A microsatellite genetic linkage map of human chromosome 18

Interactions among catechol-O-methyltransferase genotype, parenting, and sex predict children's internalizing symptoms and inhibitory control: Evidence for differential susceptibility

We used sex, observed parenting quality at 18 months, and three variants of the catechol-O-methyltransferase gene (Val158Met [rs4680], intron1 [rs737865], and 3'-untranslated region [rs165599]) to predict mothers' reports of inhibitory and attentional control (assessed at 42, 54, 72, and 84 months) and internalizing symptoms (assessed at 24, 30, 42, 48, and 54 months) in a sample of 146 children (79 male). Although the pattern for all three variants was very similar, Val158Met explained more variance in both outcomes than did intron1, the 3'-untranslated region, or a haplotype that combined all three catechol-O-methyltransferase variants. In separate models, there were significant three-way interactions among each of the variants, parenting, and sex, predicting the intercepts of inhibitory control and internalizing symptoms. Results suggested that Val158Met indexes plasticity, although this effect was moderated by sex. Parenting was positively associated with inhibitory control for methionine-methionine boys and for valine-valine/valine-methionine girls, and was negatively associated with internalizing symptoms for methionine-methionine boys. Using the "regions of significance" technique, genetic differences in inhibitory control were found for children exposed to high-quality parenting, whereas genetic differences in internalizing were found for children exposed to low-quality parenting. These findings provide evidence in support of testing for differential susceptibility across multiple outcomes.

Interactions between serotonin transporter gene haplotypes and quality of mothers' parenting predict the development of children's noncompliance

The LPR and STin2 polymorphisms of the serotonin transporter gene (SLC6A4) were combined into haplotypes that, together with quality of maternal parenting, were used to predict initial levels and linear change in children's (N = 138) noncompliance and aggression from age 18-54 months. Quality of mothers' parenting behavior was observed when children were 18 months old, and nonparental caregivers' reports of noncompliance and aggression were collected annually from 18 to 54 months of age. Quality of early parenting was negatively related to the slope of noncompliance only for children with the LPR-S/STin2-10 haplotype and to 18-month noncompliance only for children with haplotypes that did not include LPR-S. The findings support the notion that SLC6A4 haplotypes index differential susceptibility to variability in parenting quality, with certain haplotypes showing greater reactivity to both supportive and unsupportive environments. These different genetic backgrounds likely reflect an evolutionary response to variation in the parenting environment.

Polymorphisms in SLC6A4, PAH, GABRB3, and MAOB and modification of psychotic disorder features

We tested four genes [phenylalanine hydroxylase (PAH), the serotonin transporter (SLC6A4), monoamine oxidase B (MAOB), and the gamma-aminobutyric acid A receptor beta-3 subunit (GABRB3)] for their impact on five schizophrenia symptom factors: delusions, hallucinations, mania, depression, and negative symptoms. In a 90 family subset of the Irish Study of High Density Schizophrenia Families, the PAH 232 bp microsatellite allele demonstrated significant association with the delusions factor using both QTDT (F=8.0, p=.031) and QPDTPHASE (chi-square=12.54, p=.028). Also, a significant association between the GABRB3 191 bp allele and the hallucinations factor was detected using QPDTPHASE (chi-square=15.51, p=.030), but not QTDT (chi-square=2.07, p=.560).

Do A-stretches inhibit the emergence of new variants in STR loci?

Penta E and D18S51 loci are known to be highly polymorphic in humans. In this study, Japanese macaque PCR products amplified with human Penta E and D18S51 primers were examined. Electrophoresis and sequence analyses revealed that 13 Japanese macaque products amplified with human Penta E primer were homogeneous. The Japanese macaque sequence was extremely similar with human Penta E alleles except for the insertion of contiguous adenosine repeats, called 'A-stretch', at 5'-side of AAAGA repeats. Among the 11 Japanese macaque PCR products amplified with human D18S51 primers, only two variants were observed. The sequences of these Japanese macaque products were similar to those of human D18S51 alleles. However, the Japanese macaque sequences also contained the insertion of A-stretch at 5'-side of AGAA repeats. Less polymorphism in the Japanese macaque sequences, in contrast with highly polymorphic human Penta E and D18S51 loci, suggested that A-stretches might inhibit the emergence of new variants at the STR loci.

Novel linkage to chromosome 20p using latent classes of psychotic illness in 270 Irish high-density families

BACKGROUND

Several lines of evidence suggest that the clinical heterogeneity of schizophrenia is due to genetic heterogeneity. Genetic heterogeneity may decrease the signal-to-noise ratio in linkage and association studies. Therefore, linkage studies of clinically homogeneous classes of psychotic illness may result in greater power to detect at least some loci.

METHODS

Latent class analysis was used to divide psychotic subjects from 270 Irish high-density families (N = 755) into six classes based on the Operational Criteria Checklist for Psychotic Illness. We heuristically call them Bipolar, Schizoaffective, Mania, Schizomania, Deficit Syndrome, and Core Schizophrenia. The latter four had prevalences of greater than .08 and were individually tested for linkage in a 10-cM nonparametric autosomal genomewide scan. Empirical significance was determined using 200 simulated genome scans.

RESULTS

Seven regions achieved empirical criteria for suggestive significance for at least one latent class: 5q23.2-q35.3, 8q13.1-q23.1, 10q23.33-q26.3, 12q21.2-q24.32, 19q13.32-q13.43, 20p13-q22.3, and 21q11.2-q22.3. Five of 200 simulated scans resulted in seven suggestively significant loci (experiment-wide p = .03). Furthermore, at 20p13-p12.2, the Mania and Schizomania classes individually achieved criteria, whereas Deficit Syndrome had a suggestive logarithm of the odds peak 28 cM centromeric to this locus.

CONCLUSIONS

Using empirically derived, clinically homogeneous phenotypes, four chromosomal regions were suggestively linked but provided little evidence of linkage using traditional operationalized criteria. This approach was particularly fruitful on chromosome 20, which had previously yielded little evidence of linkage. Future studies of psychiatric illness may increase their ability to detect linkage or association by using clinically homogeneous phenotypes.

Significant correlation in linkage signals from genome-wide scans of schizophrenia and schizotypy

Prior family and adoption studies have suggested a genetic relationship between schizophrenia and schizotypy. However, this has never been verified using linkage methods. We therefore attempted to test for a correlation in linkage signals from genome-wide scans of schizophrenia and schizotypy. The Irish study of high-density schizophrenia families comprises 270 families with at least two members with schizophrenia or poor-outcome schizoaffective disorder (n=637). Non-psychotic relatives were assessed using the structured interview for schizotypy (n=746). A 10-cM multipoint, non-parametric, autosomal genome-wide scan of schizophrenia was performed in Merlin. A scan of a quantitative trait comprising ratings of DSM-III-R criteria for schizotypal personality disorder in non-psychotic relatives was also performed. Schizotypy logarithm of the odds (LOD) scores were regressed onto schizophrenia LOD scores at all loci, with adjustment for spatial autocorrelation. To assess empirical significance, this was also carried out using 1000 null scans of schizotypy. The number of jointly linked loci in the real data was compared to distribution of jointly linked loci in the null scans. No markers were suggestively linked to schizotypy based on strict Lander-Kruglyak criteria. Schizotypy LODs predicted schizophrenia LODs above chance expectation genome wide (empirical P=0.04). Two and four loci yielded nonparametric LOD (NPLs) >1.0 and >0.75, respectively, for both schizophrenia and schizotypy (genome-wide empirical P=0.04 and 0.02, respectively). These results suggest that at least a subset of schizophrenia susceptibility genes also affects schizotypy in non-psychotic relatives. Power may therefore be increased in molecular genetic studies of schizophrenia if they incorporate measures of schizotypy in non-psychotic relatives.

A genome-wide scan for modifier loci in schizophrenia

The purpose of this study was to detect genetic loci that influence clinical features of, but not necessarily susceptibility to, psychotic illness. In the Irish Study of High-Density Schizophrenia Families (n = 270 families, n = 1,408 individuals), subjects with non-affective psychosis were rated using the Operational Criteria Checklist for Psychotic Illness. Factor analysis identified hallucinations, delusions, and negative, manic, and depressive symptom factors. We performed autosomal genome-wide multipoint non-parametric quantitative trait locus linkage analysis, in affected individuals only, using these five factors, as well as age at onset, and course of illness. Determination of empirical significance and correction for multiple testing was implemented using 200 simulated genome scans. We also tested for pleiotropic loci by examining the sums of -log(10)'s of the empirical P values of multiple traits in selected regions. LODs of 2.42 and 2.35 were obtained near D9S934 (9q33.1) and D14S587 (14q24.2), respectively, for course of illness, and of 2.26 between D6S1040-D6S2420 (6q23.1-25.1) and age at onset. No other regions met criteria for suggestive linkage to any one trait. No loci were significant after correction for multiple testing. On 6q, however, the joint linkage of age of onset, course, delusions, and depressive symptoms resulted in a genome-wide P = 0.06. We conclude that genes located near 9q33.1 and 14q24.2 may modify the clinical course and severity of schizophrenia. A gene in 6q may affect several clinical features of illness.

Using sex-averaged genetic maps in multipoint linkage analysis when identity-by-descent status is incompletely known

The ratio of male and female genetic map distances varies dramatically across the human genome. Despite these sex differences in genetic map distances, most multipoint linkage analyses use sex-averaged genetic maps. We investigated the impact of using a sex-averaged genetic map instead of sex-specific maps for multipoint linkage analysis of affected sibling pairs when identity-by-descent states are incompletely known due to missing parental genotypes and incomplete marker heterozygosity. If either all or no parental genotypes were available, for intermarker distances of 10, 5, and 1 cM, we found no important differences in the expected maximum lod score (EMLOD) or location estimates of the disease locus between analyses that used the sex-averaged map and those that used the true sex-specific maps for female:male genetic map distance ratios 1:10 and 10:1. However, when genotypes for only one parent were available and the recombination rate was higher in females, the EMLOD using the sex-averaged map was inflated compared to the sex-specific map analysis if only mothers were genotyped and deflated if only fathers were genotyped. The inflation of the lod score when only mothers were genotyped led to markedly increased false-positive rates in some cases. The opposite was true when the recombination rate was higher in males; the EMLOD was inflated if only fathers were genotyped, and deflated if only mothers were genotyped. While the effects of missing parental genotypes were mitigated for less extreme cases of missingness, our results suggest that when possible, sex-specific maps should be used in linkage analyses.

Genetics and Genomics of Core Short Tandem Repeat Loci Used in Human Identity Testing

Over the past decade, the human identity testing community has settled on a set of core short tandem repeat (STR) loci that are widely used for DNA typing applications. A variety of commercial kits enable robust amplification of these core STR loci. A brief history is presented regarding the selection of core autosomal and Y-chromosomal STR markers. The physical location of each STR locus in the human genome is delineated and allele ranges and variants observed in human populations are summarized as are mutation rates observed from parentage testing. Internet resources for additional information on core STR loci are reviewed. Additional topics are also discussed, including potential linkage of STR loci to genetic disease-causing genes, probabilistic predictions of sample ethnicity, and desirable characteristics for additional STR loci that may be added in the future to the current core loci. These core STR loci, which form the basis for DNA databases worldwide, will continue to play an important role in forensic science for many years to come.

Clinical relevance of genetic instability in prostatic cells obtained by prostatic massage in early prostate cancer

We investigated whether genetic lesions such as loss of heterozygosity (LOH) are detected in prostatic cells obtained by prostatic massage during early diagnosis of prostate cancer (CaP) and discussed their clinical relevance. Blood and first urine voided after prostatic massage were collected in 99 patients with total prostate-specific antigen (PSA) between 4 and 10 ng ml⁻¹, prior to prostate biopsies. Presence of prostatic cells was confirmed by quantitative RT–PCR analysis of PSA mRNA. Genomic DNA was analysed for LOH on six chromosomal regions. One or more allelic deletions were found in prostatic fluid from 57 patients analysed, of whom 33 (58%) had CaP. Sensitivity and specificity of LOH detection and PSA free to total ratio <15% for positive biopsy were respectively 86.7 and 44% (P=0.002) for LOH, and 55 and 74% (P=0.006) for PSA ratio <15%. Analysis of LOH obtained from prostatic tumours revealed similar patterns compared to prostatic fluid cells in 86% of cases, confirming its accuracy. The presence of LOH of urinary prostatic cells obtained after prostatic massage is significantly associated with CaP on biopsy and may potentially help to identify a set of patients who are candidates for further prostate biopsies.

DNA studies of mono- and pseudodicentric isochromosomes 18q

The description of isochromosomes 18 has so far mainly been by cytogenetic studies and based on identical banding pattern of the two arms. However, only molecular techniques are capable to distinguish an isochromosome from a translocation, whole arm or reciprocal, between two chromosomes 18. We have used 23 PCR-based DNA polymorphisms to determine the parental origin and mechanisms of formation in four patients with isochromosomes 18q and to demonstrate that they were consistent with true isochromosomes. Three of the probands were liveborn children with clinical features characteristic of Edwards syndrome, one proband was a fetus diagnosed at prenatal diagnosis. In one case the isochromosome was monocentric with two identical q arms of maternal origin, formed by misdivision of the centromere and loss of p arm material. Another monocentric case had 47 chromosomes with isochromosomes i(18p) and i(18q) formed by maternal postzygotic centromeric misdivision and segregation of both isochromosomes, or by meiosis II centromeric misdivision and nondisjunction (without recombination in meiosis I). In two cases, the isochromosomes were dicentric with genetically identical arms composed of a part of the short and the whole long arm of chromosome 18 of paternal origin. The formation of the fused chromosomes can be explained by postzygotic exchange of sister chromatids on the short arm of chromosome 18, followed by breakage and U-shape reunion of sister chromatids.

Clinical features of psychotic disorders and polymorphisms in HT2A, DRD2, DRD4, SLC6A3 (DAT1), and BDNF: a family based association study

Schizophrenia is clinically heterogeneous and multidimensional, but it is not known whether this is due to etiological heterogeneity. Previous studies have not consistently reported association between any specific polymorphisms and clinical features of schizophrenia, and have primarily used case-control designs. We tested for the presence of association between clinical features and polymorphisms in the genes for the serotonin 2A receptor (HT2A), dopamine receptor types 2 and 4, dopamine transporter (SLC6A3), and brain-derived neurotrophic factor (BDNF). Two hundred seventy pedigrees were ascertained on the basis of having two or more members with schizophrenia or poor outcome schizoaffective disorder. Diagnoses were made using a structured interview based on the SCID. All patients were rated on the major symptoms of schizophrenia scale (MSSS), integrating clinical and course features throughout the course of illness. Factor analysis revealed positive, negative, and affective symptom factors. The program QTDT was used to implement a family-based test of association for quantitative traits, controlling for age and sex. We found suggestive evidence of association between the His452Tyr polymorphism in HT2A and affective symptoms (P = 0.02), the 172-bp allele of BDNF and negative symptoms (P = 0.04), and the 480-bp allele in SLC6A3 (= DAT1) and negative symptoms (P = 0.04). As total of 19 alleles were tested, we cannot rule out false positives. However, given prior evidence of involvement of the proteins encoded by these genes in psychopathology, our results suggest that more attention should be focused on the impact of these alleles on clinical features of schizophrenia.

Co-amplification of ENFSI-loci D3S1358, D8S1179 and D18S51: validation of new primer sequences and allelic distribution among 2874 individuals

The present communication presents a new triplex PCR co-amplifying three loci (D3S1358, D8S1179 and D18S51) recommended for STR typing by the European Network of Forensic Science Institutes (ENFSI). Twenty-two different primers were tested to optimise the PCR. Four of the six primer sequences finally chosen were self selected, the fifth was a published one and the sixth derived from a commercially available multiplex kit. Using this PCR-setup, even minimum amounts of genomic DNA are sufficient to analyse the STR loci D3S1358, D8S1179 and D18S51 in parallel. Especially in forensic casework, where DNA is mostly limited and often contaminated with enzyme inhibitors, this new PCR proved to be very advantageous. To demonstrate the reliability, buccal swabs from 2874 persons were typed not only with the new triplex PCR but also with a commercially available multiplex kit.

Genetic maps of microsatellite and single-nucleotide polymorphism markers: are the distances accurate?

Genetic maps play an important role in gene mapping. Inaccurate genetic maps can hinder gene mapping by biasing lod scores and reducing the power to map a trait to a particular region. Although sequence-based physical maps can provide a unique order for markers, they do not provide information on genetic map distances. By simulation studies, I investigated how many meioses are necessary to accurately estimate genetic map distances for maps constructed from microsatellite and single-nucleotide polymorphism (SNP) markers for various intermarker distances and marker heterozygosity. To evaluate the accuracy of the generated genetic maps, the length of the 95% confidence interval for intermarker genetic distances was examined. In addition, the power to separate two adjacent markers by a nonzero map distance was investigated. The number of meioses necessary to accurately estimate map distances depends greatly not only on intermarker distances but also on marker heterozygosity. For example, for a genetic map with intermarker distances of 0.5 cM generated with 1,000 meioses, when marker heterozygosity was high (0.90), for 96% of the markers there was a nonzero map distance between adjacent markers. However, when marker heterozygosity was low (0.32), only 48% of the markers mapped to a unique position. For identical numbers of meioses and intermarker distances, genetic maps constructed from microsatellite markers will be more precise than maps assembled from SNP markers, due to the higher levels of heterozygosity for microsatellite markers.

Genome-wide scans of three independent sets of 90 Irish multiplex schizophrenia families and follow-up of selected regions in all families provides evidence for multiple susceptibility genes

From our linkage study of Irish families with a high density of schizophrenia, we have previously reported evidence for susceptibility genes in regions 5q21-31, 6p24-21, 8p22-21, and 10p15-p11. In this report, we describe the cumulative results from independent genome scans of three a priori random subsets of 90 families each, and from multipoint analysis of all 270 families in ten regions. Of these ten regions, three (13q32, 18p11-q11, and 18q22-23) did not generate scores above the empirical baseline pairwise scan results, and one (6q13-26) generated a weak signal. Six other regions produced more positive pairwise and multipoint results. They showed the following maximum multipoint H-LOD (heterogeneity LOD) and NPL scores: 2p14-13: 0.89 (P = 0.06) and 2.08 (P = 0.02), 4q24-32: 1.84 (P = 0.007) and 1.67 (P = 0.03), 5q21-31: 2.88 (P= 0.0007), and 2.65 (P = 0.002), 6p25-24: 2.13 (P = 0.005) and 3.59 (P = 0.0005), 6p23: 2.42 (P = 0.001) and 3.07 (P = 0.001), 8p22-21: 1.57 (P = 0.01) and 2.56 (P = 0.005), 10p15-11: 2.04 (P = 0.005) and 1.78 (P = 0.03). The degree of 'internal replication' across subsets differed, with 5q, 6p, and 8p being most consistent and 2p and 10p being least consistent. On 6p, the data suggested the presence of two susceptibility genes, in 6p25-24 and 6p23-22. Very few families were positive on more than one region, and little correlation between regions was evident, suggesting substantial locus heterogeneity. The levels of statistical significance were modest, as expected from loci contributing to complex traits. However, our internal replications, when considered along with the positive results obtained in multiple other samples, suggests that most of these six regions are likely to contain genes that influence liability to schizophrenia.

The human genome project and its impact on psychiatry

There has been substantial evidence for more than three decades that the major psychiatric illnesses such as schizophrenia, bipolar disorder, autism, and alcoholism have a strong genetic basis. During the past 15 years considerable effort has been expended in trying to establish the genetic loci associated with susceptibility to these and other mental disorders using principally linkage analysis. Despite this, only a handful of specific genes have been identified, and it is now generally recognized that further advances along these lines will require the analysis of literally hundreds of affected individuals and their families. Fortunately, the emergence in the past three years of a number of new approaches and more effective tools has given new hope to those engaged in the search for the underlying genetic and environmental factors involved in causing these illnesses, which collectively are among the most serious in all societies. Chief among these new tools is the availability of the entire human genome sequence and the prospect that within the next several years the entire complement of human genes will be known and the functions of most of their protein products elucidated. In the meantime the search for susceptibility loci is being facilitated by the availability of single nucleotide polymorphisms (SNPs) and by the beginning of haplotype mapping, which tracks the distribution of clusters of SNPs that segregate as a group. Together with high throughput DNA sequencing, microarrays for whole genome scanning, advances in proteomics, and the development of more sophisticated computer programs for analyzing sequence and association data, these advances hold promise of greatly accelerating the search for the genetic basis of most mental illnesses while, at the same time, providing molecular targets for the development of new and more effective therapies.

DNA fingerprinting of sister blastomeres from human IVF embryos

BACKGROUND

Previously published single cell DNA fingerprinting systems have been plagued by high rates of allele drop-out (ADO) and preferential amplification (PA) preventing clinical application in preimplantation genetic diagnosis.

METHODS

Tetranucleotide microsatellite markers with high heterozygosity, known allelic size ranges and minimal PCR stutter artefacts were selected for chromosomes X, 13, 18 and 21 and optimized in a multiplex fluorescent (FL)-PCR format. FL-PCR products were analysed using the ABI Prism 377 DNA sequenator and Genescan software. Validation of the DNA fingerprinting system was performed on single diploid (n = 50) and aneuploid (n = 25) buccal cells and embryonic blastomeres (n = 21).

RESULTS

The optimized pentaplex PCR DNA fingerprinting system displayed a high proportion of successful amplifications (>91%) and low ADO and PA (<6%) when assessed on 50 human buccal cells. DNA fingerprints of single cells from a subject with Down's syndrome detected the expected tri-allelic pattern for the chromosome 21 marker, confirming trisomy 21. In a blind study on 21 single blastomeres, all embryos were identifiable by their unique DNA fingerprints and shared parental alleles.

CONCLUSIONS

A highly specific multiplex FL-PCR based on the amplification of five highly polymorphic microsatellite markers was developed for single cells. This finding paves the way for the development of a more complex PCR DNA fingerprinting system to assess aneuploidy and single gene mutations in IVF embryos from couples at genetic risk.

Molecular characterization of 18p deletions: evidence for a breakpoint cluster

PURPOSE

To determine the size and parental origin of the deletion in individuals with 18p- syndrome.

METHODS

Molecular and fluorescence in situ hybridization analyses of the pericentromeric region of chromosome 18 were performed on genomic DNA and chromosomes from study participants.

RESULTS

The majority of the breakpoints were located between markers D18S852 on 18p and D18S1149 on 18q, a distance of approximately 4 Mb. The parental origin of these deletions appears to be equally distributed, half maternally derived and half paternally derived.

CONCLUSION

The distributions of both the size and parental origin of the 18p deletions support the presence of a breakpoint cluster in the 18p- syndrome.

Effect of COMT Val108/158 Met genotype on frontal lobe function and risk for schizophrenia

Abnormalities of prefrontal cortical function are prominent features of schizophrenia and have been associated with genetic risk, suggesting that susceptibility genes for schizophrenia may impact on the molecular mechanisms of prefrontal function. A potential susceptibility mechanism involves regulation of prefrontal dopamine, which modulates the response of prefrontal neurons during working memory. We examined the relationship of a common functional polymorphism (Val(108/158) Met) in the catechol-O-methyltransferase (COMT) gene, which accounts for a 4-fold variation in enzyme activity and dopamine catabolism, with both prefrontally mediated cognition and prefrontal cortical physiology. In 175 patients with schizophrenia, 219 unaffected siblings, and 55 controls, COMT genotype was related in allele dosage fashion to performance on the Wisconsin Card Sorting Test of executive cognition and explained 4% of variance (P = 0.001) in frequency of perseverative errors. Consistent with other evidence that dopamine enhances prefrontal neuronal function, the load of the low-activity Met allele predicted enhanced cognitive performance. We then examined the effect of COMT genotype on prefrontal physiology during a working memory task in three separate subgroups (n = 11-16) assayed with functional MRI. Met allele load consistently predicted a more efficient physiological response in prefrontal cortex. Finally, in a family-based association analysis of 104 trios, we found a significant increase in transmission of the Val allele to the schizophrenic offspring. These data suggest that the COMT Val allele, because it increases prefrontal dopamine catabolism, impairs prefrontal cognition and physiology, and by this mechanism slightly increases risk for schizophrenia.

Analysis of epistasis in linked regions in the Irish study of high-density schizophrenia families

Epistasis may be important in the etiology of schizophrenia. Analysis of epistasis has been important in the positional cloning of a gene involved in the etiology of type II diabetes mellitus. We investigated the importance of epistasis among six linked regions in 268 multiplex pedigrees in the Irish Study of High-Density Schizophrenia Families (ISHDSF) by computing pairwise correlations between nonparametric linkage scores for narrow, intermediate, and broad diagnostic definitions. The linked regions were on chromosomes 2, 4, 5, 6, 8, and 10. No correlation reached our a priori level of statistical significance. Using this statistical approach, we did not find evidence of important epistatic effects among these six regions in the ISHDSF.

Bias in multipoint linkage analysis arising from map misspecification

Multipoint linkage analysis methods are often used in human genetic studies. Although multipoint methods increase power for a linkage analysis and will become essential if use of diallelic markers becomes widespread, the methods in use assume an accurate meiotic marker map. Unfortunately, uncertainties in estimates of between-marker meiotic distances are large. Also, sex-averaged maps are generally used, but recombination rates differ in males and females. Both these types of map misspecification can lead to lod score bias, but such bias has not previously been systematically quantified. We examine multipoint lod score bias arising from these map misspecifications, in both the presence and absence of actual linkage. We define bias as the expected difference between the lod score computed under the misspecified map and that computed under the true map. With actual linkage, any map misspecification causes negative bias in lod scores, resulting in loss of power to detect linkage. In most cases, bias is modest, only reaching clearly detectable levels when both types of misspecification are substantial. In the absence of linkage, map misspecification can cause positive or negative bias: falsely assuming a 1:1 female:male ratio always causes positive bias; using too large a distance gives a positive bias; using too small a distance gives a negative bias. This bias can inflate the false-positive rate, especially when the sample size is modest. We conclude that although current sex-averaged maps are suitable for a first-pass multipoint screen, the potential for bias from map misspecification should be evaluated in following up results from such an analysis.

Analysis of the co-amplified STR loci D1S1656, D12S391 and D18S51: population data and validation study for a highly discriminating triplex-PCR

A multiplex-PCR composed of the three highly variable STR loci D1S1656, D12S391 and D18S51 has been established. The non-overlapping fragment sizes allow allele detection using a monochrome automated laser fluorescent sequencer (A.L.F. express, Pharmacia Biotech). The typing results of the triplex-PCR showed no difference to those of singleplex-PCR. Allele frequencies were determined in a Western German population of 228 individuals from Cologne. The heterozygosities and exclusion chances (D1S1656, 0.982; D12S391, 0.979; D18S51, 0.97) are very high compared to other short tandem repeats used for forensic applications. No deviations from the Hardy-Weinberg equilibrium were found. Successful typing of DNA amounts down to 50-100 pg is possible. Mixtures of up to 1:10 can be identified. In conclusion, the high combined exclusion chance due to the well-balanced allelic distribution and its high sensitivity make this triplex-PCR a valuable tool for forensic casework.

Microsatellite variation in Central Africa: an analysis of intrapopulational and interpopulational genetic diversity

As a part of a research project on molecular variation in Central Africa, we have analyzed 10 microsatellites (CD4, CSFO, D3S1358, D18S51, D21S11, F13A1, FES, TH01, TPOX, and VWA) in the Bamileke and Ewondo from Cameroon and the Sanga and Mbenzele Pygmies from the Central African Republic (a total of 390 chromosomes). A statistically significant trend towards heterozygote deficiency was detected in the Mbenzele Pygmies. This was established through the use of powerful exact tests for the Hardy-Weinberg equilibrium. A certain degree of isolation and a small effective size may explain this finding. However, the lack of any substantial reduction in allelic diversity in the Mbenzele does not support the possibility that this group has a smaller effective size in evolutionary terms. A possible explanation based on ethnographic studies suggests that the gene flow from non-Pygmies to Pygmies could have been interrupted only in relatively recent times. The analysis of association between genotypes at pairs of independent loci indicates that the level of subheterogeneity is markedly lower in the Bamileke than in other sampled populations. This may be explained by the combined effect of larger population size, more rigid respect of clanic exogamy, and higher matrimonial mobility of the Bamileke. Finally, we have analyzed interpopulational relationships among our sampled populations and other Central African populations. The results are consistent with a previous study of protein loci (Spedini et al. 1999), which suggests the recent history of the Bamileke and Ewondo has led them to aquire a substantial genetic similarity. Furthermore, the Mbenzele Pygmies diverge from Biaka Pygmies, despite their common origin and geographical proximity. This is probably due to the differentiating effect of genetic drift, which is enhanced by the small effective size of Pygmy populations.

The hypothyroidism in an inbred kindred with congenital thyroid hormone and glucocorticoid deficiency is due to a mutation producing a truncated thyrotropin receptor

Growth and function of the thyroid and adrenal glands are maintained and controlled by thyrotropin (TSH) and adrenocorticotrophic hormone (ACTH), respectively. The action of these trophic hormones requires the presence of functional TSH and ACTH receptors. We describe a large inbred Bedouin kindred in which profound congenital hypothyroidism and hypoadrenocortisolism occurred alone or together in eight family members belonging to four nuclear families. The high serum TSH and ACTH levels in the presence of normal or hypoplastic thyroid glands and low glucocorticoid, but not mineralocorticoid concentrations, are characteristic of resistance to TSH and ACTH. Linkage analysis, using specific polymorphic markers, excluded the involvement of the ACTH receptor but not thyrotropin receptor (TSHR). A novel point mutation was identified in exon 10 of the TSHR that replaces the normal cytosine in nucleotide 2024 with a thymidine. As a result the normal arginine in codon 609 (CGA) is replaced with a stop codon (TGA). This mutation produces a truncated TSHR lacking the third intracellular and extracellular loops, the sixth and seventh transmembrane segments, and the intracytoplasmic tail. The presence of hypothyroidism did not affect the timing, severity, and manner of clinical manifestation of hypoadrenocortisolism.

Haplotype analysis of families with erythropoietic protoporphyria and novel mutations of the ferrochelatase gene

Ferrochelatase, the enzyme that catalyzes the terminal step in the heme biosynthetic pathway, is the site of the defect in the human inherited disease erythropoietic protoporphyria. Molecular genetic studies have shown that the majority of erythropoietic protoporphyria cases are transmitted in dominant fashion and that mutations underlying erythropoietic protoporphyria are heterogeneous. We performed haplotype analysis of American families that shared recurrent ferrochelatase gene mutations yet had forbearers from several European countries. This was to gain insight into whether these mutations represent mutational hotspots at the ferrochelatase gene, or propagation of ancestral alleles bearing the mutations. Two recurrent mutations were found to occur on distinctive chromosome 18 haplotypes, consistent with being hotspot mutations. On the other hand, we found three sets of two unrelated families that shared the same haplotypes bearing these mutations, which could reflect geographic dispersion of ancestral mutant alleles. In addition, we report novel mutations associated with erythropoietic protoporphyria: g(+ 1)-->t transversion of the exon 4 donor site, g(+ 1)-->a transition of the exon 6 donor site, and t(+ 2)-->a substitution at the exon 9 donor site; these mutations are predicted to cause splicing defects of the associated exons. We also identified a g(+ 5)-->a transition of the exon 1 donor site in four unrelated families with erythropoietic protoporphyria, and a G(- 1)-->A substitution at the exon 9 donor site in an additional family. The probability that these sequence changes are normal polymorphisms was virtually excluded (p < 0.0001) by their absence in 120 ferrochelatase alleles from 30 normal subjects and 30 individuals with manifested erythropoietic protoporphyria with or without a known mutation.

Preimplantation genetic diagnosis using fluorescent polymerase chain reaction: results and future developments

PURPOSE

Fluorescent polymerase chain reaction (PCR) is a multipurpose technique that can be used for diagnosing sex, single-gene defects, and trisomies as well as determining DNA fingerprints from single cells. However, its effectiveness must be assessed before clinical preimplantation genetic diagnosis (PGD) application.

METHODS

Single and multiplex fluorescent PCR was applied to single cells and blastomeres.

RESULTS

Fluorescent PCR can be used to diagnose sex from blastomeres and has been successfully applied in a clinical PGD sexing program resulting in a confirmed pregnancy. A further major advantage of fluorescent PCR is the ability to multiplex, providing multiple diagnoses and DNA fingerprints with a high reliability (approximately 75% for trisomy, 86% for DNA fingerprint) and good accuracy (70-80%). Allele dropout in multiplex PCR is approximately 20% per allele and does not appear to be associated with the fragment size.

CONCLUSIONS

Fluorescent PCR is a powerful technique for PGD, and the effects of allele dropout must be considered, particularly in multiplex PCR.

Multiple genetic diagnoses from single cells using multiplex PCR: reliability and allele dropout

We used a multiplex fluorescent PCR system containing seven primer sets on single cells from three different cell types (buccal, corneal and blastomere cells) and more than 3500 heterozygous alleles to investigate reliability and extent of allele dropout in multiplex PCRs at the single cell level. All three cell types gave similarly high reliability, accuracy and allele dropout rates, with similar reliability between singleplex and multiplex PCRs. Allele dropout was also consistent between the three cell types and did not significantly increase as allele size increased. These results indicate that multiplex fluorescent PCR is a reliable and accurate method of obtaining multiple diagnosis (eight chromosomes simultaneously) from single cells and maximizes the information available from single cell analysis.

Support for a chromosome 18p locus conferring susceptibility to functional psychoses in families with schizophrenia, by association and linkage analysis

The action of antipsychotic drugs on dopamine receptors suggests that dopaminergic signal transmission may play a role in the development of schizophrenia. We tested eight candidate genes (coding for dopamine receptors, the dopamine transporter, and G-proteins) in 59 families from Germany and Israel, for association. A P value of .00055 (.0044 when corrected for the no. of markers tested) was obtained for the intronic CA-repeat marker G-olfalpha on chromosome 18p. The value decreased to .000088 (.0007) when nine sibs with recurrent unipolar depressive disorder were included. Linkage analysis using SSLP markers densely spaced around G-olfalpha yielded a maximum two-point LOD score of 3.1 for a marker 0.5 cM distal to G-olfalpha. Multipoint analysis under the assumption of heterogeneity supported this linkage-whether the affected pheotype was defined narrowly or broadly-as did nonparametric linkage (NPL). In 12 families with exclusively maternal transmission of the disease, the NPL value also supported linkage to this marker. In order to test for association/linkage disequilibrium in the presence of linkage, the sample was restricted to independent offspring. When this sample was combined with 65 additional simplex families (each of them comprising one schizophrenic offspring and his or her parents), the 124-bp allele of G-olfalpha was transmitted 47 times and was not transmitted 21 times (P=.009). These results suggest the existence, on chromosome 18p, of a potential susceptibility locus for functional psychoses.

Important microsatellite markers in the investigation of replication errors (RER) in colorectal carcinomas

BACKGROUND

DNA replication errors (RER) have been found in hereditary nonpolyposis colorectal carcinomas and in sporadic colorectal carcinomas. The incidence of RER depends on which and how many markers are examined. The main purpose of the present study was to determine the key markers for detecting RER most efficiently.

METHODS

The RER status of 76 sporadic advanced colorectal carcinomas in the proximal colon were investigated. Seven microsatellite markers (D2S123, D3S1029, D3S1611, D2S72, TP53, Mfd26 and BAT26) were chosen to determine the RER status by PCR using the non-Rl method, because these seven markers have frequently been used in other studies and also detect RER.

RESULTS

It was found that 44.7% of sporadic colorectal advanced carcinomas in the proximal colon (34 of 76) showed RER at one or more loci. Among these 34 cases, RER was present at three or more markers (severe RER) in 22. All 22 of these cases showed RER at BAT26 and TP53. The other 12 cases with RER showed RER at one or two markers (mild RER). Eleven of these 12 cases (91%) showed RER at Mfd26 and there were one or two cases with mild RER at each of the other loci.

CONCLUSIONS

When one intends to analyze routinely a large number of cases, an analysis of two or three markers (Mfd26 and BAT26 or TP53) is considered to be sufficient for detecting mild and severe RER.

A schizophrenia locus may be located in region 10p15-p11

In our genomic scan of 265 Irish families with schizophrenia, we have thus far generated modest evidence for the presence of vulnerability genes in three chromosomal regions, i.e., 5q21-q31, 6p24-p22, and 8p22-p21. Outside of those regions, of all markers tested to date, D10S674 produced one of the highest pairwise heterogeneity lod (H-LOD) scores, 3.2 (P = 0.0004), when initially tested on a subset of 88 families. We then tested a total of 12 markers across a region of 32 centimorgans in region 10p15-p11 of all 265 families. The strongest evidence for linkage occurred assuming an intermediate phenotypic definition, and a recessive genetic model. The largest pairwise H-LOD score was found with marker D10S2443 (maximum 1.95, P = 0.005). Using multipoint H-LODs, we found a broad peak (maximum 1.91, P = 0.006) extending over the 11 centimorgans from marker D10S674 to marker D10S1426. Multipoint nonparametric linkage analysis produced a much broader peak, but with the maximum in the same location near D10S2443 (maximum z = 1.88, P = 0.03). Based on estimates from the multipoint analysis, this putative vulnerability locus appears to be segregating in 5-15% of the families studied, but this estimate should be viewed with caution. When evaluated in the context of our genome scan results, the evidence suggests the possibility of a fourth vulnerability locus for schizophrenia in these Irish families, in region 10p15-p11.

Promoter sequence, expression, and fine chromosomal mapping of the human gene (MLP) encoding the MARCKS-like protein: identification of neighboring and linked polymorphic loci for MLP and MACS and use in the evaluation of human neural tube defects

The MARCKS-like protein (MLP), also known as F52, MacMARCKS, or MARCKS-related protein, is a widely distributed substrate for protein kinase C (PKC). Recent studies using gene disruption in vivo have demonstrated the importance of both MARCKS and MLP to the development of the central nervous system; specifically, mice lacking either protein exhibit a high frequency of neural tube defects. We isolated a genomic clone for human MLP and discovered a directly linked polymorphism (MLP1) useful for genetic linkage analysis. The MLP promoter was 71% identical over 433 bp to that of the corresponding mouse gene, Mlp, with conservation of many putative transcription factor-binding sites; it was only 36% identical over 433 bp to the promoter of the human gene, MACS, which encodes the MLP homologue MARCKS. This 433-bp fragment drove expression of an MLP-beta-galactosidase transgene in a tissue-specific and developmental expression pattern that was similar to that observed for the endogenous gene, as shown by in situ hybridization histochemistry. In contrast to MACS, the MLP and Mlp promoters contain a TATA box approximately 40 bp 5' of the presumed transcription initiation site. MLP was localized to chromosome 1p34-->1pter by analysis of human-mouse somatic cell hybrid DNA and to 1p34 by fluorescence in situ hybridization. Radiation hybrid mapping of MLP placed it between genetic markers D1S511 (LOD > 3.0) and WI9232. MACS was localized to 6q21 between D6S266 (LOD > 3.0) and AFM268uh5 by the same technique. We tested the novel MLP1 polymorphism and the MACS flanking markers in a series of 43 Caucasian simplex families in which the affected child had a lumbosacral myelomeningocele. We found no evidence of linkage disequilibrium, suggesting that these loci were not major genes for spina bifida in these families. Nonetheless, the identification of linked and neighboring polymorphisms for MACS and MLP should permit similar genetic studies in other groups of patients with neural tube defects and other neurodevelopmental abnormalities.

No evidence for significant linkage between bipolar affective disorder and chromosome 18 pericentromeric markers in a large series of multiplex extended pedigrees

Bipolar affective disorder (BP) is a major neuropsychiatric disorder with high heritability and complex inheritance. Previously reported linkage between BP and DNA markers in the pericentromeric region of chromosome 18, with a parent-of-origin effect (linkage was present in pedigrees with paternal transmission and absent in pedigrees with exclusive maternal inheritance), has been a focus of interest in human genetics. We reexamined the evidence in one of the largest samples reported to date (1,013 genotyped individuals in 53 unilineal multiplex pedigrees), using 10 highly polymorphic markers and a range of parametric and nonparametric analyses. There was no evidence for significant linkage between BP and chromosome 18 pericentromeric markers in the sample as a whole, nor was there evidence for significant parent-of-origin effect (pedigrees with paternal transmission were not differentially linked to the implicated chromosomal region). Two-point LOD scores and single-locus sib-pair results gave some support for suggestive linkage, but this was not substantiated by multilocus analysis, and the results were further tempered by multiple test effects. We conclude that there is no compelling evidence for linkage between BP and chromosome 18 pericentromeric markers in this sample.

Microsatellite instability in sporadic carcinomas of the proximal colon: association with diploid DNA content, negative protein expression of p53, and distinct histomorphologic features

BACKGROUND

Microsatellite instability (MIN) seems to characterize a particular subset of sporadic colorectal adenocarcinomas with the studies indicating a better clinical outcome for patients with MIN-positive tumors than for those with MIN-negative ones. The goal of this study was to further clarify whether a genotype-specific histomorphology of the right-sided colonic carcinomas can be identified.

METHODS

MIN status, DNA content, and p53 protein expression were evaluated in cryoconserved specimens from 20 adenocarcinomas of the proximal colon and correlated to stage, grade, and other histomorphologic features. The study was restricted to tumors of the proximal colon because approximately 90% of all MIN-positive tumors were found in the proximal colon, and differences between right- and left-sided tumors cannot be excluded a priori.

RESULTS

By using four microsatellite markers, instability was detected in 35% of the tumors analyzed. The clinicopathologic features in the MIN-positive tumors were found to differ markedly from the MIN-negative tumors in their poorly differentiated histologic pattern, extracellular mucin production, and favorable lymph node and distant metastatic behavior. A marked association was found between MIN positivity and DNA diploid status, as well as negative p53 immunostaining.

CONCLUSIONS

The MIN-positive colonic carcinomas were characterized by distinct histomorphologic features that are recognizable at routine diagnostic evaluation. Poorly differentiated adenocarcinomas of the proximal colon, with only a few lymph nodes and no distant metastases at presentation, and lack of p53 accumulation are highly suggestive of being MIN positive. These tumors should be discriminated from the other poorly differentiated carcinomas, because they seem to be associated with an improved prognosis compared with the tumors without microsatellite instability.

Analysis and interpretation of mixed forensic stains using DNA STR profiling

The use of multiplex PCR and fluorescent dye technology in the automated detection and analysis of short tandem repeat loci provides not only qualitative information about the profile--i.e. which alleles are present--but can also provide quantitative information on the relative intensities of the bands, and is therefore a measure of the amount of amplified DNA. The availability of this quantitative information allows for the interpretation of mixtures in a detailed way which has not been previously possible with many other human identification systems. In this paper we present a simple approach to the resolution and analysis of mixed STR profiles resulting from the testing of mixed biological stains in forensic casework and highlight factors which can affect it. This approach requires a detailed knowledge--gained through a mixture of experiments and validation studies--of the behaviour of each locus within the multiplex systems described. We summarise the available data from previously published experimental work and validation studies to examine the general principles underlying this approach.

Autosomal dominant hypophosphatemic rickets is linked to chromosome 12p13

Autosomal dominant hypophosphatemic rickets (ADHR) is an inherited disorder of isolated renal phosphate wasting, the pathogenesis of which is unknown. We performed a genome-wide linkage study in a large kindred to determine the chromosome location of the ADHR gene. Two-point LOD scores indicate that the gene is linked to the markers D12S314 [Z(theta) = 3.15 at theta = 0.0], vWf [Z(theta) = 5.32 at theta = 0.0], and CD4 [Z(theta) = 3.53 at theta = 0.0]. Moreover, multilocus analysis indicates that the ADHR gene locus is located on chromosome 12p13 in the 18-cM interval between the flanking markers D12S100 and D12S397. These data are the first to establish a chromosomal location for the ADHR locus and to provide a framework map to further localize the gene. Such studies will permit ultimate identification of the ADHR gene and provide further insight into phosphate homeostasis.

Lack of evidence for a major locus for bipolar disorder in the pericentromeric region of chromosome 18 in Irish pedigrees

Seven families, multiply affected by bipolar mood disorder, have been collected from the Irish population and have been genotyped with microsatellite markers from the pericentromeric region of chromosome 18, a region that has been implicated as a site for a susceptibility gene for this relative common psychiatric disorder. The families significantly excluded linkage of bipolar disorder to this region under various models. Although the data provided no evidence of linkage heterogeneity among families, the number of families investigated may be too small to exclude completely the possibility of linkage in a small number of families.

Optimization of a hexaplex DNA amplification from short tandem repeat and amelogenin loci

An automated DNA profiling system based on the multiplex amplification of highly polymorphic short tandem repeat (STR) markers and the amelogenin locus was developed. Five STR loci with nonoverlapping allele size ranges have been utilized in the multiplex amplifications, including HUMD1S103, HUMTH01, HUMD21S11, HUMD18S51, and HUMFIBRA. One primer for each locus was labeled with a fluorescent dye (fluorescein) which allows detection on the single wavelength ALF DNA Sequencer (Pharmacia Biotech). As part of the detailed evaluation of the suitability of the hexaplex system for routine forensic use, the effect of variation in amplification parameters on the efficiency of the system was examined. Polymerase chain reaction amplification conditions were optimized to provide specific, robust amplification of forensic samples.

Genetic diversity at six short tandem repeat loci within the state of Victoria, Australia

A new multiplex PCR system, developed by the Forensic Science Service (FSS) in the United Kingdom, permits the coamplification and typing of six short tandem repeat (STR) loci: HUMFGA, D8S1179, HUMTHO1, HUMvWA, D18S51, D21S11 and the sex determining marker Amelogenin. Data are presented on these six STRs for two populations in the state of Victoria, Australia: Caucasian and Asian. Whilst several worldwide databases are already available for the STR loci HUMTHO1 and HUMvWA, only relatively few databases exist for D8S1179, D18S51, D21S11 and HUMFGA. Allele frequencies at each locus displayed some fluctuations between the two populations. This is particularly so for HUMTHO1. Generally, however, the most common allele at each locus was the same in all populations, at all loci. A novel D8S1179 allele was found in Asians, provisionally identified as allele 19. Results for the six loci were compared with similar data from three UK resident populations: Caucasian, Afro-Caribbean and Asian (Indian/Pakistani) populations. These indicated that ethnically similar populations display similar allele frequencies, while the Australian Asian and UK Afro-Caribbean were found to be distinct.

A patient with Edwards syndrome caused by a rare pseudodicentric chromosome 18 of paternal origin

We present an unusual case of trisomy 18 due to a pseudodicentric chromosome 18 of paternal origin. The karyotype was: 46,XY, -18, +psu dic(18)(qter-->cen-->p11.31::p11.31-->psucen-->qter). The origin of the abnormal chromosome was verified by FISH with a painting probe from chromosome 18. Absence of short-arm telomeres was shown by multicolor FISH, and the results of DNA analysis showed monosomy for loci D18S59 and D18S170 as well as paternal inheritance of the aberrant chromosome. The child's phenotype was characteristic of trisomy 18.

The complex mutation pattern of a microsatellite

DQCAR is a (CA)n microsatellite located in the HLA class II region and tightly linked to HLA-DQB1. Previous studies showed a strikingly low level of size variation in DQCAR alleles within an extensive subfamily of HLA-DQ subtypes (DQ1). DQCAR alleles in non-DQ1 subtypes showed a higher degree of size polymorphism. In this study sequence analysis demonstrates that DQ1-associated DQCAR alleles have a single C-->A nucleotide substitution interrupting the CA repeat array. Frequent CA-->GA mutations are also observed in DQ1-associated microsatellites with identical allele sizes. In contrast, DQCAR alleles associated with non-DQ1 haplotypes display a perfect CA repeat sequence and the variation in allele size is attributable only to differences in the number of CA repeats. Our results imply that several mutational mechanisms are involved in the generation of allelic diversity within the same microsatellite locus. The possibility of different mutation rates in the same locus should to be taken into account when using these markers in evolutionary and disease studies.

A chromosome 18 genetic linkage study in three large Belgian pedigrees with bipolar disorder

The contribution of genetic factors to the susceptibility for affective disorders has been firmly established. Recent reports found evidence for a susceptibility locus for affective disorders in 2 regions on chromosome 18. We describe 3 large Belgian pedigrees with multiple patients with affective disorders. Both chromosome 18 regions were investigated in the 3 families, using parametric and nonparametric segregation methods. In the pericentromeric region, all evidence was against a disease gene in our families. Also the data obtained for the distal part of 18q, argue against a genetic susceptibility factor in our sample.

Statistical analysis of data for three British ethnic groups from a new STR multiplex

Data have been collected from 602 Caucasians, 190 Afro-Caribbeans and 257 Asians of Indo/Pakistani descent who have been profiled using a new six locus short tandem repeat (STR) multiplex. The data have been analysed by conventional significance testing methods: the exact test, homozygosity, and conventional goodness of fit to Hardy-Weinberg proportions. Frequency tables are given and the expected performance in British forensic casework is discussed.

Low mutation rates of microsatellite loci in Drosophila melanogaster

Analysis of variation at microsatellite DNA loci is widely used in studies of parentage, linkage and evolutionary history. The utility of microsatellites is primarily due to high levels of allelic diversity, believed to reflect mutation rates orders of magnitude higher than base pair substitutions at single-copy genes. For humans, mice, rats and pigs, microsatellite mutation rates have been estimated at 10(-3)-10(-5). However, a recent study comparing microsatellite variation in humans with non-human primates suggests that microsatellite mutation rates may vary considerably across taxa. We measured mutation rates of 24 microsatellite loci in mutation accumulation lines of Drosophila melanogaster. Surprisingly, only a single mutation was detected after screening 157,680 allele-generations, yielding an estimated average mutation rate per locus of 6.3 x 10(-6), a mutation rate considerably lower than reported for various mammals. We propose that the comparatively low mutation rate is primarily a function of short microsatellite repeat lengths in the D. melanogaster genome.