that CHD2 might contribute to a broad spectrum of neurodevelopmental disorders. Despite growing evidence, a description of the full phenotypic spectrum of this condition is lacking. METHODS: We conducted a multicentre study to identify and characterise the clinical features associated with haploinsufficiency of CHD2. Patients with deletions of this gene were identified from among broadly ascertained clinical cohorts undergoing genomic microarray analysis for developmental delay, congenital anomalies and/or autism spectrum disorder. RESULTS: Detailed clinical assessments by clinical geneticists showed recurrent clinical symptoms, including developmental delay, intellectual disability, epilepsy, behavioural problems and autism-like features without characteristic facial gestalt or brain malformations observed on magnetic resonance imaging scans. Parental analysis showed that the deletions affecting CHD2 were de novo in all four patients, and analysis of high-resolution microarray data derived from 26,826 unaffected controls showed no deletions of this gene. CONCLUSIONS: The results of this study, in addition to our review of the literature, support a causative role of CHD2 haploinsufficiency in developmental delay, intellectual disability, epilepsy and behavioural problems, with phenotypic variability between individuals.

De novo deletion of FMN2 in a girl with mild non-syndromic intellectual disability.

We present the case of a child with mild non-syndromic intellectual disability in whom array genomic hybridization revealed a de novo heterozygous deletion involving only one gene, FMN2. FMN2 encodes FORMIN-2, a member of the formin homology family, which is primarily expressed in the developing and mature brain, and has an important role in cytoskeletal organization and actin nucleation. A heterozygous deletion of FMN2 along with 2 other genes has been recently reported in a boy with non-syndromic intellectual disability. This report provides further support for the important role of FMN2 in brain development and cognition.

MicroRNA inhibition of translation initiation in vitro by targeting the cap-binding complex eIF4F.

MicroRNAs (miRNAs) play an important role in gene regulatory networks in animals. Yet, the mechanistic details of their function in translation inhibition or messenger RNA (mRNA) destabilization remain controversial. To directly examine the earliest events in this process, we have developed an in vitro translation system using mouse Krebs-2 ascites cell-free extract that exhibits an authentic miRNA response. We show here that translation initiation, specifically the 5' cap recognition process, is repressed by endogenous let-7 miRNAs within the first 15 minutes of mRNA exposure to the extract when no destabilization of the transcript is observed. Our results indicate that inhibition of translation initiation is the earliest molecular event effected by miRNAs. Other mechanisms, such as mRNA degradation, may subsequently consolidate mRNA silencing.

Role of DNA mismatch repair genetic polymorphisms in the risk of childhood acute lymphoblastic leukaemia.

Acute lymphoblastic leukaemia (ALL) is the most common childhood cancer. Genetic variants in the coding regions of the mismatch repair genes MLH1 (Ile-219Val) and MSH3 (Arg-940Glu and Thr-1036Ala) could contribute to an individual's susceptibility as modifiers in leukaemogenesis. To investigate this possibility, we conducted a case-control study on 287 children with ALL and 320 healthy controls both of French-Canadian origin. MLH1 and MSH3 variants, when taken independently, seemed to play little or no role in the aetiology of childhood ALL. However, when the MLH1 genotypes were combined with genotypes previously shown to influence ALL susceptibility, we found that the MLH1 variant Val-219 further increases the risk of GSTM1 null and CYP1A1*2A genotypes [combined odds ratio (OR) = 6.0, P = 0.002] as well as that of CYP2E1*5 (OR = 15.8, P = 0.001). No association was found with MSH3 variants. This study suggests an association of leukaemogenesis in children with both xenobiotic metabolism and DNA repair, and thus points to the effect of environmental exposure.

UV wavelength-dependent regulation of transcription-coupled nucleotide excision repair in p53-deficient human cells.

Nucleotide excision repair (NER) prevents skin cancer by eliminating highly genotoxic cyclobutane pyrimidine dimers (CPDs) induced in DNA by the UVB component of sunlight. NER consists of two distinct but overlapping subpathways, i.e., global NER, which removes CPD from the genome overall, and transcription-coupled NER (TCNER), which removes CPD uniquely from the transcribed strand of active genes. Previous investigations have clearly established that the p53 tumor suppressor plays a crucial role in the NER process. Here we used the ligation-mediated PCR technique to demonstrate, at nucleotide resolution along two chromosomal genes in human cells, that the requirement for functional p53 in TCNER, but not in global NER, depends on incident UV wavelength. Indeed, relative to an isogenic p53 wild-type counterpart, p53-deficient human lymphoblastoid strains were shown to remove CPD significantly less efficiently along both the transcribed and nontranscribed strands of the c-jun and hprt loci after exposure to polychromatic UVB (290-320 nm). However, in contrast, after irradiation with 254-nm UV, p53 deficiency engendered less efficient CPD repair only along the nontranscribed strands of these target genes. The revelation of this intriguing wavelength-dependent phenomenon reconciles an apparent conflict between previous studies which used either UVB or 254-nm UV to claim, respectively, that p53 is required for, or plays no role whatsoever in, TCNER of CPD. Furthermore, our finding highlights a major caveat in experimental photobiology by providing a prominent example where the extensively used "nonsolar" model mutagen 254-nm UV does not accurately replicate the effects of environmentally relevant UVB.

Variable continental distribution of polymorphisms in the coding regions of DNA-repair genes.

DNA-repair pathways are critical for maintaining the integrity of the genetic material by protecting against mutations due to exposure-induced damages or replication errors. Polymorphisms in the corresponding genes may be relevant in genetic epidemiology by modifying individual cancer susceptibility or therapeutic response. We report data on the population distribution of potentially functional variants in XRCC1, APEX1, ERCC2, ERCC4, hMLH1, and hMSH3 genes among groups representing individuals of European, Middle Eastern, African, Southeast Asian and North American descent. The data indicate little interpopulation differentiation in some of these polymorphisms and typical FST values ranging from 10 to 17% at others. Low FST was observed in APEX1 and hMSH3 exon 23 in spite of their relatively high minor allele frequencies, which could suggest the effect of balancing selection. In XRCC1, hMSH3 exon 21 and hMLH1 Africa clusters either with Middle East and Europe or with Southeast Asia, which could be related to the demographic history of human populations, whereby human migrations and genetic drift rather than selection would account for the observed differences.