Alzheimer Disease and the Frontotemporal Dementias: Diseases with Cerebral Deposition of Fibrillar Proteins

ABSTRACT

• Alzheimer disease (AD), Lewy body variant of Alzheimer disease (LBV), and the frontotemporal dementias (FTD) are a class of adult-onset neurodegenerative dementias characterized by the intracellular and/or extracellular accumulation of proteins which assemble into β-pleated sheet fibrils.

• Missense mutations in the β-amyloid precursor protein (βAPP) gene on chromosome 21, in the presenilin 1 (PS1) gene on chromosome 14, and the presenilin 2 (PS2) on chromosome 1, are associated with early-onset forms of familial Alzheimer disease. Mutations in these genes result in altered processing of βAPP and the relative overproduction of either all forms of the β-amyloid peptide or specific overproduction of isoforms ending at residue 42.

• The ∈4 (Cys112Arg) variant of apolipoprotein E (APOE) is associated in a dose-dependent fashion with increased risk for late-onset Alzheimer disease (after age 55) and with Lewy body variant of Alzheimer disease. The inheritance of one or more APOE ∈4 alleles is not deterministic for AD, and the mechanism by which inheritance of one or more ∈4 alleles causes AD is unclear.

• Allelic association studies have identified several putative candidate genes in which nucleotide sequence variants have been associated with AD. These studies have received only partial replication, and the true role of these candidate genes in AD remains unclear.

• Hyperphosphorylated forms of the microtubule-associated protein Tau are consistent neuropathologic features of AD, LDV, and FTD. Coding region and splice-site mutations in the Tau gene on chromosome 17 are associated with a subtype of frontotemporal dementia (FTDP-17). These mutations probably affect filament assembly by several different mechanisms including alterations in the sequence of microtubule binding sites and alterations in the relative abundance of longer (4 repeat) isoforms of Tau relative to shorter (3 repeat) isoforms.

• The currently identified genes associated with inherited risk for these diseases represent approximately half of the probable genetic factors causing these diseases. As the remaining susceptibility genes are identified, they will likely contribute to a more complete understanding of the pathogenesis of these disorders.

INTRODUCTION

Alzheimer disease (AD), Lewy body variant of Alzheimer disease (LBV), and the frontotemporal dementias (FTD) are the three commonest causes of adult-onset dementia and together represent a unique class of neurologic diseases (MIM 104300 (AD); 127750 (LBV); 601630 (FTD)).

These diseases are linked biochemically by virtue of the fact that each is characterized by the pathologic deposition of fibrillary proteins either in cerebral neurons (Tau protein), in the cerebral extracellular space (amyloid β-peptide), or both. As is described below, current work suggests that in AD and LBV, pathologic processing of the β-amyloid precursor protein precedes abnormal Tau processing. Conversely, in some forms of FTD, abnormal processing of Tau is the initiating event causing neurodegeneration.

These diseases typically present in mid-to-late adult life with a constellation of symptoms that reflect dysfunction and degeneration of neural cells in the cerebral cortex and other selected brain regions. The clinical symptoms of these diseases vary ...