Biomarkers of probable Alzheimer’s disease

Jure Fabjan (2018) Biomarkers of probable Alzheimer’s disease. MSc thesis.

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Abstract

Dementia is becoming ever-bigger problem as the population ages. 70 % of 47 million cases of dementia is attributed to Alzheimer’s disease. Early onset Alzheimer’s disease accounts for about 1 % to 5 % of all cases. This type of the disease is typicaly inherited in a mendelian fashion. The genetic component is much weaker in cases of late onset Alzheimer’s disease. People with mild cognitive impairment usually have troubles with cognition but these troubles do not affect their everyday life. People with this diagnosis carry a higher risk for the development of Alzheimer’s disease. If a person notices a decline in cognitive abilities, although the clinical tests do not show it, a person is diagnosed with subjective cognitive complaint. The risk for developing dementia that people with subjective cognitive complaint carry is still not known. More than 20 genes were already found to probably affect the risk of developing late onset Alzheimer’s disease. The apolipoprotein E (ApoE) gene had the strongest association. From the three alleles, ApoE E4 is associated with higher risk for Alzheimer’s disease, with 3 times higher risk in heterozygotes ApoE E4/- and 8 to 12 times higher risk in homozygotes ApoE E4/E4, compared to homozygotes ApoE E3/E3. In our project, we formed two goals: I) assigning the frequencies of ApoE alleles in Slovene population of patients with Alzheimer’s disease and II) finding new potential biomarkers in preexistent databases, which could be used for the diagnosis of possible Alzheimer’s disease. In the project, we included 113 patients, treated in the clinic of Center for cognitive impairments at the Department of neurology, University medical centre, Ljubljana. We took the blood from the patients and extracted from it the DNA. We also acquired their clinical data. Because of the low number of patients, diagnosed with Alzheimer’s disease, we performed the analysis on patients, diagnosed with subjective cognitive complaint, mild cognitive impairment and cognitive decline. The three groups differed in age and results of cognitive tests, but not in the frequencies of ApoE genotypes and alleles. The genotyped population differed with statistical significance from general European population in the frequencies of ApoE genotypes. To find new potential biomarkers, we collected preexisting genome wide association studies, performed on populations of Alzheimer’s disease patients. We analyzed the results of these studies using an online tool Integratomics (http://genepark.mf.uni-lj.si/integratomics/home). The 13 regions, which shared the highest score, contained 19 genes. These were further verified in independent literature data. Of the 19 genes, only seven were described beforehand to have an association with the risk of Alzheimer’s disease. Because of a low number of participants, it was not possible to assess the frequencies of ApoE alleles in patients with Alzheimer’s disease. In the future, we would need to repeat the analysis with a higher number of participants. The integratomic analysis resulted in a list of candidate genes, which should be verified in a follow up, consisting of a genotyping of patients with Alzheimer’s disease and healthy controls. It would also be interesting to include additional non-genome wide association studies in the analysis.

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