Is being Hispanic a risk factor for non-insulin dependent diabetes mellitus (NIDDM)?

The objective of this paper is to critically assess the basis of the hypothesis that the ethnicity of Hispanics is by itself a risk factor of NIDDM. Showing that the definition of the term Hispanics has both operational and methodological problems, it is argued that in the United States, a group identified by this term is genetically, as well as culturally, heterogeneous. Further, the actual risk factors of NIDDM may simply co-vary with the ethnicity of Hispanics, so that the notion that this ethnicity is a stand-alone independent risk factor of NIDDM may be too simplistic.

Patterns of instability of expanded CAG repeats at the ERDA1 locus in general populations

A highly polymorphic CAG repeat locus, ERDA1, was recently described on human chromosome 17q21.3, with alleles as large as 50-90 repeats and without any disease association in the general population. We have studied allelic distribution at this locus in five human populations and have characterized the mutational patterns by direct observation of 731 meioses. The data show that large alleles (>/=40 CAG repeats) are generally most common in Asian populations, less common in populations of European ancestry, and least common among Africans. We have observed a high intergenerational instability (46. 3%+/-5.1%) of the large alleles. Although the mutation rate is not dependent on parental sex, paternal transmissions have predominantly resulted in contractions, whereas maternal transmissions have yielded expansions. Within this class of large alleles, the mutation rate increases concomitantly with increasing allele size, but the magnitude of repeat size change does not depend on the size of the progenitor allele. Sequencing of specific alleles reveals that the intermediate-sized alleles (30-40 repeats) have CAT/CAC interruptions within the CAG-repeat array. These results indicate that expansion and instability of trinucleotide repeats are not exclusively disease-associated phenomena. The implications of the existence of massively expanded alleles in the general populations are not yet understood.

Estimation of nonpaternity in the Mexican population of Nuevo Leon: a validation study with blood group markers

A method for estimating the general rate of nonpaternity in a population was validated using phenotype data on seven blood groups (A1A2BO, MNSs, Rh, Duffy, Lutheran, Kidd, and P) on 396 mother, child, and legal father trios from Nuevo León, Mexico. In all, 32 legal fathers were excluded as the possible father based on genetic exclusions at one or more loci (combined average exclusion probability of 0.694 for specific mother-child phenotype pairs). The maximum likelihood estimate of the general nonpaternity rate in the population was 0.118 +/- 0.020. The nonpaternity rates in Nuevo León were also seen to be inversely related with the socioeconomic status of the families, i.e., the highest in the low and the lowest in the high socioeconomic class. We further argue that with the moderately low (69.4%) power of exclusion for these seven blood group systems, the traditional critical values of paternity index (PI > or = 19) were not good indicators of true paternity, since a considerable fraction (307/364) of nonexcluded legal fathers had a paternity index below 19 based on the seven markers. Implications of these results in the context of genetic-epidemiological studies as well as for detection of true fathers for child-support adjudications are discussed, implying the need to employ a battery of genetic markers (possibly DNA-based tests) that yield a higher power of exclusion. We conclude that even though DNA markers are more informative, the probabilistic approach developed here would still be needed to estimate the true rate of nonpaternity in a population or to evaluate the precision of detecting true fathers.

Angioplasty versus coronary artery by-pass surgery: a reappraisal

Revascularisation is the main principle of treatment of obstructive coronary artery disease. This technique is available either by catheter intervention like angioplasty or by-pass surgery. The superiority of one over the other is still undetermined. In symptomatic single vessel disease angioplasty may be a better option than by-pass surgery. In two-vessel coronary artery disease angioplasty may also be preferred especially with good left ventricular function. In patients with double-vessel disease particularly involving proximal left anterior descending artery in association with diabetes mellitus surgery has better long term results. In multivessel disease by-pass surgery is a preferred option although initial results of angioplasty in this group may be very satisfactory. The incidence of further intervention either by surgery or repeat angioplasty is high in patients undergoing angioplasty in multivessel disease. Left main stem disease should be dealt with by-pass surgery. With continued advancement in the revascularisation technology of coronary artery disease both in catheter intervention and surgical fronts there is no room for unequivocal or universal strategy plan in the management of coronary artery disease. Both the techniques are complimentary to each other. Cost consideration is a major consideration in India. Choice should be made after proper evaluation of coronary anatomy, underlying clinical condition, local experience, social and especially economic circumstances.

Cationic liposome-encapsulated antisense oligonucleotide mediates efficient killing of intracellular Leishmania

Antisense oligonucleotides have been considered as inhibitors of growth of intracellular parasites such as Leishmania, but only limited inhibition has been observed in vitro. We have encapsulated an antisense oligonucleotide, complementary to the Leishmania universal miniexon sequence, in cationic liposomes. Low concentrations (4 microM) of encapsulated oligonucleotides specifically reduced the amastigote burden within cultured macrophages by 80%. This result illustrates the importance of effective delivery for efficient antiparasitic activity of antisense oligonucleotides.

The utility of short tandem repeat loci beyond human identification: implications for development of new DNA typing systems

Since the first characterization of the population genetic properties of repeat polymorphisms, the number of short tandem repeat (STR) loci validated for forensic use has now grown to at least 13. Worldwide variations of allele frequencies at these loci have been studied, showing that variations of interpopulation diversity at these loci do not compromise the power of identification of individuals. However, data collected for validation of these loci for forensic use has utility beyond human identification; the origin and past migration history of modern humans can be reconstructed from worldwide variations at these loci. Furthermore, complex forensic cases previously unresolvable can now be investigated with the help of the validated STR loci. Here, we provide the absolute power of the validated set of 13 STR loci for addressing these issues using multilocus genotype data on 1,401 individuals belonging to seven populations (US European-American, US African-American, Jamaican, Italian, Swiss, Chinese and Apache Native-American). Genomic research is discovering new classes of polymorphic loci (such as the single nucleotide polymorphisms, SNPs) and lineage markers (such as the mitochondrial DNA and Y-chromosome markers); our aim, therefore, was to determine how many SNP loci are needed to match the power of this set of 13 STR loci. We conclude that the current set of STR loci is adequate for addressing most problems of human identification (including interpretations of DNA mixtures). However, if suitable number of SNPs are used that would match the power of the STR loci, they alone cannot resolve more complex cases unless they are supplemented by the validated STR loci.

HLA haplotypes and microsatellite polymorphisms in and around the major histocompatibility complex region in a Native American population with a high prevalence of scleroderma (systemic sclerosis)

Choctaw Native Americans in southeastern Oklahoma have the highest prevalence of scleroderma or systemic sclerosis yet found (469/100,000). An Amerindian HLA DR2 haplotype (DRB1*1602) was significantly associated with scleroderma in this population in a previous study. It is not known, however, if other disease genes are linked to this HLA haplotype. The regions flanking the HLA loci were studied with polymorphic microsatellite markers. An extended HLA DR2 (DRB1*1602, DQA1*0501, DQB1*0301, DPB1*1301) haplotype that includes the class I and III regions was identified which was significantly associated with scleroderma in the Oklahoma Choctaw. No other significant associations with microsatellite marker alleles immediately flanking the HLA region were found.

Ionizing radiation and genetic risks. VI. Chronic multifactorial diseases: a review of epidemiological and genetical aspects of coronary heart disease, essential hypertension and diabetes mellitus

This paper provides a broad overview of the epidemiological and genetical aspects of common multifactorial diseases in man with focus on three well-studied ones, namely, coronary heart disease (CHD), essential hypertension (EHYT) and diabetes mellitus (DM). In contrast to mendelian diseases, for which a mutant gene either in the heterozygous or homozygous condition is generally sufficient to cause disease, for most multifactorial diseases, the concepts of genetic susceptibility' and risk factors' are more appropriate. For these diseases, genetic susceptibility is heterogeneous. The well-studied diseases such as CHD permit one to conceptualize the complex relationships between genotype and phenotype for chronic multifactorial diseases in general, namely that allelic variations in genes, through their products interacting with environmental factors, contribute to the quantitative variability of biological risk factor traits and thus ultimately to disease outcome. Two types of such allelic variations can be distinguished, namely those in genes whose mutant alleles have (i) small to moderate effects on the risk factor trait, are common in the population (polymorphic alleles) and therefore contribute substantially to the variability of biological risk factor traits and (ii) profound effects, are rare in the population and therefore contribute far less to the variability of biological risk factor traits. For all the three diseases considered in this review, a positive family history is a strong risk factor. CHD is one of the major contributors to mortality in most industrialized countries. Evidence from epidemiological studies, clinical correlations, genetic hyperlipidaemias etc., indicate that lipids play a key role in the pathogenesis of CHD. The known lipid-related risk factors include: high levels of low density lipoprotein cholesterol, low levels of high density lipoprotein cholesterol, high apoB levels (the major protein fraction of the low density lipoprotein particles) and elevated levels of Lp(a) lipoprotein. Among the risk factors which are not related to lipids are: high levels of homocysteine, low activity of paraoxonase and possibly also elevated plasma fibrinogen levels. In addition to the above, hypertension, diabetes and obesity (which themselves have genetic determinants) are important risk factors for CHD. Among the environmental risk factors are: high dietary fat intake, smoking, stress, lack of exercise etc. About 60% of the variability of the plasma cholesterol is genetic in origin. While a few genes have been identified whose mutant alleles have large effects on this trait (e.g., LDLR, familial defective apoB-100), variability in cholesterol levels among individuals in most families is influenced by allelic variation in many genes (polymorphisms) as well as environmental exposures. A proportion of this variation can be accounted for by two alleles of the apoE locus that increase (ε4) and decrease (ε2) cholesterol levels, respectively. A polymorphism at the apoB gene (XbaI) also has similar effects, but is probably not mediated through lipids. High density lipoprotein cholesterol levels are genetically influenced and are related to apoA1 and hepatic lipase (LIPC) gene functions. Mutations in the apoA1 gene are rare and there are data which suggest a role of allelic variation at or linked LIPC gene in high density lipoprotein cholesterol levels. Polymorphism at the apoA1--C3 loci is often associated with hypertriglyceridemia. The apo(a) gene which codes for Lp(a) is highly polymorphic, each allele determining a specific number of multiple tandem repeats of a unique coding sequence known as Kringle 4. The size of the gene correlates with the size of the Lp(a) protein. The smaller the size of the Lp(a) protein, the higher are the Lp(a) levels. (ABSTRACT TRUNCATED)

Crystal structure of the outer membrane active transporter FepA from Escherichia coli

Integral outer membrane receptors for iron chelates and vitamin B12 carry out specific ligand transport against a concentration gradient. Energy for active transport is obtained from the proton-motive force of the inner membrane through physical interaction with TonB-ExbB-ExbD, an inner membrane complex. Here we report the crystal structure of an active transport, outer membrane receptor at 2.4 A resolution. Two distinct functional domains are revealed: (i) a 22-stranded beta-barrel that spans the outer membrane and contains large extracellular loops which appear to function in ligand binding; and (ii) a globular N-terminal domain that folds into the barrel pore, inhibiting access to the periplasm and contributing two additional loops for potential ligand binding. These loops could provide a signaling pathway between the processes of ligand recognition and TonB-mediated transport. The blockage of the pore suggests that the N-terminal domain must undergo a conformational rearrangement to allow ligand transport into the periplasm.

Mutations in the BRCA1 gene: implications of inter-population differences for predicting the risk of radiation-induced breast cancers

The effects of cancer predisposition and increased tumorigenic radiosensitivity of the predisposed genotypes on radiation cancer risks (in the general population and in sisters and first cousins of affected probands) are studied using an autosomal dominant model of cancer predisposition and radiosensitivity. The model assumes that the predisposing alleles, which confer enhanced tumorigenic radiosensitivity, are incompletely penetrant. In addition, the model also allows for sporadic cancers, unrelated to the predisposing locus. The predictions of the model are illustrated using current estimates of BRCA1 mutant gene frequencies; the estimates of the strength of predisposition and radiosensitivity differentials used are based on animal and human studies. It is shown that, unless both the strength of predisposition and radiosensitivity differential are large (say, > 100-fold in comparison with normal homozygotes), (i) the effect of risk heterogeneity on cancer risk is marginal; (ii) dose-dependent radiation effect remains virtually the same as in a homogeneous irradiated population that has no predisposed subgroups; (iii) for the same radiation dose, relatives of affected probands show an enhancement of cancer risks; and (iv) most extra cancers in relatives can be attributed to radiosensitivity differentials. This simple model can give an upper bound of the effect of risk heterogeneity on radiation-induced breast cancer risks even when the cumulative breast cancer risk is age-dependent. Further, our model predicts that the benefits of mammography outweigh the risks.

Association of microsatellite markers near the fibrillin 1 gene on human chromosome 15q with scleroderma in a Native American population

OBJECTIVE

To localize disease genes for scleroderma, or systemic sclerosis (SSc), in a population of Choctaw Native Americans with a high prevalence of SSc, in which there is evidence of a possible founder effect.

METHODS

A candidate gene approach was used in which microsatellite alleles on human chromosomes 15q and 2q, homologous to the murine tight skin 1 (tsk1) and tsk2 loci, respectively, were analyzed in Choctaw SSc cases and race-matched normal controls for possible disease association. Genotyping first-degree relatives of the cases identified potential disease haplotypes, and haplotype frequencies were obtained by expectation-maximization and maximum-likelihood estimation methods. Simultaneously, the ancestral origins of contemporary Choctaw SSc cases were ascertained using census and historical records.

RESULTS

A multilocus 2-cM haplotype was identified on human chromosome 15q homologous to the murine tsk1 region, which showed a significantly increased frequency in SSc cases compared with controls. This haplotype contains 2 intragenic markers for the fibrillin 1 (FBN1) gene. Genealogical studies demonstrated that the SSc cases were distantly related, and their ancestry could be traced back to 5 founding families in the mid-eighteenth century. The probability that the SSc cases share this haplotype due to familial aggregation effects alone was calculated and found to be very low. There was no evidence of any microsatellite allele disturbances on chromosome 2q in the region homologous to the tsk2 locus or the region containing the interleukin-1 family.

CONCLUSION

A 2-cM haplotype on chromosome 15q that contains FBN1 is associated with scleroderma in Choctaw Native Americans from Oklahoma. This haplotype may have been inherited from common founders about 10 generations ago and may contribute to the high prevalence of SSc that is now seen.

Development of a highly polymorphic STR marker for identity testing purposes at the human androgen receptor gene (HUMARA)

We developed a non-isotopic method which improves the technical quality of the X-linked HUMARA locus typing process. The use of formamide and a low concentration of acrylamide increased resolution and sharpness of HUMARA alleles in silver-stained polyacrylamide gels. In addition, the construction of an allelic ladder containing amplified sequence of 9 alleles (even-numbered alleles) of the HUMARA locus, allows confident, rapid and precise assignment of discretely defined alleles. Allele and genotype frequencies for the HUMARA locus were determined in a French Canadian population sample. Observed genotype frequencies in females conformed to Hardy-Weinberg expectations. Furthermore, the HUMARA locus is highly polymorphic with 18 observed alleles and an heterozygosity value of 89.3%. Also, this locus has average powers of discrimination of 97.8% and 88.7% for testing samples of female and male origin, respectively. In the French Canadian population, the average probability of excluding a random man as the father in paternity analysis when both mother and daughter are tested for this locus is 88.0%. Together, the results indicate that the HUMARA locus provides a highly discriminatory system that is appropriate for the purposes of forensic identification and paternity testing involving a female child.

Simultaneous estimation of all the parameters of a stepwise mutation model

Minisatellite and microsatellite are short tandemly repetitive sequences dispersed in eukaryotic genomes, many of which are highly polymorphic due to copy number variation of the repeats. Because mutation changes copy numbers of the repeat sequences in a generalized stepwise fashion, stepwise mutation models are widely used for studying the dynamics of these loci. We propose a minimum chi-square (MCS) method for simultaneous estimation of all the parameters in a stepwise mutation model and the ancestral allelic type of a sample. The MCS estimator requires knowing the mean number of alleles of a certain size in a sample, which can be estimated using Monte Carlo samples generated by a coalescent algorithm. The method is applied to samples of seven (CA)n repeat loci from eight human populations and one chimpanzee population. The estimated values of parameters suggest that there is a general tendency for microsatellite alleles to expand in size, because (1) each mutation has a slight tendency to cause size increase and (2) the mean size increase is larger than the mean size decrease for a mutation. Our estimates also suggest that most of these CA-repeat loci evolve according to multistep mutation models rather than single-step mutation models. We also introduced several quantities for measuring the quality of the estimation of ancestral allelic type, and it appears that the majority of the estimated ancestral allelic types are reasonably accurate. Implications of our analysis and potential extensions of the method are discussed. SINCE the discovery that a large number of loci with tandemly repeated sequences in human and many eukaryote species are highly polymorphic because of copy number variation of the repeats in different individuals (Jeffreys 1985; Litt and Luty 1989; Weber and May 1989), allele size data from such loci are rapidly becoming the dominant source of genetic markers for genome mapping, forensic testing, and population studies. Loci with repeat sequences longer than 5 bp are generally referred to as minisatellite or variable number tandem repeat loci, and those with repeat sequences between 2 to 5 bp are referred to as microsatellite or short tandem repeat loci (Tautz 1993). Because mutations change the copy number of such loci in a stepwise fashion, rapid accumulation of population samples from minisatellite and microsatellite loci has resurrected the interest of the stepwise mutation model (SMM), which was popular in the 1970s.

Ionizing radiation and genetic risks. VIII. The concept of mutation component and its use in risk estimation for multifactorial diseases

Multifactorial diseases, which include the common congenital abnormalities (incidence: 6%) and chronic diseases with onset predominantly in adults (population prevalence: 65%), contribute substantially to human morbidity and mortality. Their transmission patterns do not conform to Mendelian expectations. The model most frequently used to explain their inheritance and to estimate risks to relatives is a Multifactorial Threshold Model (MTM) of disease liability. The MTM assumes that: (i) the disease is due to the joint action of a large number of genetic and environmental factors, each of which contributing a small amount of liability, (ii) the distribution of liability in the population is Gaussian and (iii) individuals whose liability exceeds a certain threshold value are affected by the disease. For most of these diseases, the number of genes involved or the environmental factors are not fully known. In the context of radiation exposures of the population, the question of the extent to which induced mutations will cause an increase in the frequencies of these diseases has remained unanswered. In this paper, we address this problem by using a modified version of MTM which incorporates mutation and selection as two additional parameters. The model assumes a finite number of gene loci and threshold of liability (hence, the designation, Finite-Locus Threshold Model or FLTM). The FLTM permits one to examine the relationship between broad-sense heritability of disease liability and mutation component (MC), the responsiveness of the disease to a change in mutation rate. Through the use of a computer program (in which mutation rate, selection, threshold, recombination rate and environmental variance are input parameters and MC and heritability of liability are output estimates), we studied the MC-heritability relationship for (i) a permanent increase in mutation rate (e.g., when the population sustains radiation exposure in every generation) and (ii) a one-time increase in mutation rate. Our investigation shows that, for a permanent increase in mutation rate of 15%, MC in the first few generations is of the order of 1-2%. This conclusion holds over a broad range of heritability values above about 30%. At equilibrium, however, MC reaches 100%. For a one-time increase in mutation rate, MC reaches its maximum value (of 1-2%) in the first generation, followed by a decline to zero in subsequent generations. These conclusions hold for so many combinations of parameter values (i.e., threshold, selection coefficient, number of loci, environmental variance, spontaneous mutation rate, increases in mutation rate, levels of 'interaction' between genes and recombination rates) that it can be considered to be relatively robust. We also investigated the biological validity of the FLTM in terms of the minimum number of loci, their mutation rates and selection coefficients needed to explain the incidence of multifactorial diseases using the theory of genetic loads. We argue that for common multifactorial diseases, selection coefficients are small in present-day human populations. Consequently, with mutation rates of the order known for Mendelian genes, the FLTM with a few loci and weak selection provides a good approximation for studying the responsiveness of multifactorial diseases to radiation exposures.

Macrophage mannosyl fucosyl receptor: its role in invasion of virulent and avirulent L. donovani promastigotes

The interaction of leishmania parasites with macrophages is known to be receptor mediated. Previous study from this laboratory (J. Parasitol. 82:632, 1996) showed the significant involvement of LPG and gp63 receptors in the recognition of virulent strains onto the macrophages. The role of carbohydrate receptors the other major receptors besides LPG and gp63 receptors, in the recognition of both virulent (strains AG83 and GE1) and avirulent (strain UR6) leishmania onto the host macrophages has been the major focus of the present investigation. Various neoglycoproteins were used as efficient ligands to preblock the carbohydrate receptors on the macrophage surface. Similarly, various sugar specific lectins were used to preblock the corresponding carbohydrate ligands on the parasite surface. When these preblocked macrophages or parasites were used to study their mode of recognition, it was obvious from the findings that avirulent leishmania promastigotes possibly use the mannosyl fucosyl receptors (MFR) more avidly for their initial attachment and subsequent internalization into the macrophages whereas the virulent leishmania exhibits limited use of this receptor. When a macrophage-like cell line (J774), lacking in MFR, was purposely selected to test the previous findings, as expected, the attachment of avirulent promastigotes (UR6) onto the cell line was found to be negligible when compared to the peritoneal macrophages. Thus, it appears that avirulent leishmania promastigotes probably utilize MFR significantly for their initial recognition and subsequent internalization by macrophages.

Ionizing radiation and genetic risks. VII. The concept of mutation component and its use in risk estimation for Mendelian diseases

The responsiveness of Mendelian diseases to an increase in the mutation rate is studied by using the concept of the mutation component (MC) of genetic diseases. Algebraic expressions to evaluate MC at any specific generation following either a one-time or a permanent increase in mutation rate are derived and are illustrated with numerical examples. For a one-time increase in mutation rate, the analysis shows that the first generation MC for autosomal dominant diseases is equal to the selection coefficient; this is also true for X-linked diseases (adjusted for the proportion of X-chromosomes in males). For autosomal recessive diseases the first generation MC is substantially smaller than that for autosomal dominants. In subsequent generations MC gradually decays to zero. Under conditions of a permanent increase in the mutation rate, the MC for autosomal dominant, X-linked and completely recessive autosomal disorders progressively increases to reach a value of one at the new equilibrium. For incompletely recessive autosomal disorders, however, the MC at equilibrium can be larger than one. The rates of approach to the new equilibrium are different for the different classes of diseases, dictated by selection and time (in generations) following radiation exposure. The effects of increases in mutation rate on MC are more pronounced for autosomal dominants, followed by X-linked and are far less for autosomal recessives. Even for autosomal dominants, the early generation effects of radiation exposures would not be appreciable unless the heterozygotes have a severely reduced fitness.

Application of a time-dependent coalescence process for inferring the history of population size changes from DNA sequence data

Distribution of pairwise differences of nucleotides from data on a sample of DNA sequences from a given segment of the genome has been used in the past to draw inferences about the past history of population size changes. However, all earlier methods assume a given model of population size changes (such as sudden expansion), parameters of which (e.g., time and amplitude of expansion) are fitted to the observed distributions of nucleotide differences among pairwise comparisons of all DNA sequences in the sample. Our theory indicates that for any time-dependent population size, N(tau) (in which time tau is counted backward from present), a time-dependent coalescence process yields the distribution, p(tau), of the time of coalescence between two DNA sequences randomly drawn from the population. Prediction of p(tau) and N(tau) requires the use of a reverse Laplace transform known to be unstable. Nevertheless, simulated data obtained from three models of monotone population change (stepwise, exponential, and logistic) indicate that the pattern of a past population size change leaves its signature on the pattern of DNA polymorphism. Application of the theory to the published mtDNA sequences indicates that the current mtDNA sequence variation is not inconsistent with a logistic growth of the human population.

Cancer predisposition, radiosensitivity and the risk of radiation-induced cancers. IV. Prediction of risks in relatives of cancer-predisposed individuals

Individuals carrying cancer-predisposing germline mutations are known to be at a higher risk for cancers than those who do not carry them. This is also true of their biological relatives because they have a higher probability of being carriers of such mutant genes than unrelated individuals in the population. Further, there are now sufficient grounds for assuming that cancer-predisposed individuals may also be at a higher risk for cancers induced by ionizing radiation. In our earlier work, we examined the impact of this heterogeneity (with respect to cancer predisposition and radiosensitivity differentials) on risks of radiation-induced cancer at the population level. This paper is focused on the question of risks of radiation-induced cancer in relatives of cancer-predisposed individuals. Using an autosomal dominant model of cancer predisposition and radiosensitivity developed earlier and applying it to breast cancer risks associated with mutations in the BRCA1 gene, we show that: (1) The risk ratio (i.e. the ratio of risk of radiation-induced cancer in relatives to that in unrelated individuals) in the population increases with the degree of biological relatedness of the relative, being higher for close than for distant relatives; incomplete penetrance of the mutant gene "dilutes" this risk ratio. (2) The proportion of excess radiation-induced cancers in relatives (i.e. the attributable fraction) is higher than in unrelated individuals. (3) In relatives, the proportion of excess cancers due to radiosensitivity differentials alone depends on the strength of predisposition, the radiosensitivity differentials assumed, the radiation dose, the proportion of cancers due to predisposition, the mutant gene frequency and the penetrance of the mutant gene. This is in contrast to the situation for unrelated individuals, for whom the above-mentioned proportion is dependent on the first three but not on the last three of these factors. Further, even when the proportion of excess cancers is small, most of it is due to radiosensitivity differential alone both in unrelated individuals and in relatives. (4) For values of predisposition strength and radiosensitivity differential <10, even when the estimated frequency of a mutant BRCA1 gene is 0.0047 and the proportion of breast cancers due to these mutations is 38% (as is the case for Ashkenazi Jewish women under age 30), the increase in breast cancer risks is only marginal even for first-degree relatives. (5) These findings support the conclusion that increases in radiation risks to relatives (compared to those in unrelated individuals), to be detectable epidemiologically, will occur only when the mutant alleles are common and the strength of predisposition and radiosensitivity differentials are conjointly dramatic.

Signatures of population expansion in microsatellite repeat data

To examine the signature of population expansion on genetic variability at microsatellite loci, we consider a population that evolves according to the time-continuous Moran model, with growing population size and mutations that follow a general asymmetric stepwise mutation model. We present calculations of expected allele-size variance and homozygosity at a locus in such a model for several variants of growth, including stepwise, exponential, and logistic growth. These calculations in particular prove that population bottleneck followed by growth in size causes an imbalance between allele size variance and heterozygosity, characterized by the variance being transiently higher than expected under equilibrium conditions. This effect is, in a sense, analogous to that demonstrated before for the infinite allele model, where the number of alleles transiently increases after a stepwise growth of population. We analyze a set of data on tetranucleotide repeats that reveals the imbalance expected under the assumption of bottleneck followed by population growth in two out of three major racial groups. The imbalance is strongest in Asians, intermediate in Europeans, and absent in Africans. This finding is consistent with previous findings by others concerning the population expansion of modern humans, with the bottleneck event being most ancient in Africans, most recent in Asians, and intermediate in Europeans. Nevertheless, the imbalance index alone cannot reliably estimate the time of initiation of population expansion.

Dynamic balance of segregation distortion and selection maintains normal allele sizes at the myotonic dystrophy locus

Myotonic dystrophy (DM), an autosomal dominant neurological disorder, is caused by CTG-repeat expansions at the DMPK locus, with affected individuals having > or = 50 repeats of this trinucleotide. Reduced reproductive fitness of affected individuals and decreased viability of congenital DM have been noted. Expanded CTG-repeat alleles are highly unstable, predominantly yielding even higher repeat sizes. Preferential transmission of longer alleles from heterozygous mothers within the normal size range of alleles also is observed. In view of these observations, it is worth examining how DM has been maintained in human populations for hundreds of generations. We present an analysis of the dynamic properties of a model of joint effects of segregation distortion and selection (intensity of which increases with allele sizes of an individual's genotype). Our mathematical formulation and numerical analyses demonstrate that a weak segregation distortion during female meiosis, together with selection of comparable intensity (within the normal allele size range), can maintain an equilibrium distribution of allele frequencies. Genetic drift, acting in conjunction with the occasional contraction of alleles by mutation, can contribute to the balance of segregation distortion and mutation, in the sense that even weaker selection can explain the observed allele frequencies. The model is applied to CTG-repeat size distributions at the DMPK locus, observed in normal individuals from world populations.

Study of three hypervariable DNA loci (D1S7; D7S22 and D12S11) in three European populations

To investigate the population genetic characteristics and genetic affinity, DNA profiles of three highly polymorphic VNTR (variable number of tandem repeats) loci (D1S7; D7S22 and D12S11) were studied in 405 individuals from three major European populations (English, Spanish and Basques). Like other studies on VNTRs, a large significant heterozygote deficiency was observed in all three populations. This decrease was ascribed to the limitation, coalescence and non-detectibility of alleles associated with the RFLP (restriction fragment length polymorphism) technique, through which the VNTR loci are genotyped. When the non-detectable alleles were taken into consideration, analyses of fragment sizes at these loci within each sample, as well as their fixed binned analyses, reveal that the assumptions of independence of allelelic occurrences within and between loci are valid for this European data. By comparing genetic variation at three VNTR loci with 17 blood groups, proteins and HLA loci in three well defined European populations, it is shown that the pattern of differentiation at these sets of loci are in general parallel especially for the hypervariable loci HLA and VNTR. Fixed-bin allele frequencies, therefore, are the best descriptions of such a database both for population genetic and forensic calculation studies. The Basques, with regard to VNTR loci, do not show any reduced genetic variability compared to other two European populations (English and Spanish).

Analysis of allele distribution for six short tandem repeat loci in the French Canadian population of Québec

Short tandem repeat (STR) loci represent a rich source of highly polymorphic markers in the human genome which are useful for the purposes of forensic identification and determination of biological relatedness of individuals. Here, as a part of an ongoing extensive study, we report the analysis of a multilocus genotype survey of 642 to 870 chromosomes in the French Canadian Caucasian population of Québec at six STR loci. The loci HUMCSF1PO, HUMTPOX, HUMTH01, HUMF13A01, HUMFESFPS, and HUMvWA were typed using two multiplex polymerase chain reactions (PCR). Amplified DNA samples were subsequently analyzed by polyacrylamide gel electrophoresis followed by silver staining. The heterozygote frequencies of the loci range from 0.614 to 0.820 (0.661 to 0.818 expected) and the number of alleles from 7 to 12 per locus. Although statistically significant deviation from Hardy-Weinberg expectations of genotype frequencies was noted at some loci by one or more tests, in general, the genotype frequencies are well estimated from the product of allele frequencies at all loci. The most frequent six-locus genotype is expected to occur in the French Canadian population with a frequency of 3.50 by 10(-5) and together, these six loci have an average probability of discrimination of 0.9999985. The study presented here indicates that these six STR loci are informative genetic markers for identity testing purposes in the French Canadian Caucasian population of Québec.