Metin Bagli

24S-hydroxycholesterol in cerebrospinal fluid is elevated in early stages of dementia

The brain is the most cholesterol-rich organ in the human body. Accumulation of excess cholesterol in hippocampal neurons promotes the cleavage of the amyloid precursor protein (APP) into amyloidogenic components with the consequence of the acceleration of neuronal degeneration. Conversion of cholesterol to 24S-hydroxycholesterol mediated by cholesterol 24S-hydroxylase (CYP46) is the major pathway for the elimination of brain cholesterol and the maintenance of brain cholesterol homeostasis. We examined whether cerebrospinal fluid (CSF) 24S-hydroxycholesterol levels differ between patients with dementia, patients with mild cognitive impairment (MCI), and cognitively intact control subjects. Plasma and CSF concentrations of 24S-hydroxycholesterol and cholesterol in 32 patients with Alzheimers disease (AD), 11 patients with vascular dementia, seven patients with MCI, and seven cognitively intact control subjects were measured by combined gas-chromatography/mass spectrometry. We show elevated concentrations of 24S-hydroxycholesterol in the CSF of AD patients and we interpret this finding as a consequence of increased cholesterol turnover in the central nervous system during neurodegeneration. The observed influence of the apolipoprotein E epsilon4 (APOE4) allele on CSF 24S-hydroxycholesterol concentrations with a gene-dosage effect suggests the existence of a link between the AD risk factor APOE4 and CNS cholesterol metabolism. The elevation of CSF 24S-hydroxycholesterol appears to occur early in the disease process, since patients with mild cognitive impairment had also increased CSF concentrations of this compound. We believe that the CSF concentration of 24S-hydroxycholesterol is altered in AD-related neurodegeneration and thus, CSF 24S-hydroxycholesterol may be a marker for monitoring the onset and progression of the disease.

A genetic variation of the inflammatory cytokine interleukin‐6 delays the initial onset and reduces the risk for sporadic Alzheimer's disease

Local inflammatory processes surrounding the amyloid plaques contribute to the progression and acceleration of the Alzheimers disease (AD)–related neurodegeneration. Interleukin‐6 (IL‐6) is an inflammatory cytokine with possible involvement in the local immune response occurring in the central nervous system of AD patients. We tested the hypothesis as to whether a genetic polymorphism of the IL‐6 gene (IL‐6) modifies the age at onset and risk for sporadic AD. Our results support an association of the C allele of the IL‐6 genotype with a delayed initial onset and reduced disease risk and indicate that genetically determined alterations of the immune response may modify the course of AD. Ann Neurol 1999;45:666–668

Plasma 24s-hydroxycholesterol: a peripheral indicator of neuronal degeneration and potential state marker for Alzheimer's disease

The conversion of brain cholesterol into 24S-hydroxycholesterol and its subsequent release into the periphery is probably an important step for the maintenance of brain cholesterol homeostasis. Recent findings suggest that plasma 24S-hydro-xycholesterol may be elevated in Alzheimers disease (AD) and vascular dementia at least at some stage of the disease, suggesting increased brain cholesterol turnover during neuro-degeneration. We investigated whether plasma 24S-hydroxy-cholesterol concentrations depend on the severity of AD and on the apolipoprotein E (apoE) genotype. Severity of AD and inheritance of the apoE4 allele were independently associated with reduced plasma 24S-hydroxycholesterol/cholesterol ratios. The results suggest that the decrease of plasma 24S-hydroxycholesterol/cholesterol in severely affected AD patients is a peripheral marker for loss of cholesterol 24S-hydroxylase in the CNS. Inheritance of the apoE4 allele may be associated with increased apoE-mediated transport of brain cholesterol to the periphery or with decreased activity of the 24S-hydroxylase. Longitudinal studies will assess the validity of the ratio plasma 24S-hydroxycholesterol/cholesterol as a state marker for AD.

A genetic variation of cathepsin D is a major risk factor for Alzheimer's disease

Cathepsin D (catD) is an intracellular acid protease possibly involved in Alzheimers disease (AD)‐related neurodegeneration through cleavage of amyloid precursor protein into amyloidogenic components. We studied whether an exonic polymorphism of the catD gene (C → T [Ala → Val] transition at position 224), which possibly influences pro‐catD secretion and intracellular maturation of the enzyme, was associated with the risk for the development of AD in 127 demented patients and 184 controls. The catD*T allele was significantly overrepresented in demented patients (11.8%) compared with nondemented controls (4.9%). Carriers of the catD*T allele had a 3.1‐fold increased risk for developing AD than noncarriers. Carriers of the apolipoprotein E (ApoE) ε4 allele (ApoE*4) had a 3.9‐fold increased risk than non‐carriers. The adjusted odds ratio for subjects with the ApoE*4 and the catD*T allele was 19.0 compared with subjects with neither of these two alleles. Our data confirm the results of a recently performed pilot study in an independent sample and suggest that the catD genotype is strongly associated with the risk for AD. Ann Neurol 2000;47:399–403

Genetic polymorphism of cathepsin D is strongly associated with the risk for developing sporadic Alzheimer's disease

The beta amyloid peptide derives from its precursor protein via proteolytic cleavage of yet unidentified proteases (beta- and gamma-secretases). Cathepsin D is an intracellular protease with in-vitro beta-secretase-like features. An exonic polymorphism of the cathepsin D gene (alanine to valine transition at position 224, exon 2) has been associated with altered enzyme function. We tested the hypothesis that this polymorphism is associated with an increased risk for Alzheimers disease in 102 demented patients, 191 healthy subjects, and 160 depressed patients. There was a highly significant overrepresentation of the cathepsin D*T allele in demented patients (14.2%) compared to non-demented controls (6.7%, P = 0.0012). Carriers of the cathepsin D*T allele had a 2.4-fold increased risk for developing AD than non-carriers. Carriers of the apolipoprotein E epsilon 4 allele had a 4.1 -fold increased risk than non-carriers. The odds ratio for subjects with the apolipoprotein E epsilon 4 and the cathepsin D*T allele was 5.9. Our data suggest that the cathepsin D genotype is strongly associated with the risk for Alzheimers disease.

Early-Onset and Late-Onset Depression Are Independent of the Genetic Polymorphism of Apolipoprotein E

The recently shown association between apolipoprotein E (APOE) genotype and depressive illness has been challenged by subsequent studies. However, controversial results may derive from the different diagnostic criteria used for depression and from the small numbers of depressed patients included in the studies. We examined the association between depression and the genetic polymorphism of APOE in a large sample of depressed patients, Alzheimer’s disease (AD) patients, and healthy controls following clear definitions for late-life depression. The cumulative incidence of depression depending on the age at onset of the first episode was examined by survival analysis. Our data do not disconfirm the hypothesis of depression sharing some common pathophysiologic features with AD, however, it seems very unlikely that the APOE genotype will elucidate the assumed common mechanisms.

Polymorphisms of the gene encoding the inflammatory cytokine interleukin-6 determine the magnitude of the increase in soluble interleukin-6 receptor levels in Alzheimer's disease

Abstract. Interleukin-6 (IL-6) is a multifunctional cytokine involved in the pathogenesis of Alzheimers disease (AD). The effects of IL-6 are mediated through a specific receptor complex made up of a ligand binding glycoprotein (gp80 or IL-6R) and a signal transducing glycoprotein (gp130). Conflicting results have been reported concerning altered IL-6 or soluble IL-6R (sIL-6R) levels in serum and CSF in AD. This study investigated whether genetic heterogeneity determines the magnitude of the difference in IL-6 and sIL-6R levels in AD. Fifty-eight AD patients and 25 control subjects were included. Plasma and CSF IL-6 and sIL-6R levels were measured and the IL-6 variable number of number repeats (IL-6vntr) and IL-6 promoter (IL-6prom) genotypes were determined. sIL-6R levels in plasma and CSF were higher in AD patients than in control subjects. This elevation was striking among non-carriers of the IL-6vntr*C allele and among subjects homozygous for the IL-6prom*C allele whereas no difference in plasma and CSF sIL-6R levels was observed among carriers of the IL-6vntr*C allele and among subjects with the IL-6prom*CG and IL-6prom*GG genotypes. We conclude that plasma and CSF levels of sIL-6R are significantly increased in AD patients and that the magnitude of increase is determined by the IL-6 genotype.

Confirmation of the association between bleomycin hydrolase genotype and Alzheimer's disease

Bleomycin hydrolase (BH), a cysteine protease from the papain superfamily, is considered to be a candidate for the β-secretase, which is presumably involved in the production of β-amyloid peptide. The G/G genotype of BH was identified as a significant risk factor for the development of Alzheimers disease (AD) in subjects not carrying the apolipoprotein E ε4 allele (apoE-ε4). However, this finding was recently challenged. We studied this polymorphism in a homogenous sample of German AD patients and controls. The over-representation of the G/G genotype in AD patients could be confirmed, however it was more pronounced in apoE-ε4 carriers. Additional studies should be undertaken to increase the confidence that the BH polymorphism is associated with AD and to explore the relationship between BH and apoE.

Steady state concentrations of clomipramine and its major metabolite desmethylclomipramine in rat brain and serum after oral administration of clomipramine

Twenty male Sprague-Dawley rats received five oral doses of clomipramine 20 mg/kg at 4-h intervals. The animals were decapitated 1, 2, 3, 5 and 12 h after the last dose for determination of clomipramine and desmethylclomipramine in serum and frontal cerebral cortex. Time dependent concentrations of clomipramine and desmethylclomipramine paralleled in serum and brain. Half-lives were similar in serum and brain with 7.8 versus 6.2 h and 5.5 versus 5.0 h for clomipramine and desmethylclomipramine, respectively. Absolute concentrations, however, were markedly higher in brain than in serum - 12.5 fold for clomipramine and 7.4 fold for desmethylclomipramine. The data indicate that serum and brain concentrations of clomipramine and its demethylated metabolite are rapidly exchanged between blood and brain. Assuming that blood and brain kinetics in man and rat are comparable, it is concluded that monitoring blood concentrations of clomipramine and desmethylclomipramine is a useful way to evaluate brain concentrations.

Allelic association between the D10S1423 marker and Alzheimer's disease in a German population

Recently a full genome survey detected an allelic association between Alzheimers disease (AD) and the D10S1423 marker on chromosome 10p12-14 (40 cM from the telomere). In this study we examined the D10S1423 marker in an ethnically homogeneous German population of 80 AD patients and two groups of controls, 168 healthy subjects and 149 depressed patients. The 234-bp allele of the D10S1423 marker showed a significant association with AD (P = 0.033). In conclusion, our results support that the D10S1423 marker is associated with an increased AD risk and provides further evidence for an AD susceptibility locus on chromosome 10.