Population survey of the human FMR1 CGG repeat substructure suggests biased polarity for the loss of AGG interruptions

Both sequence length and sequence content are important parameters in determining stability of the fragile X syndrome CGG repeat. In order to estimate the incidence of uninterrupted CGG repeats in the general population and to gain insight into mechanisms responsible for the loss and acquisition of AGG interruptions, 406 randomly selected FMR1 CGG repeat alleles from four broad ethnic groups were assayed for AGG punctuation. Among the 79 different classes of alleles detected, long uninterrupted tracts of pure repeats were rare in the general population, with only 1/406 or 0.25% found at the instability threshold (34-37 pure CGG repeats). There was no significant difference (P>0.05) in the distribution of alleles with long (>20) pure repeat tracts among the different ethnic groups, suggesting that different ethnic groups should be equally susceptible to the development of the disease. Analysis of an additional 43 alleles with total repeat lengths between 35 and 50 repeats, revealed that highly interrupted CGG repeats alleles (>2 AGG interruptions) occur preferentially at modal repeat lengths in the population, providing confirmatory evidence that the presence of AGG interruptions confers stability. A consideration of length variation of the most 3' tract of pure repeats revealed a bimodal distribution pattern with maxima at approximately 10 and 20 repeats. Only unimodal distributions with maxima 9 or 10 were observed for the 5' tract and middle CGG tract within the FMR1 CGG repeat substructure. These results suggest that the loss of the most 3' AGG interruption or its conversion to CGG is a common event in the human population, occurring by a mechanism which preserves overall repeat length. This bias for loss of the distal-most AGG interruption likely plays an important part in predisposing human alleles to the development of the X syndrome.

In utero paternity testing following alleged sexual assault. A comparison of DNA-based methods

OBJECTIVE

To compare DNA typing by both variable number of tandem repeats (VNTR) and polymerase chain reaction (PCR) methods to determine the utility of each for prenatal paternity testing following sexual assault. To consider ethical issues of limiting prenatal paternity studies.

DESIGN

Criterion standard.

SETTING

Prenatal diagnostic clinic after determination of pregnancy following an alleged sexual assault.

SUBJECTS

Ten prenatal paternity cases accepted during a 5-year period.

INTERVENTION

DNA-based paternity testing.

MAIN OUTCOME MEASURE

Inclusion or exclusion of paternity by consensual partner.

RESULTS

In all cases DNA typing using the PCR-based method provided the same conclusion as that from VNTR-based data. High probabilities of paternity were reported with both methods.

CONCLUSIONS

DNA typing with PCR using short tandem repeat loci provides a reliable method for quickly determining paternity in prenatal cases. The ethics of providing paternity testing in the context of sexual assault is discussed. The issue of providing prenatal paternity testing in consensual relationships is considered.

Allelic ladder characterization of the short tandem repeat polymorphism located in the 5' flanking region to the human coagulation factor XIII A subunit gene

The short tandem repeat (STR) polymorphism present within the 5' untranslated region of the human coagulation factor XIII A subunit gene, HUMF13A01[AAAG]n, was evaluated using an allelic ladder, i.e., a standard size marker consisting of amplified alleles from the locus. The allelic ladder was constructed by pooling 12 polymerase chain reaction (PCR)-amplified alleles identified by their differential migration in denaturing polyacrylamide gel electrophoresis. This standard marker was used to distinguish 14 different alleles observed at this locus. Sequence analyses indicate that 13 of the alleles contain 4 through 16 iterations of the tandemly repeated AAAG sequence, respectively. The remaining allele carries four repeats and displays a deletion of two consecutive nucleotides (GT), one base distal to the repeat region. The allelic ladder was employed to type 326 F13A01 chromosomes rapidly and reliably in representatives of a German Caucasian population. Population data were analyzed with respect to Hardy-Weinberg Equilibrium (HWE) and compared with those of a previously studied Houston, Texas, Caucasian population.

Evaluation of 13 short tandem repeat loci for use in personal identification applications

Personal identification by using DNA typing methodologies has been an issue in the popular and scientific press for several years. We present a PCR-based DNA-typing method using 13 unlinked short tandem repeat (STR) loci. Validation of the loci and methodology has been performed to meet standards set by the forensic community and the accrediting organization for parentage testing. Extensive statistical analysis has addressed the issues surrounding the presentation of "match" statistics. We have found STR loci to provide a rapid, sensitive, and reliable method of DNA typing for parentage testing, forensic identification, and medical diagnostics. Valid statistical analysis is generally simpler than similar analysis of RFLP-VNTR results and provides powerful statistical evidence of the low frequency of random multilocus genotype matching.

Rapid and efficient resolution of parentage by amplification of short tandem repeats

Short tandem repeat (STR) loci are highly informative polymorphic loci that are gaining popularity for identity testing. We have conducted parentage testing by using nine STR loci on 50 paternity trios that had been previously tested using VNTR loci. These nine unlinked STR loci are amplified in three multiplex reactions and, when examined for genetic informativeness, provide a combined average power of exclusion of 99.73% (Caucasian data). The informative value of the selected loci is based on extensive STR typing of four racial/ethnic populations. In 37 of the 50 cases, paternity could not be excluded by any of the loci. In the remaining 13 cases, paternity was excluded by at least two of the STR markers. The probability of paternity calculated for the alleged father of each matching trio was > 99% in 36 of the 37 inclusion cases. All data agreed with the results reported using VNTR loci and conventional Southern technology. Our studies validate the use of DNA typing with STR loci for parentage testing, thus providing an accurate, highly sensitive, and rapid assay.

Identification of repeat sequence heterogeneity at the polymorphic short tandem repeat locus HUMTH01[AATG]n and reassignment of alleles in population analysis by using a locus-specific allelic ladder

An allelic ladder containing amplified sequences of seven alleles of the polymorphic human tyrosine hydroxylase locus, HUMTH01, was constructed and employed as a standard marker. Sequence analysis of each ladder component indicates that fragments differ by integral multiples of the AATG core repeat sequence characteristic of this locus. Individual alleles are designated "5" through "11," according to the number of complete reiterations of the core repeat contained within them. Comparison of the HUMTH01 allelic ladder with DNA samples amplified at this locus revealed core repeat length heterogeneity (i.e., deletions or insertions shorter than one core repeat) within the human population. In particular, a common allele was identified which migrates more quickly than allele 10, but more slowly than allele 9, on electrophoresis through a denaturing polyacrylamide gel. Sequence analysis of this allele, designated "10-1," reveals lack of a single adenine normally present in the seventh copy of the AATG. The allelic ladder was used to reevaluate previously published population data. Results of testing for Hardy-Weinberg equilibrium and population substructure were not altered significantly by these modifications.

Genetic variation at five trimeric and tetrameric tandem repeat loci in four human population groups

Trimeric and tetrameric short tandem repeats (STRs) represent a rich source of highly polymorphic markers in the human genome that may be studied with the polymerase chain reaction (PCR). We report the analysis of a multilocus genotype survey of 97-380 chromosomes in U.S. Black, White, Mexican-American, and Asian populations at five STR loci located on chromosomes 1, 4, 11, and X. The heterozygote frequencies of the loci ranged from 0.36 to 0.91 and the number of alleles from 6 to 20 for the 20 population and locus combinations. Relative allele frequencies exhibited differences between populations and unimodal, bimodal, and complex distributions. Although deviations were noted at some locus-population test combinations, genotype data from the loci were consistent overall with Hardy-Weinberg equilibrium by three tests. Population subheterogeneity within each ethnic group was not detected by two additional tests. No mutations were detected in a total of 860 meioses for two loci studied in the CEPH kindreds and five loci studied in other families. An indirect estimate of the mutation rates gave values from 2.3 x 10(-5) to 15.9 x 10(-5) for the five loci. Higher mutation rates appear to be associated with greater numbers of tandem repeats in the core motif. The most frequent genotype for all five loci combined appears to have a frequency of 7.59 x 10(-4). Together, these results suggest that trimeric and tetrameric STR loci are useful markers for the study of new mutations and genetic linkage analysis and for application to personal identification in the medical and forensic sciences.

Linkage of the gene for an X-linked mental retardation disorder to a hypervariable (AGAT)n repeat motif within the human hypoxanthine phosphoribosyltransferase (HPRT) locus (Xq26)

We recently reported a new X-linked mental retardation (XLMR) disorder in a four-generation family of Dutch descent. Features included Dandy-Walker malformation, basal ganglia disease, and seizures. Twenty-six family members, including two living affected males and two obligate carriers, were available for study. No evidence of linkage was observed between the disease locus and RFLPs from several X-chromosome regions, including Xp21-p22 (13 markers), proximal Xq (four markers), and Xq28 (three markers). However, a new hypervariable short tandem repeat (STR) within the HPRT gene at Xq26 showed positive linkage to the disease locus, with a maximum lod score of 2.19 at a recombination fraction of 0. A second hypervariable marker in Xq26, the dinucleotide repeat XL90A3 (DXS425), showed a lod score of .84 at a recombination fraction of .11. Both the HPRT and DXS425 markers were typed in 40 CEPH families, and subsequent multipoint linkage analysis showed the following order: Xcen-DXS425-(HPRT,XLMR)-F9-qter. HPRT and these flanking markers are therefore useful for carrier detection and prenatal diagnosis in this family. This study illustrates that hypervariable STRs will be powerful tools for linkage analysis and genetic diagnosis, particularly when relatively small families are involved.

DNA typing and genetic mapping with trimeric and tetrameric tandem repeats

Tandemly reiterated sequences represent a rich source of highly polymorphic markers for genetic linkage, mapping, and personal identification. Human trimeric and tetrameric short tandem repeats (STRs) were studied for informativeness, frequency, distribution, and suitability for DNA typing and genetic mapping. The STRs were highly polymorphic and inherited stably. A STR-based multiplex PCR for personal identification is described. It features fluorescent detection of amplified products on sequencing gels, specific allele identification, simultaneous detection of independent loci, and internal size standards. Variation in allele frequencies were explored for four U.S. populations. The three STR loci (chromosomes 4, 11, and X) used in the fluorescent multiplex PCR have a combined average individualization potential of 1/500 individuals. STR loci appear common, being found every 300-500 kb on the X chromosome. The combined frequency of polymorphic trimeric and tetrameric STRs could be as high as 1 locus/20 kb. The markers should be useful for genetic mapping, as they are sequence based, and can be multiplexed with the PCR. A method enabling rapid localization of STRs and determination of their flanking DNA sequences was developed, thus simplifying the identification of polymorphic STR loci. The ease by which STRs may be identified, as well as their genetic and physical mapping utility, give them the properties of useful sequence tagged sites (STSs) for the human genome initiative.