A novel presenilin 2 mutation (V393M) in early-onset dementia with profound language impairment

Study of Alzheimer's disease- and frontotemporal dementia-associated genes in the Cretan Aging Cohort

Using exome sequencing, we analyzed 196 participants of the Cretan Aging Cohort (CAC; 95 with Alzheimer's disease [AD], 20 with mild cognitive impairment [MCI], and 81 cognitively normal controls). The APOE ε4 allele was more common in AD patients (23.2%) than in controls (7.4%; p < 0.01) and the PSEN2 p.Arg29His and p.Cys391Arg variants were found in 3 AD and 1 MCI patient, respectively. Also, we found the frontotemporal dementia (FTD)-associated TARDBP gene p.Ile383Val variant in 2 elderly patients diagnosed with AD and in 2 patients, non CAC members, with the amyotrophic lateral sclerosis/FTD phenotype. Furthermore, the p.Ser498Ala variant in the positively selected GLUD2 gene was less frequent in AD patients (2.11%) than in controls (16%; p < 0.01), suggesting a possible protective effect. While the same trend was found in another local replication cohort (n = 406) and in section of the ADNI cohort (n = 808), this finding did not reach statistical significance and therefore it should be considered preliminary. Our results attest to the value of genetic testing to study aged adults with AD phenotype.

The Clinical and Neuropathological Features of Sporadic (Late-Onset) and Genetic Forms of Alzheimer's Disease

The purpose of this review is to compare and highlight the clinical and pathological aspects of genetic versus acquired Alzheimer's disease: Down syndrome-associated Alzheimer's disease in (DSAD) and Autosomal Dominant Alzheimer's disease (ADAD) are compared with the late-onset form of the disease (LOAD). DSAD and ADAD present in a younger population and are more likely to manifest with non-amnestic (such as dysexecutive function features) in the prodromal phase or neurological features (such as seizures and paralysis) especially in ADAD. The very large variety of mutations associated with ADAD explains the wider range of phenotypes. In the LOAD, age-associated comorbidities explain many of the phenotypic differences.

Exploring the Role of PSEN Mutations in the Pathogenesis of Alzheimer's Disease

Alzheimer's disease (AD) is the most common cause of dementia. Mutations of presenilin (PSEN) genes that encode presenilin proteins have been found as the vital causal factors for early-onset familial AD (FAD). AD pathological features such as memory loss, synaptic dysfunction, and formation of plaques have been successfully mimicked in the transgenic mouse models that coexpress FAD-related presenilin and amyloid precursor protein (APP) variants. γ-Secretase (GS) is an enzyme that plays roles in catalyzing intramembranous APP proteolysis to release pathogenic amyloid beta (Aβ). It has been found that presenilins can play a role as the GS's catalytic subunit. FAD-related mutations in presenilins can modify the site of GS cleavage in a way that can elevate the production of longer and highly fibrillogenic Aβ. Presenilins can interact with β-catenin to generate presenilin complexes. Aforesaid interactions have also been studied to observe the mutational and physiological activities in the catenin signal transduction pathway. Along with APP, GS can catalyze intramembrane proteolysis of various substrates that play a vital role in synaptic function. PSEN mutations can cause FAD with autosomal dominant inheritance and early onset of the disease. In this article, we have reviewed the current progress in the analysis of PSENs and the correlation of PSEN mutations and AD pathogenesis.

A Review of the Familial Alzheimer's Disease Locus PRESENILIN 2 and Its Relationship to PRESENILIN 1

PRESENILIN 1 (PSEN1) and PRESENILIN 2 (PSEN2) genes are loci for mutations causing familial Alzheimer's disease (fAD). However, the function of these genes and how they contribute to fAD pathogenesis has not been fully determined. This review provides a summary of the overlapping and independent functions of the PRESENILINS with a focus on the lesser studied PSEN2. As a core component of the γ-secretase complex, the PSEN2 protein is involved in many γ-secretase-related physiological activities, including innate immunity, Notch signaling, autophagy, and mitochondrial function. These physiological activities have all been associated with AD progression, indicating that PSEN2 plays a particular role in AD pathogenesis.

Genetic Markers of Alzheimer's Disease

Alzheimer's disease is a complex and heterogeneous, severe neurodegenerative disorder and the predominant form of dementia, characterized by cognitive disturbances, behavioral and psychotic symptoms, progressive cognitive decline, disorientation, behavioral changes, and death. Genetic background of Alzheimer's disease differs between early-onset familial Alzheimer's disease, other cases of early-onset Alzheimer's disease, and late-onset Alzheimer's disease. Rare cases of early-onset familial Alzheimer's diseases are caused by high-penetrant mutations in genes coding for amyloid precursor protein, presenilin 1, and presenilin 2. Late-onset Alzheimer's disease is multifactorial and associated with many different genetic risk loci (>20), with the apolipoprotein E ε4 allele being a major genetic risk factor for late-onset Alzheimer's disease. Genetic and genomic studies offer insight into many additional genetic risk loci involved in the genetically complex nature of late-onset Alzheimer's disease. This review highlights the contributions of individual loci to the pathogenesis of Alzheimer's disease and suggests that their exact contribution is still not clear. Therefore, the use of genetic markers of Alzheimer's disease, for monitoring development, time course, treatment response, and prognosis of Alzheimer's disease, is still far away from the clinical application, because the contribution of genetic variations to the relative risk of developing Alzheimer's disease is limited. In the light of prediction and prevention of Alzheimer's disease, a novel approach could be found in the form of additive genetic risk scores, which combine additive effects of numerous susceptibility loci.

Neurological manifestations of autosomal dominant familial Alzheimer's disease: a comparison of the published literature with the Dominantly Inherited Alzheimer Network observational study (DIAN-OBS)

BACKGROUND

Autosomal dominant familial Alzheimer's disease (ADAD) is a rare disorder with non-amnestic neurological symptoms in some clinical presentations. We aimed to compile and compare data from symptomatic participants in the Dominantly Inherited Alzheimer Network observational study (DIAN-OBS) with those reported in the literature to estimate the prevalences of non-amnestic neurological symptoms in participants with ADAD.

METHODS

We prospectively collected data from the DIAN-OBS database, which recruited participants from study centres in the USA, Europe, and Australia, between Feb 29, 2008, and July 1, 2014. We also did a systematic review of publications to extract individual-level clinical data for symptomatic participants with ADAD. We used data for age of onset (from first report of cognitive decline), disease course from onset to death, and the presence of 13 neurological findings that have been reported in association with ADAD. Using multivariable linear regression, we investigated the prevalences of various non-amnestic neurological symptoms and the contributions of age of onset and specific mutation type on symptoms.

FINDINGS

The DIAN-OBS dataset included 107 individuals with detailed clinical data (forming the DIAN-OBS cohort). Our systematic review yielded 188 publications reporting on 1228 symptomatic individuals, with detailed neurological examination descriptions available for 753 individuals (forming the published data cohort). The most prevalent non-amnestic cognitive manifestations in participants in the DIAN-OBS cohort were those typical of mild to moderate Alzheimer's disease, including visual agnosia (55·1%, 95% CI 45·7-64·6), aphasia (57·9%, 48·6-67·3), and behavioural changes (61·7%, 51·5-70·0). Non-amnestic cognitive manifestations were less prevalent in the published data cohort (eg, visual agnosia [5·6%, 3·9-7·2], aphasia [23·0%, 20·0-26·0], and behavioural changes [31·7%, 28·4-35·1]). Prevalence of non-cognitive neurological manifestations in the DIAN-OBS cohort was low, including myoclonus and spasticity (9·3%, 95% CI 3·8-15·0), and seizures (2·8%, 0·5-5·9) and moderate for parkinsonism (11·2%, 5·3-17·1). By constrast, prevalence was higher in the published data cohort for myoclonus and spasticity (19·4%, 16·6-22·2 and 15·0%, 12·5-17·6, respectively), parkinsonism (12·5%, 10·1-15·0), and seizures (20·3%, 17·4-23·2). In an analysis of the published data cohort, ischaemic stroke was more prevalent at older ages of onset of symptoms of ADAD (odds ratio 1·09 per 1 year increase in age of onset, 95% CI 1·04-1·14, p=0·0003); and motor symptoms were more common at younger age of onset (myoclonus 0·93, 0·90-0·97, p=0·0007; seizures 0·95, 0·92-0·98, p=0·0018; corticobulbar deficits 0·91, 0·86-0·96, p=0·0012; and cerebellar ataxia 0·82, 0·74-0·91, p=0·0002). In the DIAN-OBS cohort, non-cognitive symptoms were more common at more severe stages of disease.

INTERPRETATION

The non-cognitive clinical manifestations of Alzheimer's disease seem to affect a small proportion of participants with mild to moderate ADAD, and are probably influenced by disease severity, environmental, and genetic factors. When evaluating patients with potential ADAD, clinicians should note that cognitive symptoms typical of sporadic Alzheimer's disease are the most consistent finding, with some patients manifesting non-cognitive neurological symptoms. Future work is needed to determine the environmental and genetic factors that cause these neurological symptoms.

FUNDING

National Institutes of Health and German Center for Neurodegenerative Diseases.

Mutations in presenilin 2 and its implications in Alzheimer's disease and other dementia-associated disorders

Alzheimer's disease (AD) is the most common form of dementia. Mutations in the genes encoding presenilin 1 (PSEN1), presenilin 2 (PSEN2), and amyloid precursor protein have been identified as the main genetic causes of familial AD. To date, more than 200 mutations have been described worldwide in PSEN1, which is highly homologous with PSEN2, while mutations in PSEN2 have been rarely reported. We performed a systematic review of studies describing the mutations identified in PSEN2. Most PSEN2 mutations were detected in European and in African populations. Only two were found in Korean populations. Interestingly, PSEN2 mutations appeared not only in AD patients but also in patients with other disorders, including frontotemporal dementia, dementia with Lewy bodies, breast cancer, dilated cardiomyopathy, and Parkinson's disease with dementia. Here, we have summarized the PSEN2 mutations and the potential implications of these mutations in dementia-associated disorders.

Early onset Alzheimer's disease and oxidative stress

Alzheimer's disease (AD) is the most common cause of dementia in elderly adults. It is estimated that 10% of the world's population aged more than 60-65 years could currently be affected by AD, and that in the next 20 years, there could be more than 30 million people affected by this pathology. One of the great challenges in this regard is that AD is not just a scientific problem; it is associated with major psychosocial and ethical dilemmas and has a negative impact on national economies. The neurodegenerative process that occurs in AD involves a specific nervous cell dysfunction, which leads to neuronal death. Mutations in APP, PS1, and PS2 genes are causes for early onset AD. Several animal models have demonstrated that alterations in these proteins are able to induce oxidative damage, which in turn favors the development of AD. This paper provides a review of many, although not all, of the mutations present in patients with familial Alzheimer's disease and the association between some of these mutations with both oxidative damage and the development of the pathology.

Genetics of Alzheimer disease

Alzheimer disease (AD) is the most common causes of neurodegenerative disorder in the elderly individuals. Clinically, patients initially present with short-term memory loss, subsequently followed by executive dysfunction, confusion, agitation, and behavioral disturbances. Three causative genes have been associated with autosomal dominant familial AD (APP, PSEN1, and PSEN2) and 1 genetic risk factor (APOEε4 allele). Identification of these genes has led to a number of animal models that have been useful to study the pathogenesis underlying AD. In this article, we provide an overview of the clinical and genetic features of AD.

Causes versus effects: the increasing complexities of Alzheimer's disease pathogenesis

Amyloid plaques and neurofibrillary tangles are the hallmarks of Alzheimer's disease and have been the focus of disease etiology and pathogenesis. However, in the larger picture of a complex disease, the precise etiology of the lesions per se, as well as the clinical disease, remain to be defined. In this regard, to date no single process has been identified as a useful target and treatment efforts have shown no meaningful progress. Therefore, alternative ideas that may lead to new and effective treatment options are much needed.

Alzheimer's disease phenotypes and genotypes associated with mutations in presenilin 2

Mutations in presenilin 2 are rare causes of early onset familial Alzheimer's disease. Eighteen presenilin 2 mutations have been reported, although not all have been confirmed pathogenic. Much remains to be learned about the range of phenotypes associated with these mutations. We have analysed our unique collection of 146 affected cases in 11 Volga German families, 101 who are likely to have the same N141I mutation in presenilin 2 (54 genotyped confirmed). We have also assessed the detailed neuropathologic findings in 18 autopsies from these families and reviewed the world's literature on other presenilin 2 mutations; presenting a novel mutation that is predicted to lead to a premature truncation codon. Seven presenilin 2 mutations reported in the literature have strong evidence for pathogenicity whereas others may be benign polymorphisms. One hundred and one affected persons, with sufficient historical information from the Volga German pedigrees (N141I mutation), had a mean onset age of 53.7 years+/-7.8 (range 39-75) and mean age at death of 64.2 years+/-9.8 (range 43-88). These figures overlap with and generally fall between the results from the subjects in our centre who have late onset familial Alzheimer's disease or mutations in presenilin 1. Seizures were noted in 20 (30%) of 64 subjects with detailed medical records. Two mutation carriers lived beyond age 80 without developing dementia, representing uncommon examples of decreased penetrance. Two persons had severe amyloid angiopathy and haemorrhagic stroke. Eighteen cases had detailed histopathology available and analysed at our institution. Braak stage was five or six, amyloid angiopathy and neuritic plaques were common and more than 75% had Lewy bodies in the amygdala. TAR DNA-binding protein-43 inclusions were uncommon. In addition, a 58-year-old female with a 2 year course of cognitive decline and no family history of dementia has abnormal fludeoxyglucose-positron emission tomography imaging and a novel 2 base pair deletion in presenilin 2 at nucleotide 342/343, predicted to produce a frame-shift and premature termination. We conclude that mutations in presenilin 2 are rare with only seven being well documented in the literature. The best studied N141I mutation produces an Alzheimer's disease phenotype with a wide range of onset ages overlapping both early and late onset Alzheimer's disease, often associated with seizures, high penetrance and typical Alzheimer's disease neuropathology. A novel premature termination mutation supports loss of function or haploinsufficiency as pathogenic mechanisms in presenilin 2 associated Alzheimer's disease.

Genetic testing in familial AD and FTD: mutation and phenotype spectrum in a Danish cohort

Autosomal dominantly transmitted Alzheimer's disease (AD) and frontotemporal dementia (FTD) are genetically heterogeneous disorders. To date, three genes have been identified in which mutations cause early-onset autosomal dominant inherited AD: APP, PSEN1, and PSEN2. Mutations in two genes on chromosome 17, the MAPT and the PGRN genes, are associated with autosomal dominant inherited FTD. The aim of this study was to characterize the mutation spectrum and describe genotype-phenotype correlations in families with inherited dementia. The identification of novel mutations and/or atypical genotype-phenotype correlations contributes to further characterizing the disorders. DNA-samples from the 90 index cases from a Danish referral-based cohort representing families with presumed autosomal dominant inherited AD or FTD were screened for mutations in the known genes with sequencing, denaturing high-performance liquid chromatography (DHPLC) and multiplex ligation-dependent probe amplification (MLPA) techniques. Seven presumed pathogenic mutations (two PSEN1, one PSEN2, one APP, one MAPT, and two PGRN) were identified, including a novel PSEN2 mutation (V393M). No dosage aberrations were identified.