Alzheimer’s disease (AD) can be divided into early-onset AD (EOAD) and late-onset AD (LOAD) at the age of 65, with LOAD accounting for about 94% of cases. （AD can also be divided into familial AD (FAD) and sporadic AD (SAD). FAD is predominant in EOAD, while SAD is predominant in LOAD.
Mutation studies of FAD
Early-onset FAD is mostly autosomal dominant, characterized by low incidence, early onset, rapid progression and severe consequences. The study of AD has entered the new field of molecular genetics.
About 50% of FAD is caused by mutations in APP and three genes, presenilins 1 (PS1) and 2 (PS2), located on chromosome 14. Among them, familial AD caused by APP mutation accounts for 10%-15%, PS1 mutation accounts for 70%-85%, and PS2 mutation is less than 5%. Currently there are more than 231 pathogenic mutations in these 3 genes, including 185 in PS1, 13 in PS2 and 33 in APP, and new mutations are reported in the AD mutation database every year.
Mutations in APP gene
The APP protein encoded by APP is processed and hydrolyzed to produce β amyloid (Aβ). Under pathological conditions, APP is catabolized by β-secretase and γ-secretase to produce Aβ40 and Aβ42, of which Aβ42 is closely associated with Aβ deposition and neuronal degeneration in the brain of AD patients.
Most mutations in the APP gene are located in the secretase cleavage sites of exons 16 and 17 or in the APP transmembrane region, and mutations can be screened by sequence analysis of exons 16 and 17 or sequencing of the entire coding region.
APP mutations alter APP processing, leading to the production of Aβ42 with neurotoxic effects and reduced Aβ40. This process can selectively contribute to apoptosis or death of some neuronal cells by triggering multiple pathological mechanisms, ultimately leading to the development of AD. For example, the missense
APP Swedish mutations (APPswe: APPK670N and M671L), London mutation (APPlon: APP V717I) and L723P found in Australian families can lead to increased Aβ42 levels and development of AD.
PS1 gene mutations
The PS1 and PS2 genes encode PS proteins that are important components of γ-secretase and play an important role in the production of Aβ.
PS1 mutations are the most important cause of early-onset FAD. PS1 mutations cause deletion of the hydrophilic loop domain of the encoded protein, resulting in a change in its conformation, which in turn affects the activity of γ-secretase and increases Aβ42 production.
Most PS1 mutations are missense mutations and can be detected by sequencing of the coding region and associated intron regions. Jia Jianping’s group at Xuanwu Hospital of Capital Medical University identified two new PS1 mutations (Val97Leu and Ala136Gly missense mutations) in 218 members of seven Han Chinese families with AD, and established a Val97Leu transgenic mouse model to initially confirm its pathogenicity.
PS2 gene mutation
PS2 protein has a high degree of homology (80.5%) with PS1, which is also involved in the cleavage of Aβ. PS2 gene mutations have a lower epistasis compared to PS1 and therefore may be modified by other genes or influenced by environmental factors.
PS2 can act on the hydrolysis process of APP through its effect on C-terminal peptide hydrolase, causing increased production of aggregated Aβ42 and deposition, forming neuroinflammatory spots, and increasing intracellular calcium, exacerbating oxygen radical production and promoting a decrease in mitochondrial membrane potential, thus causing apoptosis.
PS2 mutations have been identified in only a few families, whereas PS1 mutations have been identified in several hundred families, and PS2 gene mutations are often detected by sequencing the coding region to detect missense mutations.
New FAD-causing genes to be discovered
APP, PS1 and PS2 mutations were not detected in about half of the FAD lines, suggesting that many new pathogenic genes remain to be discovered. Traditional linkage analysis and in situ cloning methods are difficult to find new pathogenic genes, and with the high mutation rate in exons of genetic diseases, whole-exome sequencing is far easier, more economical and efficient than performing whole-genome sequencing.
Whole-exome sequencing refers to a genomic analysis method that uses sequence capture technology to capture and enrich DNA from exonic regions of the whole genome and then perform high-throughput sequencing of all exons, which has been gradually and widely used to search for causative or susceptibility genes in Mendelian inherited diseases and other complex diseases.
Therefore, the combined application of whole-exome sequencing and linkage analysis methods to study AD families may lead to the identification of new FAD causative genes and allow further investigation of their pathogenic mechanisms.
Study of genetic variants in SAD
The etiology of SAD is complex, involving genetic, environmental, metabolic and viral factors. The apolipoprotein E (APOE) gene is currently the only recognized gene for SAD susceptibility.
APOE gene polymorphisms
APOE includes three alleles ε2, ε3, and ε4, of which ε2 plays a protective role and reduces the risk of AD and delays the age of onset; ε4 is a risk factor for AD and has a dose-dependent relationship with the incidence of AD.
APOEε4 allele is involved in regulating Aβ production and affects the clearance of Aβ by astrocytes and neurons, promoting amyloid formation and its deposition in the brain, and further leading to neuroinflammatory patches and neuronal death.
At the same time, APOE4 protein does not effectively maintain tau protein-microtubule protein connections, leading to abnormal phosphorylation of tau protein, which reduces the ability of microtubule assembly. The ensuing impairment of axoplasmic flow results in the accumulation of transmitters and some neuronal components that are not rapidly degraded within the affected neurons, leading to reduced neurological function and loss until neuronal destruction.
Other susceptibility genes for SAD
The APOEε4 allele explains less than 50% of the genetic variation in SAD, suggesting that there are other genetic factors involved in the pathogenesis of AD.
Jia’s group screened multiple polymorphic loci of more than 20 candidate genes involved in multiple aspects of AD pathogenesis, especially for the production, degradation, clearance and deposition of Aβ protein and tau protein metabolism, including promoter regions, coding regions and some non-coding regions. The results identified APP, PS1, PS2, β-secretase ( BACE1 ), progerin enhancer 2 ( PEN-2 ), propharyngeal defective protein 1 ( APH-1 ), Nicastrin, insulin degrading enzyme ( IDE ), low density lipoprotein receptor related protein ( LRP ) and α2M as genetic susceptibility genes for SAD in Chinese Han population; and also identified related genetic More than 20 genetic susceptibility loci were identified, and more than 10 promoter region variant loci were found to be involved in the pathogenesis of AD by altering the transcriptional activity of genes, leading to abnormal Aβ and tau metabolism.
SAD is a complex genetic disease, and it is likely that the combined action of multiple loci on different chromosomes leads to the development of the disease. One or several variants may have only weak genetic effects, and therefore, single-gene association study methods are difficult to detect all common variants. This is the reason why the vast majority of genetic polymorphic loci are not validated in different populations. Therefore, there is still an urgent need for further in-depth genetic studies of SAD, which is important to fully reveal the pathogenesis of AD and thus effectively prevent and treat SAD.
GWAS research
Advances in science and technology have led to unprecedented development of methods to find susceptibility genes for complex genetic diseases. In recent years, several new genetic loci associated with AD have been identified through candidate genes and genome-wide scans. The emergence of genome-wide association studies (GWAS) has made it possible for researchers to search for susceptibility genes on a genome-wide scale, bringing a new perspective to the search for the genetic mechanism of SAD.
Researchers around the world have applied GWAS to discover multiple AD-associated susceptibility regions that had not been identified by candidate gene association studies, and several new AD susceptibility genes have surfaced. Among them, a 2011 GWAS of US and European populations identified several polymorphic loci in MS4A4, CD2AP, CD33 and EPHA1 associated with AD onset. 2007 GWAS study reported 6 loci in GAB2 gene associated with AD onset, while a 2008 GWAS of Canadian and UK populations showed that 2 loci in A 2008 GWAS of Canadian and British populations showed that two polymorphic loci and two loci of unknown function in the GOLPH2 gene were associated with the development of SAD. Several other genome-wide association studies in US, UK, Canadian, Belgian, Finnish, Italian and Spanish populations reported multiple polymorphic loci in CLU, PICALM, LRAT, PCDH11X, CHRNA7, TNK1, GALP, PCK1, PGBD1, LMNA and TRAK2 to be associated with the development of SAD. .
These findings have brought breakthroughs in the study of AD susceptibility genes, but because of the strong genetic and phenotypic heterogeneity of SAD in different ethnic groups and regions, as well as the differences in the frequency distribution of each allele of single nucleotide polymorphism (SNP) loci in different populations, population association studies for the above GWAS identified loci associated with SAD have been conducted in different geographical In recent years, Prof. Tan Lan in Qingdao, China, has conducted population-based association studies on SAD-related loci. In recent years, Professor Tanlan Qingdao has validated the positive loci reported by foreign GWAS in Chinese populations, which is of great significance for validating the reliability of GWAS and discovering the susceptibility genes for SAD. At present, there is no report of AD-related GWAS in Asia, but Prof. Jia Jianping’s group has established a large clinical resource base of AD patients, and has started to conduct AD-related GWAS in Chinese Han population, with the aim of discovering the genetic pattern of SAD and identifying AD gene variants in Chinese population, which will have important implications.