Linkage and mutational analysis of familial Alzheimer disease kindreds for the APP gene region

A large number of familial Alzheimer disease (FAD) kindreds were examined to determine whether mutations in the amyloid precursor protein (APP) gene could be responsible for the disease. Previous studies have identified three mutations at APP codon 717 which are pathogenic for Alzheimer disease (AD). Samples from affected subjects were examined for mutations in exons 16 and 17 of the APP gene. A combination of direct sequencing and single-strand conformational polymorphism analysis was used. Sporadic AD and normal controls were also examined by the same methods. Five sequence variants were identified. One variant at APP codon 693 resulted in a Glu-->Gly change. This is the same codon as the hereditary cerebral hemorrhage with amyloidosis-Dutch type Glu-->Gln mutation. Another single-base change at APP codon 708 did not alter the amino acid encoded at this site. Two point mutations and a 6-bp deletion were identified in the intronic sequences surrounding exon 17. None of the variants could be unambiguously determined to be responsible for FAD. The larger families were also analyzed by testing for linkage of FAD to a highly polymorphic short tandem repeat marker (D21S210) that is tightly linked to APP. Highly negative LOD scores were obtained for the family groups tested, and linkage was formally excluded beyond theta = .10 for the Volga German kindreds, theta = .20 for early-onset non-Volga Germans, and theta = .10 for late-onset families. LOD scores for linkage of FAD to markers centromeric to APP (D21S1/S11, D21S13, and D21S215) were also negative in the three family groups. These studies show that APP mutations account for AD in only a small fraction of FAD kindreds.     

Genetic heterogeneity of early-onset familial breast cancer

Summary A gene for early-onset familial breast cancer has recently been mapped to the chromosome 17q12–23 region. In order to confirm the gene location, we have tested an extensive early-onset breast cancer family with 4 markers in this chromosome region. Linkage was negative with all 4 markers. This study suggests that there is genetic heterogeneity among early-onset breast cancer families.     

Genetic susceptibility to Alzheimer disease

Alzheimer disease (AD) is the leading cause of dementia in the elderly. Less than a decade ago, it was questioned as to whether or not genes were even involved in anything but rare early onset AD. Since that time, using a variety of genetic epidemiological and molecular biological techniques, four loci have been identified that play a role in the genetic susceptibility of AD, AD presents as a prototype of the power of genetic techniques in defining the etiology of a complex disease.     

Genetic heterogeneity in neural tube defects.

In 1985-1987, the authors attempted to ascertain all cases of confirmed neural tube defects (NTD) in California and Illinois, not only among live-born infants (postnatal) but also cases ascertained during pregnancy (prenatal). Mothers of both prenatal and postnatal NTD cases were interviewed within 5 months. Among postnatal NTD cases, 14.9% (45/303) had anomalies not ordinarily associated with NTD. The frequency of non-NTD related anomalies was 9.4% (5/53) in anencephaly, 0/3 in craniorachischisis, 22.9% (8/35) in encephalocele, 14.5% (27/186) in spina bifida, 20% (1/5) in multiple NTD cases and 19% (4/21) in other NTDs. However, relatively few postnatal NTD cases had known multiple malformation patterns; Meckel-Gruber syndrome was the most common, with 2 postnatal cases, and 3 additional prenatal cases. Maternal age, paternal age and birth order in postnatal cases were 26.7 +/- 5.4 SD, 28.9 +/- 5.8 and 2.8 +/- 1.8, respectively. These characteristics were similar in prenatal NTD cases (27.9 +/- 6.0, 30.1 +/- 6.3, 2.5 +/- 1.5, respectively). We also found no differences in parental ages among different types of NTD. Frequency of prior spontaneous abortion differed neither between postnatal NTD (9.3%) and postnatal controls (8.1%), nor between prenatal NTD (10.7%) and prenatal control (8.7%). Loss rates in the pregnancy immediately prior to the index NTD cases were not significantly higher than in control subjects. The high frequency of non-NTD associated malformations (14.9%) indicates the caution must be exercised before assuming that a given NTD case is polygenic-multifactorial in etiology, especially cases of encephalocele.     

Genetic dissection of familial Parkinson's disease

In the past few years, the genetic contribution to Parkinson's disease (PD) has gained major attention and has resulted in the identification of the first mutant gene, called α-synuclein, involved in the pathogenesis of autosomal-dominant PD. α-Synuclein is a major component of Lewy bodies, which are a neuropathological feature of PD. Furthermore, deletions in the parkin gene have been identified as the primary cause in rare forms of autosomal-recessive juvenile PD. The elucidation of polygenic changes in the dopamine pathway, mitochondrial dysfunction, and metabolism of xenobiotics is now technically possible by means of association and genotype studies. The increasing knowledge of the pathogenesis of PD at a molecular level will have important implications for the development of individual therapeutic strategies to prevent disease progression.     

Linkage studies in familial Alzheimer disease: Evidence for chromosome 19 linkage

A genetic component in the etiology of Alzheimer disease (AD) has been supported by indirect evidence for several years, with autosomal dominant inheritance with age-dependent penetrance being suggested to explain the familial aggregation of affecteds. St. George Hyslop et al. reported linkage of familial AD (FAD) in four early-onset families (mean age at onset [M] less than 50 years). Subsequent studies have been inconsistent in their results; Goate et al. also reported positive lod scores. However, both Pericak-Vance et al.'s study of a series of mainly late-onset FAD families (M greater than 60 years) and Schellenberg et al.'s study failed to confirm linkage to chromosome 21 (CH21). These various studies suggest the possibility of genetic heterogeneity, with some families linked to CH21 and others unlocalized. Recently, St. George Hyslop et al. extended their analysis to include additional families. The extended analyses supported their earlier finding of linkage to CH21, while showing strong evidence of heterogeneity between early-onset (M less than 65 years) and late-onset (M greater than 60 years) FAD families. Because our families did not show linkage to CH21, we undertook a genomic search for an additional locus for FAD. Because of both the confounding factor of late age at onset of FAD and the lack of clear evidence of Mendelian transmission in some of our families, we employed the affected-pedigree-member (APM) method of linkage analysis as an initial screen for possible linkage. Using this method, we identified two regions suggesting linkage: the proximal long arm of chromosome 19 (CH19) and the CH21 region of FAD linkage reported by St. George Hyslop et al. Application of standard likelihood (LOD score) analysis to these data support the possibility of an FAD gene locate on CH19, particularly in the late-onset FAD families. These data further suggest genetic heterogeneity and delineate this region of CH19 as an area needing additional investigation in FAD.     

Mitochondrial defects and oxidative stress in Alzheimer disease and Parkinson disease

Alzheimer disease (AD) and Parkinson disease (PD) are the two most common age-related neurodegenerative diseases characterized by prominent neurodegeneration in selective neural systems. Although a small fraction of AD and PD cases exhibit evidence of heritability, among which many genes have been identified, the majority are sporadic without known causes. Molecular mechanisms underlying neurodegeneration and pathogenesis of these diseases remain elusive. Convincing evidence demonstrates oxidative stress as a prominent feature in AD and PD and links oxidative stress to the development of neuronal death and neural dysfunction, which suggests a key pathogenic role for oxidative stress in both AD and PD. Notably, mitochondrial dysfunction is also a prominent feature in these diseases, which is likely to be of critical importance in the genesis and amplification of reactive oxygen species and the pathophysiology of these diseases. In this review, we focus on changes in mitochondrial DNA and mitochondrial dynamics, two aspects critical to the maintenance of mitochondrial homeostasis and function, in relationship with oxidative stress in the pathogenesis of AD and PD.     

Chromosome 14 and late-onset familial Alzheimer disease (FAD)

Familial Alzheimer disease (FAD) is genetically heterogeneous. Two loci responsible for early-onset FAD have been identified: the amyloid precursor protein gene on chromosome 21 and the as-yet-unidentified locus on chromosome 14. The genetics of late-onset FAD is unresolved. Maximum-likelihood, affected-pedigree-member (APM), and sib-pair analyses were used, in 49 families with a mean age at onset ≥60 years, to determine whether the chromosome 14 locus is responsible for late-onset FAD. The markers used were D14S53, D14S43, and D14S52. The LOD score method was used to test for linkage of late-onset FAD to the chromosome 14 markers, under three different models: age-dependent penetrance, an affected-only analysis, and age-dependent penetrance with allowance for possible age-dependent sporadic cases. No evidence for linkage was obtained under any of these conditions for the late-onset kindreds, and strong evidence against linkage (LOD score ≤ –2.0) to this region was obtained. Heterogeneity tests of the LOD score results for the combined group of families (early onset, Volga Germans, and late onset) favored the hypothesis of linkage to chromosome 14 with genetic heterogeneity. The positive results are primarily from early-onset families. APM analysis gave significant evidence for linkage of D14S43 and D14S52 to FAD in early-onset kindreds (P < .02). No evidence for linkage was found for the entire late-onset family group. Significant evidence for linkage to D14S52, however, was found for a subgroup of families of intermediate age at onset (mean age at onset ≥60 years and <70 years). These results indicate that the chromosome 14 locus is not responsible for Alzheimer disease in most late-onset FAD kindreds but could play a role in a subset of these kindreds.     

Genetic heterogeneity and localization of a familial breast-ovarian cancer gene on chromosome 17q12-q21.

In a study of 31 breast cancer families and 12 breast-ovarian cancer families, we have obtained clear evidence of linkage to markers on chromosome 17q in the families with ovarian cancer (maximum lod score 3.34 at theta = .04) but only weak evidence in those without ovarian cancer. Recombinant events indicate that the gene lies between D17S588 and D17S250.     

Estimating the probability for major gene Alzheimer disease.

Alzheimer disease (AD) is neuropsychiatric illness caused by multiple etiologies. Prediction of whether AD is genetically based in a given family is problematic because of censoring bias among unaffected relatives as a consequence of the late onset of the disorder, diagnostic uncertainties, heterogeneity, and limited information in a single family. We have developed a method based on Bayesian probability to compute values for a continuous variable that ranks AD families as having a major gene form of AD (MGAD). In addition, we have compared the Bayesian method with a maximum-likelihood approach. These methods incorporate sex- and age-adjusted risk estimates and allow for phenocopies and familial clustering of age at onset. Agreement is high between the two approaches for ranking families as MGAD (Spearman rank [r] = .92). When either method is used, the numerical outcomes are sensitive to assumptions of the gene frequency and cumulative incidence of the disease in the population. Consequently, risk estimates should be used cautiously for counseling purposes; however, there are numerous valid applications of these procedures in genetic and epidemiological studies.     

Linkage analysis of familial Alzheimer disease, using chromosome 21 markers

Chromosome 21 markers were tested for linkage to familial Alzheimer disease (FAD) in 48 kindreds. These families had multiple cases of Alzheimer disease (AD) in 2 or more generations with family age-at-onset means (M) ranging from 41 to 83 years. Included in this group are seven Volga German families which are thought to be genetically homogeneous with respect to FAD. Autopsy documentation of AD was available for 32 families. Linkage to the 21 q11-q21 region was tested using D21S16, D21S13, D21S110, D21S1/S11, and the APP gene as genetic markers. When linkage results for all the families were summed, the LOD scores for these markers were consistently negative and the entire region was formally excluded. Linkage results were also summed for the following family groups; late-onset (M greater than 60), early-onset (M less than or equal to 60), Volga Germans (M = 56), and early-onset non-Volga Germans (M less than or equal to 60). For the first three groups, LOD scores were negative for this region. For the early-onset non-Volga German group (six families), small positive LOD scores of Zmax = 0.78 (recombination fraction theta = .15), Zmax = 0.27 (theta = .15), and Zmax = 0.64 (theta = .0), were observed for D21S13, D21S16, and D21S110, respectively. The remainder of the long arm of chromosome 21 was tested for linkage to FAD using seven markers spanning the q22 region. Results for these markers were also predominantly negative. Thus it is highly unlikely that a chromosome 21 gene is responsible for late-onset FAD and at least some forms of early-onset FAD represented by the Volga German kindreds.     

CHIP is associated with Parkin,a gene responsible for familial Parkinson's disease,and enhances its ubiquitin ligase activity

Unfolded Pael receptor (Pael-R) is a substrate of the E3 ubiquitin ligase Parkin. Accumulation of Pael-R in the endoplasmic reticulum (ER) of dopaminergic neurons induces ER stress leading to neurodegeneration. Here, we show that CHIP, Hsp70, Parkin, and Pael-R formed a complex in vitro and in vivo. The amount of CHIP in the complex was increased during ER stress. CHIP promoted the dissociation of Hsp70 from Parkin and Pael-R, thus facilitating Parkin-mediated Pael-R ubiquitination. Moreover, CHIP enhanced Parkin-mediated in vitro ubiquitination of Pael-R in the absence of Hsp70. Furthermore, CHIP enhanced the ability of Parkin to inhibit cell death induced by Pael-R. Taken together, these results indicate that CHIP is a mammalian E4-like molecule that positively regulates Parkin E3 activity.     

Genetic study of familial cases of Alzheimer's disease

A small number (1-5%) of Alzheimer's disease (AD) cases associated with the early-onset form of the disease (EOAD) appears to be transmitted as a pure genetic, autosomal dominant trait. To date, three genes responsible for familial EOAD have been identified in the human genome: amyloid precursor protein (APP), presenilin 1 (PS1), and presenilin 2 (PS2). Mutations in these genes account for a significant fraction (18 to 50%) of familial cases of early onset AD. The mutations affect APP processing causing increased production of the toxic Abeta42 peptide. According to the "amyloid cascade hypothesis", aggregation of the Abeta42 peptide in brain is a primary event in AD pathogenesis. In our study of twenty AD patients with a positive family history of dementia, 15% (3 of 20) of the cases could be explained by coding sequence mutations in the PS1 gene. Although a frequency of PS1 mutations is less than 2% in the whole population of AD patients, their detection has a significant diagnostic value for both genetic counseling and treatment in families with AD.     

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

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.     

Localization of the gene responsible for familial benign polycythemia to chromosome 11q23.

Familial benign polycythemia (FBP) (OMIM 263400) is a rare autosomal recessive condition characterized by erythrocytosis, normal leukocyte and platelet counts, normal uric acid level, and usually increased erythropoietin production. There is a high incidence of this disorder in Chuvashia (Russian Federation), probably due to a founder effect. In an attempt to locate the gene responsible for this disorder, we have carried out linkage studies in 12 Chuvash families, with 35 affected and 32 unaffected members. Linkage to the erythropoietin and erythropoietin receptor loci was excluded, and the FBP gene was assigned to the region of chromosome 11q23 between D11S4142 and D11S1356, with a maximal lod score of 6.61.     

Genetic heterogeneity of polycystic kidney disease in Bulgaria

Linkage analysis was performed on 22 Bulgarian families with polycystic kidney disease (PKD) ascertained through the hemodialysis centers of two medical schools. A total of 128 affected and 59 unaffected individuals, and 54 spouses have been investigated using eight polymorphic markers linked to PKD1 and nine markers to PKD2. The results demonstrate locus heterogeneity with 0.67 as the maximum likelihood value of alpha, i.e., the proportion of families linked to PKD1. In five families, the results suggest linkage to PKD2, and observed recombinants place the gene between loci D4S1544 and D4S1542. In one family, two double recombinants for closely linked markers on chromosome 16 and on chromosome 4 give evidence for the lack of link-age to either PKD1 or PKD2, thus suggesting the involvement of a third locus. Analysis of clinical data in the PKD1 group versus the unlinked group shows no significant differences in the severity of the disease.