Margaret J. Smith

Soluble pool of Aβ amyloid as a determinant of severity of neurodegeneration in Alzheimer's disease

Genetic evidence strongly supports the view that Aβ amyloid production is central to the cause of Alzheimers disease. The kinetics, compartmentation, and form of Aβ and its temporal relation to the neurodegenerative process remain uncertain. The levels of soluble and insoluble Aβ were determined by using western blot techniques, and the findings were assessed in relation to indices of severity of disease. The mean level of soluble Aβ is increased threefold in Alzheimers disease and correlates highly with markers of disease severity. In contrast, the level of insoluble Aβ (also a measure of total amyloid load) is found only to discriminate Alzheimers disease from controls, and does not correlate with disease severity or numbers of amyloid plaques. These findings support the concept of several interacting pools of Aβ, that is, a large relatively static insoluble pool that is derived from a constantly turning over smaller soluble pool. The latter may exist in both intracellular and extracellular compartments, and contain the basic forms of Aβ that cause neurodegeneration. Reducing the levels of these soluble Aβ species by threefold to levels found in normal controls might prove to be a goal of future therapeutic intervention.

Variable phenotype of Alzheimer's disease with spastic paraparesis

A variant form of Alzheimers disease (AD), in which spastic paraparesis (SP) precedes dementia, is characterised by large, noncored, weakly neuritic Aβ‐amyloid plaques resembling cotton wool balls and is caused by genomic deletion of presenilin 1 exon 9. A pedigree with a 5.9 kb exon 9 deletion shows a phenotypic spectrum including subjects with typical AD or with SP and numerous cotton wool plaques. In SP subjects, dementia onset is delayed and modified. This phenotypic variation suggests that modifying factors are associated with exon 9 deletions. Ann Neurol 2001;49:125–129

Memory decline in healthy older people Implications for identifying mild cognitive impairment

Background: Criteria for mild cognitive impairment require objective evidence of a memory deficit but do not require objective evidence of memory decline. Application of these criteria may therefore result in the misclassification of older patients with memory decline as normal because their neuropsychological test performance at a single point in time may be within normal limits. This study aimed to identify and characterize older people with memory decline. Method: Word list delayed recall (WLDR) test performance was assessed on five occasions during a 2-year period in a cohort of healthy older individuals. Older people with declining (n = 35) and nondeclining (n = 66) WLDR scores were identified. Both subgroups were then compared on apoE genotype, Clinical Dementia Rating, and neuropsychological test performance at the fifth assessment. Results: Thirty-four percent of the group with declining memory recorded a Clinical Dementia Rating of 0.5, compared with 5% of the nondeclining memory group. No between-group differences were observed in cognitive domains other than memory, self-reported cognitive failures, or the proportion of each group carrying the apoE epsilon 4 allele. Conclusions: A large proportion of healthy older individuals show memory decline, which may represent the early stages of a potentially more severe cognitive impairment. Further investigation is necessary to determine the relationship between apoE genotype, self-reported cognitive impairment, and memory decline in older people.

Chemical cleavage of mismatch: A new look at an established method

Chemical cleavage of mismatch (CCM), also known as chemical mismatch cleavage (CMC) or the HOT (hydroxylamine/osmium tetroxide) chemical method, has been used for detection of sequence variability with many systems since it was first described. Recently, adaptation to fluorescence‐based detection systems has fundamentally changed both the execution and analysis of CCM. This review will outline major advances in the methodology of CCM, from the advent of PCR through fluorescent analysis, and includes applications and modifications of CCM. Hum Mutat 11:345–353, 1998.

Creutzfeldt-Jakob disease in Australia 1970-1999.

Objective: To ascertain all persons who developed a transmissible spongiform encephalopathy (TSE) within Australia during the 30-year period 1970 to 1999 through a comprehensive national surveillance program and subject the group to detailed epidemiologic analysis. Methods: Cases were ascertained through reviews of morbidity separation coding data from all university-affiliated tertiary referral hospitals, as well as the centralized data bases of state and territory health departments, regular national death certificate searches, and semiannual mailout questionnaires to all neurologists and pathologists throughout Australia. Prospective monitoring commenced in September 1993. Results: A total of 387 patients were confirmed as having TSE during this epoch. The majority of cases were sporadic Creutzfeldt–Jakob disease (CJD) (90.7%), with 7.2% heredofamilial and 2.1% iatrogenic. Over this 30-year period, the national average annual sporadic CJD incidence rate per million progressively increased from 0.31 for the decade 1970 through 1979 to 0.77 for 1980 through 1989, reaching 1.03 for 1990 through 1999. Death certificates were found to have a false-positive rate of 11.5% and sensitivity of 83.0% for sporadic CJD. Conclusions: Within Australia, there has been a gradual increase in the incidence of transmissible spongiform encephalopathy over the three-decade period 1970 through 1999, peaking in 1999 at 1.4/million/year for sporadic Creutzfeldt–Jakob disease. This increase is believed secondary to improved case ascertainment. Variant Creutzfeldt–Jakob disease was not identified during this period. Age- and sex-adjusted comparisons showed a decline in incidence rates in the elderly in both sexes, usually from age 74 years. Death certificates were a useful but imperfect method of case detection.

Alternative transcripts of presenilin-1 associated with frontotemporal dementia.

&NA; We have analyzed the expression of Alzheimers disease‐associated presenilin 1 (PS1) in various neurodegenerative disorders. Western blotting identified PS1 N‐ and C‐terminal fragments similarly in the cortex of controls, Parkinson, Huntington and schizophrenia subjects. Additional PS1 immunoreactive species of 42 and 46 kDa were present in six out of seven cases of sporadic frontotemporal dementia (FTD) and these were particularly prominent in two cases. RT‐PCR analysis using nested primers showed the presence of PS1 gene products with deletions within the exon 4–8 region. Our results suggest that alternative transcription of PS1 may be associated with FTD.

Novel prion protein insert mutation associated with prolonged neurodegenerative illness

Background: Mutations in the prion protein gene (PRNP) are found in approximately 13 to 15% of persons classified as dying from a transmissible spongiform encephalopathy. Point and octapeptide repeat insert and deletion mutations are described in the open reading frame (ORF) of PRNP. The authors present a clinicopathologic study of a patient with a family history of a lengthy and progressive neurodegenerative disorder associated with a novel large octapeptide repeat insert mutation. Methods: Neuropathologic examination, including immunohistochemistry for the prion protein, was undertaken. The ORF of PRNP was amplified by PCR, cloned, and sequenced. Homogenate of cerebral tissue underwent Western blot analysis for the prion protein before and after proteinase K treatment. Results: The proband died after a 16-year illness commencing at age 29 years. Confident premortem clinical diagnosis was not achieved despite a brain biopsy. Autopsy examination of the brain confirmed a spongiform encephalopathy. Prion protein immunohistochemistry revealed occasional granular deposits in the cerebellar granular layer. The proband was found to harbor a novel PRNP 168 base pair (bp) insert mutation. Conclusion: The authors have identified a novel 168 bp octapeptide repeat insert mutation. Prion protein immunohistochemistry differs from previous cases harboring seven octapeptide repeat and other long insert mutations. Optimization of PRNP analysis, especially PCR conditions, is essential to avoid overlooking this type of mutation and delay the correct molecular genetic diagnosis.

Creutzfeldt-Jakob disease cluster in an Australian rural city

Through the Australian National Creutzfeldt‐Jakob Disease Registry, 6 pathologically confirmed sporadic cases were recognized over a 13‐year period in persons who had been long‐term residents of a moderate‐sized rural city, whereas the expected number was 0.923. An extensive investigation could not find any point‐source or case‐to‐case transmission links. This occurrence is highly statistically significant (p = 0.0027) when viewed in isolation and remains significant (p < 0.02) when only the cases that arose after the cluster was recognized were taken into account. However, a more conservative statistical analysis suggests that such a grouping could have arisen by chance in at least one population group of this size when the whole country is taken into consideration.

Early-onset Alzheimer's disease caused by a novel mutation at codon 219 of the presenilin-1 gene.

Mutations in the presenilin 1 (PS1) gene are responsible for approximately 50% of early onset autosomal-dominant Alzheimers disease cases. A PCR based mutation detection method, chemical cleavage of mismatch, was used to detect a novel PS1 mutation in the coding sequence of the PS1 gene. Sequencing confirmed a T to C transition altering a leucine to proline at codon 219 of the PS1 gene. This is a novel mutation in exon 7 of the PS1 gene occurring outside the transmembrane regions of IV and V.

Correct heteroduplex formation for mutation detection analysis

AbstractBackground: The majority of mutation detection methods for unknown mutations are polymerase chain reaction (PCR)-based methods dependent on the formation of heteroduplexes between wild-type and mutant strands of DNA. Methods and Results: This report discusses the difficulties associated with forming heteroduplexes with a large DNA fragment and the implications for subsequent mutation detection by the chemical cleavage of mismatch technique and other methods reliant on heteroduplex formation. It was found that the size and sequence context of the fragment being investigated inhibited correct heteroduplex formation. The problem was overcome by dividing the sequence into two overlapping fragments. Conclusions: Early identification of this problem in other fragments will help with the rapid optimization of PCR-based mutation detection methods.