Molecular genetic analysis of early onset familial Alzheimer's disease

Today, results of molecular genetic analyses of familial Alzheimer's disease are not only confusing to those not directly involved in this particular research field but, they are also extremely troublesome to those trying to elucidate the genetic basis of this devastating neurological disorder. Dr. St. George-Hyslop has provided an excellent review of the current research state, outlining the major problems hindering the molecular geneticists in reaching their final goal, namely isolation and characterization of the gene(s) predisposing to familial Alzheimer's disease. We feel confident that with additional work and further advances, many of these obstacles will be overcome and the genetic role in AD more clearly delineated.     

Clinical features of early onset,familial Alzheimer's disease linked to chromosome 14

Most congenital cutaneous hemangiomas are a sporadic occurrence. Hemangiomas have been found in association with coarctation of the aorta and a right aortic arch. A separate association has been noted of midline ventral defects with hemangiomas. We report on a patient with multiple hemangiomas, coarctation of the aorta and a right aortic arch, a supraumbilical midabdominal raphé and sternal cleft. Our patient represents an overlap between these two conditions. Review of the literature identified four additional patients with a similar combination of anomalies. The clinical overlap between these 5 patients suggests that they are variants of the same condition and represent a spectrum of defects that includes hemangiomas, midline ventral defects, aortic arch abnormalities and brain malformation. © 1995 Wiley-Liss, Inc.     

Whole genome analysis in a consanguineous family with early onset Alzheimer's disease

Early-onset Alzheimer's disease (EOAD) is a clinically and genetically heterogeneous condition in which the typical features appear significantly earlier in life (before 65 years). Mutations in three genes (PSEN1, PSEN2, and APP) have been identified in autosomal dominant forms of EOAD. However, in about 50% of Mendelian cases and in most of the sporadic EOAD patients, no mutations have been found. We present clinical characteristics of an Israeli family comprising two affected siblings with EOAD born to neurologically healthy parents who were first cousins (both parents died after 90 years old). Sequence analysis of PSEN1, PSEN2, APP, TAU, PGRN, and PRNP failed to reveal any mutations in the affected siblings. Because the disease in this family is consistent with an autosomal recessive mode of inheritance we identified all homozygous regions identical by descent (IBD) in both siblings, by high-density SNP genotyping. We provide here the first catalog of autozygosity in EOAD and suggest that the regions identified are excellent candidate loci for a recessive genetic lesion causing this disease.     

Mean age of onset in familial Alzheimer's disease is determined by amyloid beta 42

More than 130 known mutations in the presenilin-1 (PS1) gene result in familial Alzheimer's disease (FAD) with a mutation specific age of disease onset. These mutations increase amyloid beta 42 (A beta42) levels, and this increase has been validated in recent years as one pathogenic factor in FAD. However, further malfunctions of mutant presenilin-1 are discussed as well. In order to assess the weight of A beta42 regarding the pathogenesis of FAD, we expressed mutant forms of PS1 (30-65 years onset age) in COS-7 cells and analyzed amyloid beta levels by a novel ELISA. We found a strong correlation (r = 0.98; p<0.001) between the A beta40/42-ratio and mean age of disease onset indicating a substantial extent of A beta42 contribution to FAD pathology. Our data strongly suggest that A beta42 is the decisive factor for age of onset in FAD.     

Chromosome fragility in Alzheimer's disease

We present cytogenetic findings for 12 patients with Alzheimer's Disease (AD) mean age 75.8 ± 6.01 years and 35 normal age and sex matched controls (mean age 74.8 ± 4.04 years). The study, undertaken due to reports of increased fragments and chromosome breakage in individuals with AD, was performed blind on coded peripheral blood specimens and the allocation of AD or control was not known to the cytogenetic staff until the end of the study. Both the AD group and the controls have been very carefully selected and both underwent the same clinical assessment and screening procedures which included CT scanning.
Chromosomes were analysed after 72 h cultures, using deprived medium TC199 which is known to enhance the appearance of fragile sites. A minimum of 50 cells was examined in each case and any rearrangement found was classified as ctg, csg, ctb, csb and the chromosome in which it occurred was recorded. Analysis of results showed that there was no statistically significant difference between the AD group and the controls for either the total occurrence of breaks, the type of aberration or the chromosome(s) involved. In both groups the commonest break was in 3p.     

Molecular diagnosis of autosomal dominant early onset Alzheimer's disease: an update

Background: Autosomal dominant early onset Alzheimer''s disease (ADEOAD) is genetically heterogeneous. Mutations of the amyloid precursor protein (APP), presenilin 1 (PSEN1), and presenilin 2 (PSEN2) genes have been identified. Objective: To further clarify the respective contribution of these genes to ADEOAD. Methods: 31 novel families were investigated. They were ascertained using stringent criteria (the occurrence of probable or definite cases of Alzheimer''s disease with onset before 60 years of age in three generations). All cases fulfilled the NINCDS-ADRDA criteria for probable or definite Alzheimer''s disease. The entire coding regions of PSEN1 and PSEN2 genes and exons 16 and 17 of APP gene were sequenced from genomic DNA Results: PSEN1 mutations, including eight previously unreported mutations, were detected in 24 of the 31 families, and APP mutations were found in five families. In this sample, the mean ages of disease onset in PSEN1 and APP mutation carriers were 41.7 and 51.2 years, respectively. Conclusions: Combining these data with previously published data, yielding 65 ADEOAD families, 66% of the cases were attributable to PSEN1 mutations and 16% to APP mutations, while 18% remained unexplained.     

Cholinergic activity correlates with reserve proxies in Alzheimer's disease

The clinical expression of Alzheimer's disease (AD) occurs as neuropathology exceeds the brain “reserve capacity.” A possible association between the cholinergic system and reserve is suggested by preclinical observations that the cholinergic system allows cortical plasticity and by clinical observations of variable responses to cholinergic treatments depending on the patient’s educational level. The aim of this study was to investigate the association of reserve proxies, that is, education and occupation, with acetylcholinesterase (AChE) activity, measured voxelwise by [¹¹C]-MP4A and positron emission tomography (PET), in 9 healthy controls (HC), 7 patients with early probable AD, and 9 subjects with mild cognitive impairment (MCI) at the time of PET imaging, who progressed to AD at follow-up (prodromal AD). The analysis of prodromal and early AD showed positive correlations between education and AChE activity in the hippocampus, bilaterally, and between occupation and AChE activity in the right posterior cingulate gyrus. The significant correlation between AChE activity in structures belonging to the memory network and reserve proxies suggests that the brain reserve in AD is associated with a preserved/stimulated cholinergic neurotransmission.     

Molecular genetic analysis of familial early-onset Alzheimer's disease linked to chromosome 14q24.3

Genetic linkage studies have indicated that chromosome 14q24.3harbours a major locus for early-onset (onset age <65 years)Alzheimer's disease (AD3). Positional cloning efforts have identifieda novel gene S182 or presenilin 1 as the AD3 gene. We have mappedS182 in the AD3 candidate region between D14S277 and D14S284defined by genetic linkage studies in the two chromosome 14linked, early-onset AD families AD/A and AD/B. We have shownthat S182 is expressed in lymphoblasts and have determined thecomplete cDNA in both brain and lymphoblasts by RT-PCR sequencing.S182 is alternatively spliced in both brain and lymphoblastswithin a putative phosphorylation site located 5' in the codingregion. We identified two novel mutations, Ile143Thr and Gly384Alalocated in, respectively, the second transmembrane domain andin the sixth hydrophilic loop of the putative transmembranestructure of S182. As families AD/A and AD/B have a very similarAD phenotype our observation of two mutations in functionallydifferent domains suggest that onset age and severity of ADmay not be very helpful predictors of the location of putativeS182 mutations.     

Linkage of familial alzheimer disease to chromosome 14 in two large early-onset pedigrees: Effects of marker allele frequencies on lod scores

Alzheimer disease (AD) is a devastating neurodegenerative disease leading to global dementia. In addition to sporadic forms of AD, familial forms (FAD) have been recognized. Mutations in the amyloid precursor protein (APP) gene on chromosome (CHR) 21 have been shown to cause early-onset AD in a small number of pedigrees. Recently, linkage to markers on CHR 14 has been established in several early-onset FAD pedigrees. We now report lod scores for CHR 14 markers in two large early-onset FAD pedigrees. Pairwise linkage analysis suggested that in these pedigrees the mutation is tightly linked to the loci D14S43 and D14S53. However, assumptions regarding marker allele frequencies had a major and often unpredictable effect on calculated lod scores. Therefore, caution needs to be exercised when single pedigrees are analyzed with marker allele frequencies determined from the literature or from a pool of spouses. © 1993 Wiley-Liss, Inc.     

Acceleration of cortical thinning in familial Alzheimer's disease

Background

MRI in presymptomatic autosomal dominant Alzheimer's disease mutation carriers (MC) provides an opportunity to detect changes that pre-date symptoms or clinical diagnosis. We used automated cortical thickness (CTh) measurement to compare the grey matter of such a group with cognitively normal controls.

Methods

9 presymptomatic mutation carriers (4 PSEN1, 5 APP) and 25 healthy, age and sex-matched controls underwent longitudinal volumetric MRI brain imaging. CTh measurement was performed across the whole brain using a validated, automated technique. Four regions of interest (ROI) (entorhinal cortex (ERC), parahippocampal gyrus (PHG), posterior cingulate cortex and precuneus) and two control regions (paracentral and pericalcarine) were selected on the basis of imaging data in existing Alzheimer's disease (AD) literature. Linear mixed models were used to describe normal ageing in controls and the extent to which mean CTh in cases differed from controls according to time since clinical diagnosis, adjusting for normal ageing.

Results

An accelerating decline in CTh was observed across all ROI in the MC group. No such decline was demonstrated in the control regions for the MC group. Relative to controls, and adjusting for normal ageing, there was evidence (p = 0.05, one-sided test) of lower CTh in the posterior cingulate up to 1.8 years prior to diagnosis and in the precuneus up to 4.1 years prior to diagnosis in the MC group.

Discussion

Automated CTh analysis is a relatively practical, rapid and effective technique for assessing subtle structural change in AD. There is evidence that cortical thickness is reduced in mutation carriers a number of years prior to clinical diagnosis.     

Allelic association at the D14S43 locus in early onset Alzheimer's disease

The D14S43 marker is closely linked to the major gene for early onset autosomal dominant Alzheimer's disease on chromosome 14. Allelic frequencies at the D14S43 locus were compared in 113 familial and isolated cases of early onset Alzheimer's disease (<60 years of age at onset) (EOAD) and 109 unaffected individuals of the same geographic origin. Allele 7 was significantly (P = 0.033) more frequent in type 1 EOAD patients (13.2%), defined by the presence of at least another first degree relative with EOAD, than in controls (4.1%). Since an autosomal dominant gene is probably responsible for type 1 patients, allelic association may reflect linkage disequilibrium at the D14S43 locus. This would mean that some patients share a common ancestral mutation. However, since multiple tests were carried out, this result must be interpreted with caution, and needs confirmation in an independent sample. © 1995 Wiley-Liss, Inc.     

Telomere shortening in T cells correlates with Alzheimer's disease status

Telomeres, the repeated sequences that cap chromosome ends, undergo shortening with each cell division, and therefore serve as markers of a cell's replicative history. In vivo, clonal expansion of T cells during immune responses to both foreign and autoantigens is associated with telomere shortening. To investigate possible immune alterations in Alzheimer's disease (AD) that might impact current vaccine-based therapeutic strategies, we analyzed telomere lengths in immune cell populations from AD patients. Our data show a significant telomere shortening in PBMC from AD versus controls (P=0.04). Importantly, telomere length of T cells, but not of B cells or monocytes, correlated with AD disease status, measured by Mini Mental Status Exam (MMSE) scores (P=0.025). T cell telomere length also inversely correlated with serum levels of the proinflammatory cytokine TNFalpha (a clinical marker of disease status), with the proportion of CD8+ T cells lacking expression of the CD28 costimulatory molecule, and with apoptosis. These findings suggest an immune involvement in AD pathogenesis.     

Age at onset and life table risks in genetic counselling for Huntington''s disease.

Age related genetic risk data for carrying the gene for Huntington's disease in relatives at risk, based on a previously documented life table approach, are presented in a form allowing ready access for use in genetic counselling. Figures are given for first degree relatives and for second degree relatives showing varying age combinations of consultand and intervening parent at risk. Data are also given for the risk of developing Huntington's disease over varying finite periods in relation to age. The availability of this information in tabular form should help those involved in genetic counselling for this disorder in providing accurate risk estimates to relatives; the data are also of importance for combination with genotype information in predictive testing.     

A genetic cluster of early onset Parkinson's disease in a Colombian population.

We previously identified in two families with early onset Parkinson's Disease (PD) from the isolated population of Antioquia (Colombia), a parkin Cys212Tyr substitution caused by a G736A mutation. This mutation was subsequently observed in a Spanish family, suggesting that it could have been taken to Antioquia by Spanish immigrants. Here we screened for the G736A mutation in additional Antioquian early onset PD cases and used haplotype analysis to investigate the relationship between Spanish and Antioquian G736A chromosomes. We confirmed the occurrence of an extensive founder effect in Antioquia. Thirteen individuals (10 homozygotes) from seven nuclear families were identified with the G736A mutation. Genealogical investigations demonstrated the existence of shared ancestors between six of these families four to five generations ago and no evidence of Spanish ancestry during this period. A second parkin mutation (a duplication of exon 3), was detected in the three G736A heterozygote carriers. Haplotype data exclude a recent common ancestry between the Spanish and Antioquian patients studied here and is consistent with the introduction of the G736A mutation in Antioquia during early colonial times (about 16 generations ago).     

Increased fMRI signal with age in familial Alzheimer's disease mutation carriers

Although many Alzheimer's disease (AD) patients have a family history of the disease, it is rarely inherited in a predictable way. Functional magnetic resonance imaging (fMRI) studies of nondemented adults carrying familial AD mutations provide an opportunity to prospectively identify brain differences associated with early AD-related changes. We compared fMRI activity of 18 nondemented autosomal dominant AD mutation carriers with fMRI activity in eight of their noncarrier relatives as they performed a novelty encoding task in which they viewed novel and repeated images. Because age of disease onset is relatively consistent within families, we also correlated fMRI activity with subjects' distance from the median age of diagnosis for their family. Mutation carriers did not show significantly different voxelwise fMRI activity from noncarriers as a group. However, as they approached their family age of disease diagnosis, only mutation carriers showed increased fMRI activity in the fusiform and middle temporal gyri. This suggests that during novelty encoding, increased fMRI activity in the temporal lobe may relate to incipient AD processes.     

Age of onset in siblings of persons with juvenile Huntinqton disease

The age of onset distribution of Huntington disease (HD) has been defined for siblings of patients who exhibit the disease before age 20. The mean age of onset for the siblings of juvenile onset cases is 26.8 years, which is significantly less than the mean age of onset (39.8) observed in siblings of non-juvenile cases. We have shown that the curve for age of onset in the general affected population is significantly different from that of the juvenile sibships. Furthermore, the significant regression equation suggests that the 'expected age of onset' of a sibling can be predicted from a knowledge of the age of onset of the juvenile proband. This information can be used to predict a range of age of onset for those sibs of juvenile patients who are likely to be asymptomatic heterozygotes with DNA polymorphism studies.     

Very early onset Huntington''s disease: genetic mechanism and risk to siblings

A study of very early onset Huntington's disease (VEOHD) has shown that at least 38% of gene-carrying sibs also develop symptoms before the age of 10, thus improving the genetic risk for those sibs who remain healthy. The prevalence of VEOHD among sibs shows that mutation during spermatogenesis is most unlikely to account for these uncommon cases. The data suggest that two mechanisms contribute to VEOHD: modification by many genes (individually of small effect), and an epigenetic mechanism occurring when transmission is through a series of males.     

Cognitive correlates of alterations in acetylcholinesterase in Alzheimer's disease

We recently reported findings of modest loss of cortical acetylcholinesterase (AChE) activity in patients with overall mild Alzheimer's disease (AD) using N-[11C]methyl-pi-peridin-4-yl propionate ([11C]PMP) AChE positron emission tomography (PET). To determine cognitive correlates of in vivo cortical AChE activity in patients with mild to moderate AD (n=15), and in normal controls (NC, n=12) using [11C]PMP AChE PET imaging. Mean cortical AChE activity in the AD subjects was mildly reduced (-11.1%) compared to the control subjects (P<0.05). Analysis of the cognitive data showed that mean cortical AChE activity was significantly associated with performance on a test of attention and working memory (WAIS-III Digit Span, R=0.46, P=0.01) but not with tests of delayed short or long-term memory functions. Similar findings were present when the analysis was limited to the temporal cortex. Cortical AChE activity is more robustly associated with functions of attention and working memory compared to performance on primary memory tests in AD.     

Subicular dendritic arborization in Alzheimer's disease correlates with neurofibrillary tangle density

Intracellular accumulation of PHFtau in Alzheimer's disease (AD) disrupts the neuronal cytoskeleton and other neuronal machinery and contributes to axonal and dendritic degeneration, and neuronal death. Furthermore, amyloid-beta (Abeta) has been reported to be toxic to neurons and neurites. While loss of presynaptic elements is an established feature of AD, the nature and extent of dendritic degeneration has been infrequently studied. We investigated MAP2-immunoreactive dendrites using a novel method of high-throughput quantification and also measured cortical thickness and the densities of NeuN-immunoreactive neurons, PHFtau neurofibrillary tangles (NFTs), and Abeta plaque burden in the subiculum in AD and elderly controls. Corrected for atrophy, the "dendritic arborization index" was significantly reduced by up to 66% in all three layers of the subiculum. Laminar thickness was reduced by an average 33% and there was a marked reduction in neuron density of approximately 50%. As expected, NFTs and Abeta plaques were significantly increased in AD. Dendritic arborization indices negatively correlated with NFT densities while no significant correlations were found with Abeta plaque densities. The pattern of dendritic loss in the subiculum and the correlations with NFT densities respectively suggest that deafferentation and intrinsic neurofibrillary degeneration both may contribute to dendritic loss in AD.