Genetic Study of Alzheimer's Disease in Saudi Population

A global view of the genetic basis of Alzheimer disease

The risk of Alzheimer disease (AD) increases with age, family history and informative genetic variants. Sadly, there is still no cure or means of prevention. As in other complex diseases, uncovering genetic causes of AD could identify underlying pathological mechanisms and lead to potential treatments. Rare, autosomal dominant forms of AD occur in middle age as a result of highly penetrant genetic mutations, but the most common form of AD occurs later in life. Large-scale, genome-wide analyses indicate that 70 or more genes or loci contribute to AD. One of the major factors limiting progress is that most genetic data have been obtained from non-Hispanic white individuals in Europe and North America, preventing the development of personalized approaches to AD in individuals of other ethnicities. Fortunately, emerging genetic data from other regions - including Africa, Asia, India and South America - are now providing information on the disease from a broader range of ethnicities. Here, we summarize the current knowledge on AD genetics in populations across the world. We predominantly focus on replicated genetic discoveries but also include studies in ethnic groups where replication might not be feasible. We attempt to identify gaps that need to be addressed to achieve a complete picture of the genetic and molecular factors that drive AD in individuals across the globe.

Double Mutations in a Patient with Early-Onset Alzheimer's Disease in Korea: An APP Val551Met and a PSEN2 His169Asn

The etiology of early-onset Alzheimer's disease (EOAD) is associated with alterations in the production of amyloid beta (Aβ) species caused by mutations in the APP, PSEN1, and PSEN2 genes. Mutations affect intra- or inter-molecular interactions and processes between the γ-secretase complex and amyloid precursor protein (APP), leading to the aberrant sequential cleavage of Aβ species. A 64-year-old woman presented with progressive memory decline, mild right hippocampal atrophy, and a family history of Alzheimer's dementia (AD). Whole exome sequencing was performed to evaluate AD-related gene mutations, which were verified by Sanger sequencing. A mutation-caused structural alteration of APP was predicted using in silico prediction programs. Two AD-related mutations, in APP (rs761339914; c.G1651A; p.V551M) and PSEN2 (rs533813519; c.C505A; p.H169N), were identified. The APP Val551Met mutation in the E2 domain may influence APP homodimerization through changes in intramolecular interactions between adjacent amino acids, altering Aβ production. The second mutation was PSEN2 His169Asn mutation, which was previously reported in five EOAD patients from Korea and China, with a relatively high frequency in the East Asian population. According to a previous report, the presenilin 2 protein was predicted to result in a major helical torsion by PSEN2 His169Asn mutation. Notably, the co-existence of APP Val551Met and PSEN2 His169Asn may induce a synergistic effect by both mutations. Future functional studies are needed to clarify the pathological effects of these double mutations.

Finding memo: versatile interactions of the VPS10p-Domain receptors in Alzheimer’s disease

The family of VPS10p-Domain (D) receptors comprises five members named SorLA, Sortilin, SorCS1, SorCS2 and SorCS3. While their physiological roles remain incompletely resolved, they have been recognized for their signaling engagements and trafficking abilities, navigating a number of molecules between endosome, Golgi compartments, and the cell surface. Strikingly, recent studies connected all the VPS10p-D receptors to Alzheimer’s disease (AD) development. In addition, they have been also associated with diseases comorbid with AD such as diabetes mellitus and major depressive disorder. This systematic review elaborates on genetic, functional, and mechanistic insights into how dysfunction in VPS10p-D receptors may contribute to AD etiology, AD onset diversity, and AD comorbidities. Starting with their functions in controlling cellular trafficking of amyloid precursor protein and the metabolism of the amyloid beta peptide, we present and exemplify how these receptors, despite being structurally similar, regulate various and distinct cellular events involved in AD. This includes a plethora of signaling crosstalks that impact on neuronal survival, neuronal wiring, neuronal polarity, and synaptic plasticity. Signaling activities of the VPS10p-D receptors are especially linked, but not limited to, the regulation of neuronal fitness and apoptosis via their physical interaction with pro- and mature neurotrophins and their receptors. By compiling the functional versatility of VPS10p-D receptors and their interactions with AD-related pathways, we aim to further propel the AD research towards VPS10p-D receptor family, knowledge that may lead to new diagnostic markers and therapeutic strategies for AD patients.

Genetic Variants Associated With Alzheimer Disease in the 22 Arab Countries: A Systematic Review

BACKGROUND AND AIMS

Alzheimer disease (AD) is a progressive and complex neurodegenerative disease. Approximately 70% of AD risk is attributed to genetic risk factors, including variants in amyloid precursor protein (APP), presenilin 1 (PSEN1), and presenilin 2 (PSEN2) genes. Several studies have revealed a considerable number of candidate loci and genes for AD among different ethnic populations. However, the outcomes of these studies have been inconsistent. In this study, we aimed to investigate the spectrum of variants that are associated with the onset and development of AD among 22 Arab countries.

METHODOLOGY

We systematically searched 4 literature databases (Science Direct, Scopus, PubMed, and Web of Science) from the date of inception until July 2020 using various search terms to obtain all the reported genetic data on Arab AD cases.

RESULTS

In total, 18 studies were included, comprising a total of 2173 individuals, of whom 888 were clinically diagnosed AD patients and were genetically tested for genes and variants associated with AD. A total of 27 variants in 8 genes were found to be associated with AD. Of these variants, 17 were unique to the Arab population and 10 were shared with other ethnic groups.

CONCLUSIONS

There is a dearth of studies on the genetics of AD in the Arab world. There seems to be distinctive genetic and clinical susceptibility profiles for Arab patients with AD.

A Comprehensive Analysis of Unique and Recurrent Copy Number Variations in Alzheimer's Disease and its Related Disorders

BACKGROUND

Copy number variations (CNVs) play an important role in the genetic etiology of various neurological disorders, including Alzheimer's disease (AD). Type 2 diabetes mellitus (T2DM) and major depressive disorder (MDD) were shown to have share mechanisms and signaling pathways with AD.

OBJECTIVE

We aimed to assess CNVs regions that may harbor genes contributing to AD, T2DM, and MDD in 67 Saudi familial and sporadic AD patients, with no alterations in the known genes of AD and genotyped previously for APOE.

METHODS

DNA was analyzed using the CytoScan-HD array. Two layers of filtering criteria were applied. All the identified CNVs were checked in the Database of Genomic Variants (DGV).

RESULTS

A total of 1086 CNVs (565 gains and 521 losses) were identified in our study. We found 73 CNVs harboring genes that may be associated with AD, T2DM or MDD. Nineteen CNVs were novel. Most importantly, 42 CNVs were unique in our studied cohort existing only in one patient. Two large gains on chromosomes 1 and 13 harbored genes implicated in the studied disorders. We identified CNVs in genes that encode proteins involved in the metabolism of amyloid-β peptide (AGRN, APBA2, CR1, CR2, IGF2R, KIAA0125, MBP, RER1, RTN4R, VDR and WISPI) or Tau proteins (CACNAIC, CELF2, DUSP22, HTRA1 and SLC2A14).

CONCLUSION

The present work provided information on the presence of CNVs related to AD, T2DM, and MDD in Saudi Alzheimer's patients.

A novel variant c.3706C>T p.(Avg 1236Cys) in the ABCA7 gene in a Saudi patient with susceptibility to Alzheimer's disease 9

Alzheimer's disease (AD) is the most common cause of dementia with around 50 million people suffering from this disease worldwide. Mutations in the ATP-binding cassette sub-family A member 7 (ABCA7) have been reported to cause susceptibility to AD 9 (OMIM #608907). In this study, we report a novel variant in ABCA7 in a Saudi patient with susceptibility to AD 9 and a strong family history of neurodegenerative disorders, which may be explained by the same variant. We studied a single 57-year-old female patient with typical symptoms of AD supported by MRI findings from a Saudi family with a positive history of a similar disease in multiple individuals. The case study was conducted in King Abdulaziz Medical City in Jeddah, Saudi Arabia. Whole-exome sequencing identified the novel heterozygous variant c.3706C>T p.(Avg 1236Cys) in the ABCA7 gene, which leads to an amino acid exchange. Furthermore, bioinformatics in silico programs predict a pathogenic effect for this variant. To the best of our knowledge, the variant has not been described in the literature so far as evidenced by a thorough literature review using multiple databases such as Ovid, Medline, EMBASE, ProQuest, Science Direct, Google Scholar, and PubMed. In this article, we reported a middle-aged Saudi woman with a novel variant in ABCA7 who had clinical features of both AD and Parkinson's disease. Given the reported function of this gene, it is most likely that it is etiological and pathological because of the presenting complex neurological disease due to decreased clearance of β-amyloid and α-Synuclein. We illustrate the importance of this interesting gene that could be implicated in several neurodegenerative disorders.

Amyloidosis causes downregulation of SorLA, SorCS1 and SorCS3 expression in mice

Accumulation of β-amyloid peptide (Aβ) is regarded as a primary cause of Alzheimer's disease (AD). Aβ is derived by sequential cleavage of the amyloid precursor protein (APP). Alterations in the subcellular targeting of APP are thought to affect the degree of Aβ production. Sorting receptors, such as SorLA, convey subcellular targeting of APP. Dysfunction of SorLA, and likely of the related receptors SorCS1 and SorCS3, cause AD. Nevertheless, disease progression could also provoke altered expression of the receptors. Here, we assessed if Aβ plaque formation promotes altered expression of SorLA, SorCS1 and SorCS3. We analyzed transcript levels during aging and after amyloidosis in brain areas characterized by early amyloid plaque formation in an AD mouse model (APPPS1) and wild types. We observed stable expression levels during aging (1-12 months). After plaque formation, SorCS1 and SorLA expression were markedly reduced in the frontal cerebral cortex and to a minor extent in the hippocampus, whereas SorCS3 expression was solely reduced in the frontal cerebral cortex. Our results indicate that disease progression, associated with Aβ accumulation, can negatively regulate expression of the receptors.

Binding between Prion Protein and Aβ Oligomers Contributes to the Pathogenesis of Alzheimer's Disease

A plethora of evidence suggests that protein misfolding and aggregation are underlying mechanisms of various neurodegenerative diseases, such as prion diseases and Alzheimer's disease (AD). Like prion diseases, AD has been considered as an infectious disease in the past decades as it shows strain specificity and transmission potential. Although it remains elusive how protein aggregation leads to AD, it is becoming clear that cellular prion protein (PrP^C) plays an important role in AD pathogenesis. Here, we briefly reviewed AD pathogenesis and focused on recent progresses how PrP^C contributed to AD development. In addition, we proposed a potential mechanism to explain why infectious agents, such as viruses, conduce AD pathogenesis. Microbe infections cause Aβ deposition and upregulation of PrP^C, which lead to high affinity binding between Aβ oligomers and PrP^C. The interaction between PrP^C and Aβ oligomers in turn activates the Fyn signaling cascade, resulting in neuron death in the central nervous system (CNS). Thus, silencing PrP^C expression may turn out be an effective treatment for PrP^C dependent AD.