P. St George-Hyslop

Consensus Report of the Working Group on : Molecular and Biochemical Markers of Alzheimer's Disease

The ideal biomarker for Alzheimers disease (AD) should detect a fundamental feature of neuropathology and be validated in neuropathologically-confirmed cases; it should have a sensitivity >80% for detecting AD and a specificity of >80% for distinguishing other dementias; it should be reliable, reproducible, non-invasive, simple to perform, and inexpensive. Recommended steps to establish a biomarker include confirmation by at least two independent studies conducted by qualified investigators with the results published in peer-reviewed journals. Our review of current candidate markers indicates that for suspected early-onset familial AD, it is appropriate to search for mutations in the presenilin 1, presenilin 2, and amyloid precursor protein genes. Individuals with these mutations typically have increased levels of the amyloid Abeta42 peptide in plasma and decreased levels of APPs in cerebrospinal fluid. In late-onset and sporadic AD, these measures are not useful, but detecting an apolipoprotein E e4 allele can add confidence to the clinical diagnosis. Among the other proposed molecular and biochemical markers for sporadic AD, cerebrospinal fluid assays showing low levels of Abeta42 and high levels of tau come closest to fulfilling criteria for a useful biomarker.The ideal biomarker for Alzheimers disease (AD) should detect a fundamental feature of neuropathology and be validated in neuropathologically-confirmed cases: it should have a sensitivity >80% for detecting AD and a specificity of >80% for distinguishing other dementias: it should be reliable, reproducible non-invasive, simple to perform, and inexpensive. Recommended steps to establish a biomarker include confirmation by at least two independent studies conducted by qualified investigators with the results published in peer-reviewed journals. Our review of current candidate markers indicates that for suspected early-onset familial AD. it is appropriate to search for mutations in the presenilin 1, presenilin 2, and amyloid precursor protein genes. Individuals with these mutations typically have increased levels of the amyloid Aβ 42 peptide in plasma and decreased levels of APPs in cerebrospinal fluid. In late-onset and sporadic AD. these measures are not useful. but detecting an apolipoprotein E e4 allele can add confidence to the clinical diagnosis. Among the other proposed molecular and biochemical markers for sporadic AD. cerebrospinal fluid assays showing low levels of Aβ 42 and high levels of tau come closest to fulfilling criteria for a useful biomarker.

The Presenilin 1 Protein Is a Component of a High Molecular Weight Intracellular Complex That Contains β-Catenin

The presenilin (PS) genes associated with Alzheimer disease encode polytopic transmembrane proteins which undergo physiologic endoproteolytic cleavage to generate stable NH2- and COOH-terminal fragments (NTF or CTF) which co-localize in intracellular membranes, but are tightly regulated in their stoichiometry and abundance. We have used linear glycerol velocity and discontinuous sucrose gradient analysis to investigate the distribution and native conformation of PS1 and PS2 during this regulated processing in cultured cells and in brain. The PS1 NTF and CTF co-localize in the endoplasmic reticulum (ER) and in the Golgi apparatus, where they are components of a ∼250-kDa complex. This complex also contains β-catenin but not β-amyloid precursor protein (APP). In contrast, the PS1 holoprotein precursor is predominantly localized to the rough ER and smooth ER, where it is a component of a ∼180-kDa native complex. PS2 forms similar but independent complexes. Restricted incorporation of the presenilin NTF and CTF along with a potentially functional ligand (β-catenin) into a multimeric complex in the ER and Golgi apparatus may provide an explanation for the regulated accumulation of the NTF and CTF.

Corticobasal degeneration and progressive supranuclear palsy share a common tau haplotype

Objective: To analyze the association of polymorphisms in the tau gene with pathologically confirmed corticobasal degeneration (CBD). Background: The authors previously described an extended tau haplotype (H1) that covers the human tau gene and is associated with the development of progressive supranuclear palsy (PSP). The authors now extend this analysis to CBD, a neurodegenerative condition with clinical and neuropathologic similarities to PSP. Like PSP, CBD is associated with accumulation of aggregates containing the 4-repeat isoforms of tau. Because of difficulty in diagnosis of CBD, the authors only analyzed cases with pathologically confirmed CBD. Methods: The authors collected 57 unrelated, neuropathologically confirmed cases of CBD. Tau sequencing in these cases failed to show the presence of pathogenic mutations. Polymorphisms that spanned the tau gene were analyzed in all CBD cases and controls. Results: Analyzing tau polymorphisms in CBD cases showed that the frequency of H1 and H1/H1 was significantly increased when analyzing all cases and when separating by country of origin. H1 frequency in all CBD cases was 0.921, compared with a control frequency of 0.766 (X2 = 9.1, p = 0.00255 [1df], OR 3.56 [8.43 > CI 95% > 1.53]). The H1/H1 frequency was also significantly higher at 0.842 compared with 0.596 in age-matched controls (X2 = 17.42, p = 0.00016, 2df), OR 3.61 [7.05 > CI 95% > 1.85]). Conclusions: The CBD tau association described here suggests that PSP and CBD share a similar cause, although the pathogenic mechanism behind the two diseases leads to a different clinical and pathologic phenotype.

Genetic linkage of von Recklinghausen neurofibromatosis to the nerve growth factor receptor gene

von Recklinghausen neurofibromatosis (VRNF) is one of the most common inherited disorders affecting the human nervous system. VRNF is transmitted as an autosomal dominant defect with high penetrance but variable expressivity. The disorder is characterized clinically by hyperpigmented patches of skin (café au lait macules, axillary freckles) and by multiple tumors of peripheral nerve, spinal nerve roots, and brain (neurofibromas, optic gliomas). These tumors can cause disfigurement, paralysis, blindness, and death. We have determined the chromosomal location of the VRNF gene by genetic linkage analysis using DNA markers. The VRNF gene is genetically linked to the locus encoding nerve growth factor receptor, located on the long arm of chromosome 17 in the region 17q12----17q22. However, crossovers with the VRNF locus suggest that a mutation in the nerve growth factor receptor gene itself is unlikely to be the fundamental defect responsible for the VRNF phenotype.

Screening for PS1 mutations in a referral-based series of AD cases: 21 novel mutations

Background: Mutations in the presenilin-1 gene (PS1) account for a majority of patients with early-onset familial AD. However, the clinical indications and algorithms for genetic testing in dementia are still evolving. Methods: The entire open reading frame of the PS1 gene was sequenced in a series of 414 consecutive patients referred for diagnostic testing, including 372 patients with AD and 42 asymptomatic persons with a strong family history of AD. Results: Forty-eight independent patients screened had a PS1 mutation including 21 novel mutations. In addition, 3% of subjects (11/413) had a known polymorphism, the Glu318Gly substitution. The majority of the mutations were missense substitutions but there were three insertions and Δexon 10 mutation. With six exceptions (codons 35, 178, 352, 354, 358, and 365) most of the mutations occurred at residues conserved in the homologous PS2 gene or in PS1 of other species. Conclusions: Eleven percent of a referral-based series of patients with AD can be explained by coding sequence mutations in the PS1 gene. The high frequency of PS1 mutations in this study indicates that screening for PS1 mutations in AD is likely to be successful, especially when directed at patients with a positive family history with onset before 60 years (90% of those with PS1 mutations were affected by age 60 years). This will also have significance for the secondary identification of at-risk relatives who might be candidates for future prophylactic therapies for AD.

Familial frontotemporal dementia with ubiquitin-positive, tau-negative inclusions

Objective: To describe the clinical features, neuropathology, and genetic studies in a family with autosomal dominant frontotemporal dementia (FTD). Background: Clinical Pick’s disease, or FTD with parkinsonism, has been described in several families linked to chromosome 17 (FTDP-17). Most of these have shown tau protein mutations. The clinical and pathologic variations in these families resemble the spectrum of sporadic FTD or “Pick complex.” Methods: Clinical and behavioral analysis of the affected members with extensive histochemical and neuropathologic description of three cases, genetic analysis of three clinically affected members and seven at risk members to assess linkage to chromosome 17, and sequencing of the tau gene in two patients were performed. Results: The clinical pattern shows a highly stereotypic disinhibition dementia with late extrapyramidal features, progressive mutism, and terminal dysphagia in three generations of affected individuals. Neuropathology showed frontotemporal atrophy, and microscopically tau- and synuclein-negative and ubiquitin-positive neuronal inclusions, in the background of superficial cortical spongiosis, neuronal loss, and gliosis. Tau expression was restricted to oligodendroglia. All exons and surrounding introns of the tau gene were sequenced, and no mutation or disease-related polymorphisms were detected in either of two affected pedigree members. Conclusion: This family with autosomal dominant frontotemporal dementia (FTD) shows no tau expression in neurons. The ubiquitin-positive, tau-negative inclusions have been described before in FTD with and without motor neuron disease, but not in a familial form. The clinical and some pathologic features are similar to those of several of the families included in descriptions of FTD with parkinsonism linked to chromosome 17, but the linkage to tau has been excluded. The defect in this family, however, could be functionally related to tau mutations.

A prospective study of the clinical utility of ApoE genotype in the prediction of outcome in patients with memory impairment

Article abstract-Given the relationship between the presence of ApoE epsilon 4 and Alzheimers disease (AD), we studied whether knowledge of epsilon 4 status would predict which memory-impaired patients would develop AD over time. One hundred seven patients who presented with memory impairment but not dementia were referred to the study by their family physicians. These patients were followed prospectively over a 2-year period. Twenty-nine patients developed AD, while 78 did not develop dementia. We found that ApoE genotype was a reliable prognostic indicator of who developed AD in this group only when memory test performance was included in the predictive model. These findings indicate the limitations of ApoE genotyping in isolation as a prognostic indicator of AD. Because this study included prospectively selected patients who were followed longitudinally, our findings are likely to have more relevance in the clinical setting than those obtained from currently available retrospective studies. NEUROLOGY 1996;46: 149-154

Nicastrin binds to membrane-tethered Notch

The presenilins and nicastrin, a type 1 transmembrane glycoprotein, form high molecular weight complexes that are involved in cleaving the β-amyloid precursor protein (βAPP) and Notch in their transmembrane domains. The former process (termed γ-secretase cleavage) generates amyloid β-peptide (Aβ), which is involved in the pathogenesis of Alzheimers disease. The latter process (termed S3-site cleavage) generates Notch intracellular domain (NICD), which is involved in intercellular signalling. Nicastrin binds both full-length βAPP and the substrates of γ-secretase (C99- and C83-βAPP fragments), and modulates the activity of γ-secretase. Although absence of the Caenorhabditis elegans nicastrin homologue (aph-2) is known to cause an embryonic-lethal glp-1 phenotype, the role of nicastrin in this process has not been explored. Here we report that nicastrin binds to membrane-tethered forms of Notch (substrates for S3-site cleavage of Notch), and that, although mutations in the conserved 312–369 domain of nicastrin strongly modulate γ-secretase, they only weakly modulate the S3-site cleavage of Notch. Thus, nicastrin has a similar role in processing Notch and βAPP, but the 312–369 domain may have differential effects on these activities. In addition, we report that the Notch and βAPP pathways do not significantly compete with each other.

Alzheimer's disease Interaction of apolipoprotein E genotype, family history of dementia, gender, education, ethnicity, and age of onset

We evaluated 197 patients with predominantly late-onset Alzheimers disease (AD) who belonged to several ethnic groups and analyzed the relationship of age of onset of AD to the presence or absence of several risk factors in this entire group of patients. The apolipoprotein E (apoE) epsilon 4 allele frequency, which was 29% in all patients (compared with the reported population mean of 13.7%, p < 0.001, did not vary significantly between ethnic groups but declined significantly with increasing age. The apoE epsilon 2 allele frequency was 3%, compared with the reported population mean of 7.4% (p = 0.001). The frequency of a positive family history of dementia in first-degree relatives (FH+) (overall 45%) did not vary significantly between ethnic groups. ApoE epsilon 4-positive (epsilon 4+) patients tended to have a higher FH+ rate (58%) than apoE epsilon 4-negative (epsilon 4-) patients (40%) (p = 0.02). When the potential risk factors of gender, education, FH+ status, and epsilon 4+ status were examined together in a multiple linear-regression analysis, FH+ and epsilon 4+ status (but not gender or education) were significant (they were both associated with an earlier age of onset of AD). In a post-hoc analysis, we found a reduced age of onset in women, but not men, who were both FH+ and epsilon 4+. Additionally, those probands who were epsilon 4+ were more likely to inherit the disease from their mothers than their fathers. The mechanism by which epsilon 4+ and FH+ status operate as risk factors may be by their effect on the age of onset of AD. NEUROLOGY 1996;46: 1575-1579

Presenilins Interact with Armadillo Proteins Including Neural‐Specific Plakophilin‐Related Protein and β‐Catenin

Abstract : Missense substitutions in the presenilin 1 (PS1) and presenilin 2 (PS2) proteins are associated with early‐onset familial Alzheimers disease. We have used yeast‐two‐hybrid and coimmunoprecipitation methods to show that the large cytoplasmic loop domains of PS1 and PS2 interact specifically with three members of the armadillo protein family, including β‐catenin, p0071, and a novel neuronal‐specific armadillo protein—neural plakophilin‐related armadillo protein (NPRAP). The PS1 : NPRAP interaction occurs between the arm repeats of NPRAP and residues 372‐399 at the C‐terminal end of the large cytoplasmic loop of PS1. The latter residues contain a single arm‐like domain and are highly conserved in the presenilins, suggesting that they form a functional armadillo protein binding site for the presenilins.