Clinical Genetics and Genetic Counseling in Alzheimer Disease
Department of Psychiatry, Duke University Medical Center, Durham, NC 27710. Annals of internal medicine
(Impact Factor: 17.81).
11/1991; 115(8):601-6. DOI: 10.7326/0003-4819-601
Because of growing evidence for genetic causes of Alzheimer disease, clinicians are often asked about risks for this disorder among relatives of patients. This paper briefly reviews the evidence for genetic and other causes of Alzheimer disease, including specifically its typical late-onset variant. Several recent developments in the molecular genetics of early-onset Alzheimer disease are described briefly. These findings, however, may not apply to more typical, late-onset Alzheimer disease. Genetic studies of the latter disorder are complicated by the censorship of any inherited predisposition to disease by death from other causes. Hence, heritable disease may be apparent only in families with large numbers of elderly relatives at risk. Relying on empiric observations of risk among relatives from several recent studies of pooled Alzheimer families, curves are presented describing theoretic age-specific risks in relatives of patients and unaffected controls. Real (current) lifetime risks of Alzheimer disease are then estimated by modification of the theoretic figures with corresponding current age-specific mortality characteristics of the U.S. population. The lifetime incidence among relatives of patients with Alzheimer disease is estimated to be 19%, or three to four times the risk among control relatives. Because both mortality of and risk for Alzheimer disease change rapidly with advancing age, it may be possible to reduce the current morbidity from Alzheimer disease by 50% if onset can be postponed by only 5 years.
Available from: James Mortimer
- "For example , it has long been known that there is a markedly increased cumulative risk of dementia among first-degree relatives of individuals with AD. Life table analyses have shown a cumulative risk of dementia to firstdegree relatives of AD cases of approximately 50% by age 90, while relatives of purported control subjects had a much lower cumulative risk  . Because only about one-third of people meeting neuropathological criteria for AD present with dementia prior to death    , (for discussion of neuropathological criteria for AD, see ; for discussion of predementia and AD neuropathology, see ), some individuals classified as controls will also carry the disease . "
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ABSTRACT: This team takes the position that what is commonly referred to as non-familial Alzheimer's disease (AD) is predominantly due to genetic factors. Population-based studies suggest that genetic factors cause the majority of cases that begin after age 60. There are several lines of evidence supporting this position: Data from the Nun Study suggest that the risk for AD is largely established by early adulthood, implying that later adult exposures likely play only a small role in causation. Family studies show that first-degree blood relatives of persons with non-familial AD have a substantially increased risk of AD relative to controls. Twin studies suggest that the heritability of AD exceeds 60%. Environmental factors, such as socioeconomic status, education, and head injury, are strong risk factors for AD only in individuals with a genetic predisposition. The APOE genotype is a powerful risk factor for AD and accounts for as much as 50%. There are numerous other candidate genes with strong associations with AD that presumably explain the remaining population risk. This paper further reviews the mechanisms associated with AD causation for APOE and other candidate genes and implications for the development of prevention strategies.
Available from: Roger L. Albin
- "In typical AD cases, the late onset of disease prevents the ascertainment of inheritance patterns by conventional pedigree tracing and linkage analysis. Recent studies of typical AD, however, have found that there is a high incidence of dementia in first degree relatives of AD patients (Breitner, 1991). Breituer (1991) has recently summarized the literature examining the incidence of dementia in first degree relatives of AD victims and argues convincingly that AD is inherited in an autosomal dominant fashion with age dependent penetrance and maximum cumulative risk of developing AD around age 90. "
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ABSTRACT: The antagonistic pleiotropy theory of senescence is the most convincing theoretical explanation of the existence of aging. As yet, no locus or allele has been identified in a wild population with the features predicted by the pleiotropic theory. Human genetic diseases offer the opportunity to identify potentially pleiotropic alleles/loci. Four human genetic diseases--Huntington's disease, idiopathic hemochromatosis, myotonic dystrophy, and Alzheimer's disease--may exhibit pleiotropic effects and further study of these diseases might result in the identification of pleiotropic genes causing aging. Inability to find an early life selective benefit associated with these disease-causing alleles would favor the major alternative genetic explanation for aging, the mutation accumulation theory.
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ABSTRACT: The twin method for investigating genetic and environmental causes of disease has been applied mostly in early-onset illnesses. Analysis of late-onset disorders requires reexamination of common assumptions about the relation between genetic causes and the degree of concordance expected. This paper considers Alzheimer disease (AD) as an example of a late-onset disorder with putative genetic factors. For argument it employs the strong hypothesis that AD is an autosomal dominant trait with age-dependent expression, as described by a previously published parametric model. That model encompasses 2 principal variants of disease: a rare form with onset in middle life, and a more common late-onset type which is nonetheless eventually fully penetrant. The present work then specifies the probability that, when a given member of a twin pair (the proband) is affected, an identical or fraternal co-twin also shows the disease. Such probability is expressed as a function of the age at onset of the proband and the current age of the pair. Even under strong working assumptions regarding genetic influence, the expected proportion of identical co-twins actually affected with AD will not exceed 40% until the subjects are about 80 years old. Therefore, except in very old subjects, modest twin concordance is a feeble argument against genetic causes, or in favor of exclusively environmental ones. In this sense the interpretation of results of twin studies in AD and other late-onset disorders differs substantially from studies of diseases with early onset.
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