Showing papers in "American Journal of Human Genetics in 1996"

Parametric and nonparametric linkage analysis: a unified multipoint approach.

Abstract: In complex disease studies, it is crucial to perform multipoint linkage analysis with many markers and to use robust nonparametric methods that take account of all pedigree information. Currently available methods fall short in both regards. In this paper, we describe how to extract complete multipoint inheritance information from general pedigrees of moderate size. This information is captured in the multipoint inheritance distribution, which provides a framework for a unified approach to both parametric and nonparametric methods of linkage analysis. Specifically, the approach includes the following: (1) Rapid exact computation of multipoint LOD scores involving dozens of highly polymorphic markers, even in the presence of loops and missing data. (2) Non-parametric linkage (NPL) analysis, a powerful new approach to pedigree analysis. We show that NPL is robust to uncertainty about mode of inheritance, is much more powerful than commonly used nonparametric methods, and loses little power relative to parametric linkage analysis. NPL thus appears to be the method of choice for pedigree studies of complex traits. (3) Information-content mapping, which measures the fraction of the total inheritance information extracted by the available marker data and points out the regions in which typing additional markers is most useful. (4) Maximum-likelihood reconstruction of many-marker haplotypes, even in pedigrees with missing data. We have implemented NPL analysis, LOD-score computation, information-content mapping, and haplotype reconstruction in a new computer package, GENEHUNTER. The package allows efficient multipoint analysis of pedigree data to be performed rapidly in a single user-friendly environment.

Descent graphs in pedigree analysis: applications to haplotyping, location scores, and marker-sharing statistics.

Abstract: The introduction of stochastic methods in pedigree analysis has enabled geneticists to tackle computations intractable by standard deterministic methods. Until now these stochastic techniques have worked by running a Markov chain on the set of genetic descent states of a pedigree. Each descent state specifies the paths of gene flow in the pedigree and the founder alleles dropped down each path. The current paper follows up on a suggestion by Elizabeth Thompson that genetic descent graphs offer a more appropriate space for executing a Markov chain. A descent graph specifies the paths of gene flow but not the particular founder alleles traveling down the paths. This paper explores algorithms for implementing Thompson's suggestion for codominant markers in the context of automatic haplotyping, estimating location scores, and computing gene-clustering statistics for robust linkage analysis. Realistic numerical examples demonstrate the feasibility of the algorithms.

The TDT and other family-based tests for linkage disequilibrium and association.

Abstract: The transmission/disequilibrium test (TDT) was introduced several years ago by Spielman et al. as a test for linkage between a complex disease and a genetic marker. The original intended use of the TDT was to test for linkage with a marker located near a candidate gene, in cases where disease association already had been found. However, even if prior evidence for association is absent, the TDT is valid and can be used to test any marker (or a set of markers) for which data are available from parents and one or more affected offspring. Other tests that focus on association itself - but that, like the TDT, are applied to data from nuclear families - also have been proposed. In addition, several papers have appeared recently that discuss extensions and properties of these tests and of the TDT. Since the aim of both the TDT and the association tests is to locate genes that contribute to disease susceptibility, a number of questions about their differences have arisen. In this review, we compare the properties of the TDT with those of family-based tests of association, and we comment on issues regarding the use of these tests. 22 refs., 2 tabs.

Origin and evolution of Native American mtDNA variation: a reappraisal.

Abstract: The timing and number of prehistoric migrations involved in the settlement of the American continent is subject to intense debate. Here, we reanalyze Native American control region mtDNA data and demonstrate that only an appropriate phylogenetic analysis accompanied by an appreciation of demographic factors allows us to discern different migrations and to estimate their ages. Reappraising 574 mtDNA control region sequences from aboriginal Siberians and Native Americans, we confirm in agreement with linguistic, archaeological and climatic evidence that (i) the major wave of migration brought one population, ancestral to the Amerinds, from northeastern Siberia to America 20,000-25,000 years ago and (ii) a rapid expansion of a Beringian source population took place at the end of the Younger Dryas glacial phase approximately 11,300 years ago, ancestral to present Eskimo and Na-Dene populations.

Molecular genetic analysis in mild hyperhomocysteinemia: a common mutation in the methylenetetrahydrofolate reductase gene is a genetic risk factor for cardiovascular disease.

Abstract: Mild hyperhomocysteinemia is an established risk factor for cardiovascular disease. Genetic aberrations in the cystathionine beta-synthase (CBS) and methylenetetrahydrofolate reductase (MTHFR) genes may account for reduced enzyme activities and elevated plasma homocysteine levels. In 15 unrelated Dutch patients with homozygous CBS deficiency, we observed the 833T-->C (I278T) mutation in 50% of the alleles. Very recently, we identified a common mutation (677C-->T; A-->V) in the MTHFR gene, which, in homozygous state, is responsible for the thermolabile phenotype and which is associated with decreased specific MTHRF activity and elevated homocysteine levels. We screened 60 cardiovascular patients and 111 controls for these two mutations, to determine whether these mutations are risk factors for premature cardiovascular disease. Heterozygosity for the 833T-->C mutation in the CBS gene was observed in one individual of the control group but was absent in patients with premature cardiovascular disease. Homozygosity for the 677C-->T mutation in the MTHFR gene was found in (15%) of 60 cardiovascular patients and in only 6 (approximately 5%) of 111 control individuals (odds ratio 3.1 [95% confidence interval 1.0-9.2]). Because of both the high prevalence of the 833T-->C mutation among homozygotes for CBS deficiency and its absence in 60 cardiovascular patients, we may conclude that heterozygosity for CBS deficiency does not appear to be involved in premature cardiovascular disease. However, a frequent homozygous mutation in the MTHFR gene is associated with a threefold increase in risk for premature cardiovascular disease.

Phenotypic characterization of individuals with 30-40 CAG repeats in the Huntington disease (HD) gene reveals HD cases with 36 repeats and apparently normal elderly individuals with 36-39 repeats.

Abstract: Abnormal CAG expansions in the IT-15 gene are associated with Huntington disease (HD). In the diagnostic setting it is necessary to define the limits of the CAG size ranges on normal and HD-associated chromosomes. Most large analyses that defined the limits of the normal and pathological size ranges employed PCR assays, which included the CAG repeats and a CCG repeat tract that was thought to be invariant. Many of these experiments found an overlap between the normal and disease size ranges. Subsequent findings that the CCG repeats vary by 8 trinucleotide lengths suggested that the limits of the normal and disease size ranges should be reevaluated with assays that exclude the CCG polymorphism. Since patients with between 30 and 40 repeats are rare, a consortium was assembled to collect such individuals. All 178 samples were reanalyzed in Cambridge by using assays specific for the CAG repeats. We have optimized methods for reliable sizing of CAG repeats and show cases that demonstrate the dangers of using PCR assays that include both the CAG and CCG polymorphisms. Seven HD patients had 36 repeats, which confirms that this allele is associated with disease. Individuals without apparent symptoms or signs of HD were found at 36 repeats (aged 74, 78, 79, and 87 years), 37 repeats (aged 69 years), 38 repeats (aged 69 and 90 years), and 39 repeats (aged 67, 90, and 95 years). The detailed case histories of an exceptional case from this series will be presented: a 95-year-old man with 39 repeats who did not have classical features of HD. The apparently healthy survival into old age of some individuals with 36-39 repeats suggests that the HD mutation may not always be fully penetrant.

The relationship between trinucleotide (GAA) repeat length and clinical features in Friedreich ataxia.

Abstract: Friedreich ataxia (FA) is associated with the expansion of a GAA trinucleotide repeat in the first intron of the X25 gene. We found both alleles expanded in 67 FA patients from 48 Italian families. Five patients from three families were compound heterozygotes with expansion on one allele and an isoleucine-->phenylalanine change at position 154 on the other one. We found neither expansions nor point mutations in three patients. The length of FA alleles ranged from 201 to 1,186 repeat units, with no overlap with the normal range, and showed a negatively skewed distribution with a peak between 800 and 1,000 repeats. The FA repeat showed meiotic instability with a median variation of 150 repeats. The lengths of both larger and smaller alleles in each patient inversely correlated with age at onset of the disorder. Smaller alleles showed the best correlation, accounting for approximately 50% of the variation of age at onset. Mean allele length was significantly higher in patients with diabetes and in those with cardiomyopathy.

The role of HLA class II genes in insulin-dependent diabetes mellitus: molecular analysis of 180 Caucasian, multiplex families.

Abstract: We report here our analysis of HLA class II alleles in 180 Caucasian nuclear families with at least two children with insulin-dependent diabetes mellitus (IDDM). DRB1, DQA1, DQB1, and DPB1 genotypes were determined with PCR/sequence-specific oligonucleotide probe typing methods. The data allowed unambiguous determination of four-locus haplotypes in all but three of the families. Consistent with other studies, our data indicate an increase in DR3/DR4, DR3/DR3, and DR4/DR4 genotypes in patients compared to controls. In addition, we found an increase in DR1/DR4, DR1/DR3, and DR4/DR8 genotypes. While the frequency of DQB1*0302 on DR4 haplotypes is dramatically increased in DR3/DR4 patients, DR4 haplotypes in DR1/DR4 patients exhibit frequencies of DQB1*0302 and DQB1*0301 more closely resembling those in control populations. The protective effect of DR2 is evident in this data set and is limited to the common DRB1*1501-DQB1*0602 haplotype. Most DR2+ patients carry the less common DR2 haplotype DRB1*1601-DQB1*0502, which is not decreased in patients relative to controls. DPB1 also appears to play a role in disease susceptibility. DPB1*0301 is increased in patients (P < .001) and may contribute to the disease risk of a number of different DR-DQ haplotypes. DPB1*0101, found almost exclusively on DR3 haplotypes in patients, is slightly increased, and maternal transmissions of DRB1*0301-DPB1*0101 haplotypes to affected children occur twice as frequently as do paternal transmissions. Transmissions of DR3 haplotypes carrying other DPB1 alleles occur at approximately equal maternal and paternal frequencies. The complex, multigenic nature of HLA class II-associated IDDM susceptibility is evident from these data.

Paleolithic and neolithic lineages in the European mitochondrial gene pool

Abstract: Phylogenetic and diversity analysis of the mtDNA control region sequence variation of 821 individuals from Europe and the Middle East distinguishes five major lineage groups with different internal diversities and divergence times. Consideration of the diversities and geographic distribution of these groups within Europe and the Middle East leads to the conclusion that ancestors of the great majority of modern, extant lineages entered Europe during the Upper Paleolithic. A further set of lineages arrived from the Middle East much later, and their age and geographic distribution within Europe correlates well with archaeological evidence for two culturally and geographically distinct Neolithic colonization events that are associated with the spread of agriculture. It follows from this interpretation that the major extant lineages throughout Europe predate the Neolithic expansion and that the spread of agriculture was a substantially indigenous development accompanied by only a relatively minor component of contemporary Middle Eastern agriculturalists. There is no evidence of any surviving Neanderthal lineages among modern Europeans.

Thiopurine S-methyltransferase deficiency : Two nucleotide transitions define the most prevalent mutant allele associated with loss of catalytic activity in Caucasians

Abstract: The autosomal recessive trait of thiopurine S-methytransferase (TPMT) deficiency is associated with severe hematopoietic toxicity when patients are treated with standard doses of mercaptopurine, azathioprine, or thioguanine To define the molecular mechanism of this genetic polymorphism, we cloned and characterized the cDNA of a TPMT-deficient patient, which revealed a novel mutant allele (TPMT*3) containing two nucleotide transitions (G460-->A and A719-->G) producing amino acid changes at codons 154 (Ala-->Thr) and 240 (Tyr--> Cys), differing from the rare mutant TPMT allele we previously identified (ie, TPMT*2 with only G238-->C) Site-directed mutagenesis and heterologous expression established that either TPMT*3 mutation alone leads to a reduction in catalytic activity (G460-->A, ninefold reduction; A719-->G, 14-fold reduction), while the presence of both mutations leads to complete loss of activity Using mutation specific PCR-RFLP analysis, the TPMT*3 allele was detected in genomic DNA from approximately 75 percent of unrelated white subjects with heterozygous phenotypes, indicating that TPMT*3 is the most prevalent mutant allele associated with TPMT-deficiency in Caucasians

The retrieval of ancient human DNA sequences

Abstract: DNA was extracted from approximately 600-year-old human remains found at an archaeological site in the southwestern United States, and mtDNA fragments were amplified by PCR. When these fragments were sequenced directly, multiple sequences seemed to be present. From three representative individuals, DNA fragments of different lengths were quantified and short overlapping amplification products cloned. When amplifications started from <40 molecules, clones contained several different sequences. In contrast, when they were initiated by a few thousand molecules, unambiguous and reproducible results were achieved. These results show that more experimental work than is often applied is necessary to ensure that DNA sequences amplified from ancient human remains are authentic. In particular, quantitation of the numbers of amplifiable molecules is a useful tool to determine the role of contaminating contemporary molecules and PCR errors in amplifications from ancient DNA.

Orofacial clefts, parental cigarette smoking, and transforming growth factor-alpha gene variants

Abstract: Results of studies to determine whether women who smoke during early pregnancy are at increased risk of delivering infants with orofacial clefts have been mixed, and recently a gene-environment interaction between maternal smoking, transforming growth factor-alpha (TGFa), and clefting has been reported. Using a large population-based case-control study, we investigated whether parental periconceptional cigarette smoking was associated with an increased risk for having offspring with orofacial clefts. We also investigated the influence of genetic variation of the TGFa locus on the relation between smoking and clefting. Parental smoking information was obtained from telephone interviews with mothers of 731 (84.7% of eligible) orofacial cleft case infants and with mothers of 734 (78.2%) nonmalformed control infants. DNA was obtained from newborn screening blood spots and genotyped for the allelic variants of TGFa. We found that risks associated with maternal smoking were most elevated for isolated cleft lip with or without cleft palate, (odds ratio 2.1 [95% confidence interval 1.3-3.6]) and for isolated cleft palate (odds ratio 2.2 [1.1-4.5]) when mothers smoked > or =20 cigarettes/d. Analyses controlling for the potential influence of other variables did not reveal substantially different results. Clefting risks were even greater for infants with the TGFa allele previously associated with clefting whose mothers smoked > or =20 cigarettes/d. These risks for white infants ranged from 3-fold to 11-fold across phenotypic groups. Paternal smoking was not associated with clefting among the offspring of nonsmoking mothers, and passive smoke exposures were associated with at most slightly increased risks. This study offers evidence that the risk for orofacial clefting in infants may be influenced by maternal smoke exposures alone as well as in combination (gene-environment interaction) with the presence of the uncommon TGFa allele.

Allelic loss is frequent in tuberous sclerosis kidney lesions but rare in brain lesions.

Abstract: Tuberous sclerosis (TSC) is an autosomal dominant disorder characterized by seizures, mental retardation, and hamartomatous lesions. Although hamartomas can occur in almost any organ, they are most common in the brain, kidney, heart, and skin. Allelic loss or loss of heterozygosity (LOH) in TSC lesions has previously been reported on chromosomes 16p13 and 9q34, the locations of the TSC2 and TSC1 genes, respectively, suggesting that the TSC genes act as tumor-suppressor genes. In our study, 87 lesions from 47 TSC patients were analyzed for LOH in the TSC1 and TSC2 chromosomal regions. Three findings resulted from this analysis. First, we confirmed that the TSC1 critical region is distal to D9S149. Second, we found LOH more frequently on chromosome 16p13 than on 9q34. Of the 28 patients with angiomyolipomas or rhabdomyomas, 16p13 LOH was detected in lesions from 12 (57%) of 21 informative patients, while 9q34 LOH was detected in lesions from only 1 patient (4%). This could indicate that TSC2 tumors are more likely than TSC1 tumors to require surgical resection or that TSC2 is more common than TSC1 in our patient population. It is also possible that small regions of 9q34 LOH were missed. Lastly, LOH was found in 56% of renal angiomyolipomas and cardiac rhabdomyormas but in only 4% of TSC brain lesions. This suggests that brain lesions can result from different pathogenic mechanisms than kidney and heart lesions.

Gender difference in apolipoprotein E-associated risk for familial Alzheimer disease: a possible clue to the higher incidence of Alzheimer disease in women.

Abstract: Late-onset Alzheimer disease (AD) is associated with the apolipoprotein E (APOE)-epsilon4 allele. In late-onset familial AD, women have a significantly higher risk of developing the disease than do men. The aim of this study was to determine whether the gender difference in familial AD is a function of APOE genotype. We studied 58 late-onset familial AD kindreds. Kaplan-Meier survival analysis was used to assess genotype-specific distributions of age at onset. Odds ratios were estimated by logistic regression with adjustment for age and by conditional logistic regression with stratification on families. All methods detected a significant gender difference for the epsilon4 heterozygous genotype. In women, epsilon4 heterozygotes had higher risk than those without epsilon4; there was no significant difference between epsilon4 heterozygotes and epsilon4 homozygotes. In men, epsilon4 heterozygotes had lower risk than epsilon4 homozygotes; there was not significant difference between epsilon4 heterozygotes and those without epsilon4. A direct comparison of epsilon4 heterozygous men and women revealed a significant twofold increased risk in women. We confirmed these results in 15 autopsy-confirmed AD kindreds from the National Cell Repository at Indiana University Alzheimer Disease Center. These observations are consistent with the increased incidence of familial AD in women and may be a critical clue to the role of gender in the pathogenesis of AD.

mtDNA polymorphism in East Asian Populations, with special reference to the peopling of Japan.

Abstract: Nucleotide sequences of the major noncoding (D-loop) region of human mtDNA from five East Asian populations including mainland Japanese, Ainu, Ryukyuans, Koreans, and Chinese were analyzed. On the basis of a comparison of 482-bp sequences in 293 East Asians, 207 different sequence types were observed. Of these, 189 were unique to their respective populations, whereas 18 were shared between two or three populations. Among the shared types, eight were found in common between the mainland Japanese and Koreans, which is the largest number in the comparison. The intergenic COII/tRNA(Lys) 9-bp deletion was observed in every East Asian population with varying frequencies. The D-loop sequence variation suggests that the deletion event occurred only once in the ancestry of East Asians. Phylogenetic analysis revealed that East Asian lineages were classified into at least 18 monophyletic clusters, though lineages from the five populations were completely intermingled in the phylogenetic tree. However, we assigned 14 of the 18 clusters for their specificity on the basis of the population from which the maximum number of individuals in each cluster was derived. Of note is the finding that 50% of the mainland Japanese had continental specificity in which Chinese or Koreans were dominant, while < 20% of either Ryukyuans or Ainu possessed continental specificity. Phylogenetic analysis of the entire human population revealed the closest genetic affinity between the mainland Japanese and Koreans. Thus, the results of this study are compatible with the hybridization model on the origin of modern Japanese. It is suggested that approximately 65% of the gene pool in mainland Japanese was derived from the continental gene flow after the Yayoi Age.

Type of mutation in the neurofibromatosis type 2 gene (NF2) frequently determines severity of disease.

Abstract: The gene predisposing to neurofibromatosis type 2 (NF2) on human chromosome 22 has revealed a wide variety of different mutations in NF2 individuals. These patients display a marked variability in clinical presentation, ranging from very severe disease with numerous tumors at a young age to a relatively mild condition much later in life. To investigate whether this phenotypic heterogeneity is determined by the type of mutation in NF2, we have collected clinical information on 111 NF2 cases from 73 different families on whom we have performed mutation screening in this gene. Sixty-seven individuals (56.2%) from 41 of these kindreds revealed 36 different putative disease-causing mutations. These include 26 proposed protein-truncating alterations (frameshift deletions/insertions and nonsense mutations), 6 splice-site mutations, 2 missense mutations, 1 base substitution in the 3' UTR of the NF2 cDNA, and a single 3-bp in-frame insertion. Seventeen of these mutations are novel, whereas the remaining 19 have been described previously in other NF2 individuals or sporadic tumors. When individuals harboring protein-truncating mutations are compared with cases with single codon alterations, a significant correlation (P < .001) with clinical outcome is observed. Twenty-four of 28 patients with mutations that cause premature truncation of the NF2 protein, schwannomin, present with severe phenotypes. In contrast, all 16 cases from three families with mutations that affect only a single amino acid have mild NF2. These data provide conclusive evidence that a phenotype/genotype correlation exists for certain NF2 mutations.

Osteoporosis-pseudoglioma syndrome, a disorder affecting skeletal strength and vision, is assigned to chromosome region 11q12-13

Abstract: Osteoporosis-pseudoglioma syndrome (OPS) is an autosomal recessive disorder characterized by severe juvenile-onset osteoporosis and congenital or juvenile-onset blindness. The pathogenic mechanism is not known. Clinical, biochemical, and microscopic analyses suggest that OPS may be a disorder of matrix homeostasis rather than a disorder of matrix structure. Consequently, identification of the OPS gene and its protein product could provide insights regarding common osteoporotic conditions, such as postmenopausal and senile osteoporosis. As a first step toward determining the cause of OPS, we utilized a combination of traditional linkage analysis and homozygosity mapping to assign the OPS locus to chromosome region 11q12-13. Mapping was accomplished by analyzing 16 DNA samples (seven affected individuals) from three different consanguineous kindreds. Studies in 10 additional families narrowed the candidate region, supported locus homogeneity, and did not detect founder effects. The OPS locus maps to a 13-cM interval between D11S1298 and D11S971 and most likely lies in a 3-cM region between GSTP1 and D11S1296. At present, no strong candidate genes colocalize with OPS.

How rapidly does the human mitochondrial genome evolve

Abstract: The results of an empirical nucleotide-sequencing approach indicate that the evolution of the human mitochondrial noncoding D-loop is both more rapid and more complex than is revealed by standard phylogenetic approaches. The nucleotide sequence of the D-loop region of the mitochondrial genome was determined for 45 members of a large matrilineal Leber hereditary optic neuropathy pedigree. Two germ-line mutations have arisen in members of one branch of the family, thereby leading to triplasmic descendants with three mitochondrial genotypes. Segregation toward the homoplasmic state can occur within a single generation in some of these descendants, a result that suggests rapid fixation of mitochondrial mutations as a result of developmental bottlenecking. However, slow segregation was observed in other offspring, and therefore no single or simple pattern of segregation can be generalized from the available data. Evidence for rare mtDNA recombination within the D-loop was obtained for one family member. In addition to these germ-line mutations, a somatic mutation was found in the D-loop of one family member. When this genealogical approach was applied to the nucleotide sequences of mitochondrial coding regions, the results again indicated a very rapid rate of evolution.

Identification of new polymorphisms of the angiotensin I-converting enzyme (ACE) gene, and study of their relationship to plasma ACE levels by two-QTL segregation-linkage analysis.

Abstract: Plasma angiotensin I-converting enzyme (ACE) levels are highly genetically determined. A previous segregation-linkage analysis suggested the existence of a functional mutation located within or close to the ACE locus, in almost complete linkage desequilibrium (LD) with the ACE insertion/deletion (I/D) polymorphism and accounting for half the ACE variance. In order to identify the functional variant at the molecular level, we compared ACE gene sequences between four subjects selected for having contrasted ACE levels and I/D genotypes. We identified 10 new polymorphisms, among which 8 were genotyped in 95 healthy nuclear families, in addition to the I/D polymorphism. These polymorphisms could be divided into two groups: five polymorphisms in the 5' region and three in the coding sequence and the 3' UTR. Within each group, polymorphisms were in nearly complete association, whereas polymorphisms from the two groups were in strong negative LD. After adjustment for the I/D polymorphism, all polymorphisms of the 5' group remained significantly associated with ACE levels, which suggests the existence of two quantitative trait loci (QTL) acting additively on ACE levels. Segregation-linkage analyses including one or two ACE-linked QTLs in LD with two ACE markers were performed to test this hypothesis. The two QTLs and the two markers were assumed to be in complete LD. Results supported the existence of two ACE-linked QTLs, which would explain 38% and 49% of the ACE variance in parents and offspring, respectively. One of these QTLs might be the I/D polymorphism itself or the newly characterized 4656(CT)2/3 polymorphism. The second QTL would have a frequency of approximately .20, which is incompatible with any of the yet-identified polymorphisms. More extensive sequencing and extended analyses in larger samples and in other populations will be necessary to characterize definitely the functional variants.

Heritability of human brain functioning as assessed by electroencephalography

Abstract: To study the genetic and environmental contributions to individual differences in CNS functioning, the electroencephalogram (EEG) was measured in 213 twin pairs age 16 years. EEG was measured in 91 MZ and 122 DZ twins. To quantify sex differences in the genetic architecture, EEG was measured in female and male same-sex twins and in opposite-sex twins. EEG was recorded on 14 scalp positions during quiet resting with eyes closed. Spectral powers were calculated for four frequency bands: delta, theta, alpha, and beta. Twin correlations pointed toward high genetic influences for all these powers and scalp locations. Model fitting confirmed these findings; the largest part of the variance of the EEG is explained by additive genetic factors. The averaged heritabilites for the delta, theta, alpha and beta frequencies was 76%, 89%, 89%, and 86%, respectively. Multivariate analyses suggested that the same genes for EEG alpha rhythm were expressed in different brain areas in the left and right hemisphere. This study shows that brain functioning, as indexed by rhythmic brain-electrical activity, is one of the most heritable characteristics in humans.

Association between nondisjunction and maternal age in meiosis-II human oocytes.

Abstract: The relationship between advanced maternal age and increased risk of trisomic offspring is well known clinically but not clearly understood at the level of the oocyte. A total of 383 oocytes that failed fertilization from 107 patients undergoing in vitro fertilization were analyzed by FISH using X-, 18-, and 13/21-chromosome probes simultaneously. The corresponding polar bodies were also analyzed in 188 of these oocytes. The chromosomes in the oocyte and first polar body complement each other and provide an internal control to differentiate between aneuploidy and technical errors. Two mechanisms of nondisjunction were determined. First, nondisjunction of bivalent chromosomes resulting in two univalents going to the same pole and, second, nondisjunction by premature chromatid separation (predivision) of univalent chromosomes producing either a balanced (2 + 2) or unbalanced (3 + 1) distribution of chromatids into the first polar body and M-II oocytes. Balanced predivision of chromatids, previously proposed as a major mechanism of aneuploidy, was found to increase significantly with time in culture (P or = 40 years of age, a significant increase (P < .001) in nondisjunction of full dyads was found in the oocytes with analyzable polar bodies and no FISH errors. Premature predivision of chromatids was also found to cause nondisjunction, but it did not increase with maternal age.

Gene structure, DNA methylation, and imprinted expression of the human SNRPN gene.

Abstract: The human SNRPN (small nuclear ribonucleoprotein polypeptide N) gene is one of a gene family that encode proteins involved in pre-mRNA splicing and maps to the smallest deletion region involved in the Prader-Willi syndrome (PWS) within chromosome 15q11-q13. Paternal only expression of SNRPN has previously been demonstrated by use of cell lines from PWS patients (maternal allele only) and Angelman syndrome (AS) patients (paternal allele only). We have characterized two previously unidentified 5' exons of the SNRPN gene and demonstrate that exons -1 and 0 are included in the full-length transcript. This gene is expressed in a wide range of somatic tissues and at high, approximately equal levels in all regions of the brain. Both the first exon of SNRPN (exon -1) and the putative transcription start site are embedded within a CpG island. This CpG island is extensively methylated on the repressed maternal allele and is unmethylated on the expressed paternal allele, in a wide range of fetal and adult somatic cells. This provides a quick and highly reliable diagnostic assay for PWS and AS, which is based on DNA-methylation analysis that has been tested on > 100 patients in a variety of tissues. Conversely, several CpG sites approximately 22 kb downstream of the transcription start site in intron 5 are preferentially methylated on the expressed paternal allele in somatic tissues and male germ cells, whereas these same sites are unmethylated in fetal oocytes. These findings are consistent with a key role for DNA methylation in the imprinted inheritance and subsequent gene expression of the human SNRPN gene.

Molecular definition of the chromosome 7 deletion in Williams syndrome and parent-of-origin effects on growth.

Abstract: Williams syndrome (WS) is a developmental disorder with variable phenotypic expression associated, in most cases, with a hemizygous deletion of part of chromosomal band 7q11.23 that includes the elastin gene (ELN). We have investigated the frequency and size of the deletions, determined the parental origin, and correlated the molecular results with the clinical findings in 65 WS patients. Hemizygosity at the ELN locus was established by typing of two intragenic polymorphisms, quantitative Southern analysis, and/or FISH. Polymorphic markers covering the deletion and flanking regions were ordered by a combination of genetic and physical mapping. Genotyping of WS patients and available parents for 13 polymorphisms revealed that of 65 clinically defined WS patients, 61 (94%) had a deletion of the ELN locus and were also hemizygous (or noninformative) at loci D7S489B, D7S2476, D7S613, D7S2472, and D7S1870. None of the four patients without ELN deletion was hemizygous at any of the polymorphic loci studied. All patients were heterozygous (or noninformative) for centromeric (D7S1816, D7S1483, and D7S653) and telomeric (D7S489A, D7S675, and D7S669) flanking loci. The genetic distance between the most-centromeric deleted locus, D7S489B, and the most-telomeric one, D7S1870, is 2 cM. The breakpoints cluster at approximately 1 cM to either side of ELN. In 39 families informative for parental origin, all deletions were de novo, and 18 were paternally and 21 maternally derived. Comparison of clinical data, collected in a standardized quantifiable format, revealed significantly more severe growth retardation and microcephaly in the maternal deletion group. An imprinted locus, silent on the paternal chromosome and contributing to statural growth, may be affected by the deletion.

Autosomal dominant spinocerebellar ataxia with sensory axonal neuropathy (SCA4): clinical description and genetic localization to chromosome 16q22.1.

Abstract: The hereditary ataxias represent a clinically and genetically heterogeneous group of neurodegenerative disorders. Various classification schemes based on clinical criteria are being replaced as molecular characterization of the ataxias proceeds; so far, seven distinct autosomal dominant hereditary ataxias have been genetically mapped in the human genome. We report linkage to chromosome 16q22.1 for one of these genes (SCA4) in a five-generation family with an autosomal dominant, late-onset spinocerebellar ataxia; the gene is tightly linked to the microsatellite marker D16S397 (LOD score = 5.93 at theta = .00). In addition, we present clinical and electrophysiological data regarding the distinct and previously unreported phenotype consisting of ataxia with the invariant presence of a prominent axonal sensory neuropathy.

Guanidinoacetate methyltransferase deficiency: the first inborn error of creatine metabolism in man.

Abstract: In two children with an accumulation of guanidinoacetate in brain and a deficiency of creatine in blood, a severe deficiency of guanidinoacetate methyltransferase (GAMT) activity was detected in the liver. Two mutant GAMT alleles were identified that carried a single base substitution within a 5' splice site or a 13-nt insertion and gave rise to four mutant transcripts. Three of the transcripts encode truncated polypeptides that lack a residue known to be critical for catalytic activity of GAMT. Deficiency of GAMT is the first inborn error of creatine metabolism. It causes a severe developmental delay and extrapyramidal symptoms in early infancy and is treatable by oral substitution with creatine.

Genetic heterogeneity in Niemann-Pick C disease: a study using somatic cell hybridization and linkage analysis.

Abstract: The primary molecular defect underlying Niemann-Pick C disease (NPC) is still unknown. A wide spectrum of clinical and biochemical phenotypes has previously been documented. Indication of genetic heterogeneity has recently been provided for one patient. In the present study, somatic cell hybridization experiments were carried out on skin fibroblast cultures from 32 unrelated NPC patients covering the range of known clinical and biochemical phenotypes. The criterion for complementation was the restoration of a normal intracellular fluorescent pattern in polykaryons stained with filipin to document cholesterol distribution. Crosses between the various cell lines revealed a major complementation group comprising 27 unrelated patients and a second minor group comprising 5 patients. Linkage analysis in one multiplex family belonging to the minor complementation group showed that the mutated gene does not map to the 18q11-12 region assigned to the major gene. Patients in the first group spanned the whole spectrum of clinical and cellular phenotypes. No consistent clinical or biochemical phenotypes was associated with the second complementation group. Three of the five group 2 patients, however, presented with a new rare phenotype associated with severe pulmonary involvement leading to death within the first year of life. No biochemical abnormality specific of either group could be demonstrated with regard to tissue lipid storage pattern, intralysosomal cholesterol storage, and regulation of cholesterol homeostasis. Mutations affecting at least two different genes have thus been shown to underlie NPC. The two gene products may function together or sequentially in a common metabolic pathway affecting intracellular cholesterol transport.

Germ-line mutations in the neurofibromatosis 2 gene: correlations with disease severity and retinal abnormalities.

Abstract: Neurofibromatosis 2 (NF2) features bilateral vestibular schwannomas, other benign neural tumors, and cataracts. Patients in some families develop many tumors at an early age and have rapid clinical progression, whereas in other families, patients may not have symptoms until much later and vestibular schwannomas may be the only tumors. The NF2 gene has been cloned from chromosome 22q; most identified germ-line mutations result in a truncated protein and severe NF2. To look for additional mutations and clinical correlations, we used SSCP analysis to screen DNA from 32 unrelated patients. We identified 20 different mutations in 21 patients (66%): 10 nonsense mutations, 2 frameshifts, 7 splice-site mutations, and 1 large in-frame deletion. Clinical information on 47 patients from the 21 families included ages at onset and at diagnosis, numbers of meningiomas, spinal and skin tumors, and presence of cataracts and retinal abnormalities. We compared clinical findings in patients with nonsense or frameshift mutations to those with splice-site mutations. When each patient was considered as an independent random event, the two groups differed (P < or = .05) for nearly every variable. Patients with nonsense or frameshift mutations were younger at onset and at diagnosis and had a higher frequency and mean number of tumors, supporting the correlation between nonsense and frameshift mutations and severe NF2. When each family was considered as an independent random event, statistically significant differences between the two groups were observed only for mean ages at onset and at diagnosis. A larger data set is needed to resolve these discrepancies. We observed retinal hamartomas and/or epiretinal membranes in nine patients from five families with four different nonsense mutations. This finding, which may represent a new genotype-phenotype correlation, merits further study.