Novel PSEN1 mutations (H214N and R220P) associated with familial Alzheimer's disease identified by targeted exome sequencing

PSEN1 His214Asn Mutation in a Korean Patient with Familial EOAD and the Importance of Histidine-Tryptophan Interactions in TM-4 Stability

A pathogenic mutation in presenilin-1 (PSEN1), His214Asn, was found in a male patient with memory decline at the age of 41 in Korea for the first time. The proband patient was associated with a positive family history from his father, paternal aunt, and paternal grandmother without genetic testing. He was diagnosed with early onset Alzheimer's disease (EOAD). PSEN1 His214Asn was initially reported in an Italian family, where the patient developed phenotypes similar to the current proband patient. Magnetic resonance imaging (MRI) scans revealed a mild hippocampal atrophy. The amyloid positron emission tomography (amyloid-PET) was positive, along with the positive test results of the increased amyloid ß (Aβ) oligomerization tendency with blood. The PSEN1 His214 amino acid position plays a significant role in the gamma-secretase function, especially from three additional reported mutations in this residue: His214Asp, His214Tyr, and His214Arg. The structure prediction model revealed that PSEN1 protein His214 may interact with Trp215 of His-Trp cation-π interaction, and the mutations of His214 would destroy this interaction. The His-Trp cation-π interaction between His214 and Trp215 would play a crucial structural role in stabilizing the 4th transmembrane domain of PSEN1 protein, especially when aromatic residues were often reported in the membrane interface of the lipid-extracellular region of alpha helices or beta sheets. The His214Asn would alter the cleavage dynamics of gamma-secretase from the disappeared interactions between His214 and Trp215 inside of the helix, resulting in elevated amyloid production. Hence, the increased Aβ was reflected in the increased Aβ oligomerization tendency and the accumulations of Aβ in the brain from amyloid-PET, leading to EOAD.

Elucidating the Protonation State of the γ-Secretase Catalytic Dyad

γ-Secretase (GS) is an intramembrane aspartyl protease that participates in the sequential cleavage of C99 to generate different isoforms of the amyloid-β (Aβ) peptides that are associated with the development of Alzheimer's disease. Due to its importance in the proteolytic processing of C99 by GS, we performed pH replica exchange molecular dynamics (pH-REMD) simulations of GS in its apo and substrate-bound forms to sample the protonation states of the catalytic dyad. We found that the catalytic dyad is deprotonated at physiological pH in our apo form, but the presence of the substrate at the active site displaces its monoprotonated state toward physiological pH. Our results show that Asp257 acts as the general base and Asp385 as the general acid during the cleavage mechanism. We identified different amino acids such as Lys265, Arg269, and the PAL motif interacting with the catalytic dyad and promoting changes in its acid-base behavior. Finally, we also found a significant pK_a shift of Glu280 related to the internalization of TM6-CT in the GS-apo form. Our study provides critical mechanistic insight into the GS mechanism and the basis for future research on the genesis of Aβ peptides and the development of Alzheimer's disease.

Mutation Analysis of the Genes Linked to Early Onset Alzheimer's Disease and Frontotemporal Lobar Degeneration

A lot of effort has been done to unravel the genetics underlying early-onset Alzheimer's disease (AD) and frontotemporal lobar degeneration (FTLD). However, many familial early-onset dementia (EOD) cases still show an unclear genetic background. The aim of this study was to evaluate the role of the known causative mutations and possible pathogenic variants associated with AD and FTLD in a Finnish EOD cohort. The cohort consisted of 39 patients (mean age at onset 54.8 years, range 39-65) with a positive family history of dementia or an atypical or rapidly progressive course of the disease. None of the patients carried the C9orf72 hexanucleotide repeat expansion. Mutations and variants in APP, PSEN1, PSEN2, MAPT, GRN, VCP, CHMP2B, FUS, TARDBP, TREM2, TMEM106B, UBQLN2, SOD1, PRNP, UBQLN1, and BIN1 were screened by using a targeted next generation sequencing panel. Two previously reported pathogenic mutations (PSEN1 p.His163Arg and MAPT p.Arg406Trp) were identified in the cohort. Both patients had familial dementia with an atypical early onset phenotype. In addition, a heterozygous p.Arg71Trp mutation in PSEN2 with an uncertain pathogenic nature was identified in a patient with neuropathologically confirmed AD. In conclusion, targeted investigation of the known dementia-linked genes is worthwhile in patients with onset age under 55 and a positive family history, as well as in patients with atypical features.

Next Generation Sequencing Analysis in Early Onset Dementia Patients

Background:

Early onset dementias (EOD) are rare neurodegenerative dementias that present before 65 years. Genetic factors have a substantially higher pathogenetic contribution in EOD patients than in late onset dementia.

Objective:

To identify known and/or novel rare variants in major candidate genes associated to EOD by high-throughput sequencing. Common-risk variants of apolipoprotein E (APOE) and prion protein (PRNP) genes were also assessed.

Methods:

We studied 22 EOD patients recruited in Memory Clinics, in the context of studies investigating genetic forms of dementia. Two methodological approaches were applied for the target-Next Generation Sequencing (NGS) analysis of these patients. In addition, we performed progranulin plasma dosage, C9Orf72 hexanucleotide repeat expansion analysis, and APOE genotyping.

Results:

We detected three rare known pathogenic mutations in the GRN and PSEN2 genes and eleven unknown-impact mutations in the GRN, VCP, MAPT, FUS, TREM2, and NOTCH3 genes. Six patients were carriers of only common risk variants (APOE and PRNP), and one did not show any risk mutation/variant. Overall, 69% (n = 9) of our early onset Alzheimer’s disease (EAOD) patients, compared with 34% (n = 13) of sporadic late onset Alzheimer’s disease (LOAD) patients and 27% (n = 73) of non-affected controls (ADNI, whole genome data), were carriers of at least two rare/common risk variants in the analyzed candidate genes panel, excluding the full penetrant mutations.

Conclusion:

This study suggests that EOD patients without full penetrant mutations are characterized by higher probability to carry polygenic risk alleles that patients with LOAD forms. This finding is in line with recently reported evidence, thus suggesting that the genetic risk factors identified in LOAD might modulate the risk also in EOAD.

Translational Research in Alzheimer's and Prion Diseases

Translational neuroscience integrates the knowledge derived by basic neuroscience with the development of new diagnostic and therapeutic tools that may be applied to clinical practice in neurological diseases. This information can be used to improve clinical trial designs and outcomes that will accelerate drug development, and to discover novel biomarkers which can be efficiently employed to early recognize neurological disorders and provide information regarding the effects of drugs on the underlying disease biology. Alzheimer’s disease (AD) and prion disease are two classes of neurodegenerative disorders characterized by incomplete knowledge of the molecular mechanisms underlying their occurrence and the lack of valid biomarkers and effective treatments. For these reasons, the design of therapies that prevent or delay the onset, slow the progression, or improve the symptoms associated to these disorders is urgently needed. During the last few decades, translational research provided a framework for advancing development of new diagnostic devices and promising disease-modifying therapies for patients with prion encephalopathies and AD. In this review, we provide present evidence of how supportive can be the translational approach to the study of dementias and show some results of our preclinical studies which have been translated to the clinical application following the ‘bed-to-bench-and-back’ research model.

Alzheimer's Disease and Frontotemporal Dementia: The Current State of Genetics and Genetic Testing Since the Advent of Next-Generation Sequencing

The advent of next-generation sequencing has changed genetic diagnostics, allowing clinicians to test concurrently for phenotypically overlapping conditions such as Alzheimer's disease (AD) and frontotemporal dementia (FTD). However, to interpret genetic results, clinicians require an understanding of the benefits and limitations of different genetic technologies, such as the inability to detect large repeat expansions in such diseases as C9orf72-associated FTD and amyotrophic lateral sclerosis. Other types of mutations such as large deletions or duplications and triple repeat expansions may also go undetected. Additionally, the concurrent testing of multiple genes or the whole exome increases the likelihood of discovering variants of unknown significance. Our goal here is to review the current knowledge about the genetics of AD and FTD and suggest up-to-date guidelines for genetic testing for these dementias. Despite the improvements in diagnosis due to biomarkers testing, AD and FTD can have overlapping symptoms. When used appropriately, genetic testing can elucidate the diagnosis and specific etiology of the disease, as well as provide information for the family and determine eligibility for clinical trials. Prior to ordering genetic testing, clinicians must determine the appropriate genes to test, the types of mutations that occur in these genes, and the best type of genetic test to use. Without this analysis, interpretation of genetic results will be difficult. Patients should be counseled about the benefits and limitations of different types of genetic tests so they can make an informed decision about testing.

Gene mutations in a Han Chinese Alzheimer's disease cohort

OBJECTIVE

Alzheimer's disease (AD) is the most common form of dementia characterized by memory loss at disease onset. The gene mutations in the amyloid precursor protein (APP), presenilin 1 (PSEN1), and presenilin 2 (PSEN2) are the frequent causes of AD. However, the clinical and genetic features of AD overlap with other neurodegenerative diseases. The present study aimed to identify the clinical and genetic characteristics in a Han Chinese AD cohort.

METHODS

Detailed clinical assessment was applied to all the patients. We screened amyloid precursor protein (APP), PSEN1, PSEN2, and microtubule-associated protein tau (MAPT) genes were assessed in 83 sporadic AD patients by Sanger sequencing. A total of 25 probands from families with AD were subjected to next-generation sequencing on 53 dementia-associated genes to capture the target region, and Sanger sequencing was used to detect the variants in the DNA sequence.

RESULTS

PSEN1 p.L226R was found in an early-onset AD (EOAD) family characterized by language impairment at disease onset, a novel probably pathogenetic variant (p.D534H) was identified in a frontal-temporal dementia gene, TANK-binding kinase 1 (TBK1) with a typical AD phenotype in a late-onset AD (LOAD) family, and a PSEN2p.H169N mutation and two benign MAPT (p.Q230R and p.V48L) mutations were detected in three EOAD patients.

CONCLUSIONS

Thus, five variants were identified in a Han Chinese cohort. In the present study, a novel, probably damaging FTLD gene TBK1variant with a typical AD phenotype was detected. Also, the phenotypic characteristics of PSEN1 p.L226R, a PSEN2pathogenic mutation, and two likely benign MAPT variants were described. Hence, screening for mutations in other dementia genes could be further explored in clinically diagnosed AD patients.

CRISPR/Cas9 therapeutics: a cure for cancer and other genetic diseases

Cancer is caused by a series of alterations in genome and epigenome mostly resulting in activation of oncogenes or inactivation of cancer suppressor genes. Genetic engineering has become pivotal in the treatment of cancer and other genetic diseases, especially the formerly-niche use of clustered regularly interspaced short palindromic repeats (CRISPR) associated with Cas9. In defining its superior use, we have followed the recent advances that have been made in producing CRISPR/Cas9 as a therapy of choice. We also provide important genetic mutations where CRISPRs can be repurposed to create adaptive immunity to fight carcinomas and edit genetic mutations causing it. Meanwhile, challenges to CRISPR technology are also discussed with emphasis on ability of pathogens to evolve against CRISPRs. We follow the recent developments on the function of CRISPRs with different carriers which can efficiently deliver it to target cells; furthermore, analogous technologies are also discussed along CRISPRs, including zinc-finger nuclease (ZFN) and transcription activator-like effector nucleases (TALENs). Moreover, progress in clinical applications of CRISPR therapeutics is reviewed; in effect, patients can have lower morbidity and/or mortality from the therapeutic method with least possible side-effects.

Role of Genetics and Epigenetics in the Pathogenesis of Alzheimer's Disease and Frontotemporal Dementia

Alzheimer's disease (AD) and frontotemporal dementia (FTD) represent the first cause of dementia in senile and pre-senile population, respectively. A percentage of cases have a genetic cause, inherited with an autosomal dominant pattern of transmission. The majority of cases, however, derive from complex interactions between a number of genetic and environmental factors. Gene variants may act as risk or protective factors. Their combination with a variety of environmental exposures may result in increased susceptibility to these diseases or may influence their course. The scenario is even more complicated considering the effect of epigenetics, which encompasses mechanisms able to alter the expression of genes without altering the DNA sequence. In this review, an overview of the current genetic and epigenetic progresses in AD and FTD will be provided, with particular focus on 1) causative genes, 2) genetic risk factors and disease modifiers, and 3) epigenetics, including methylation, non-coding RNAs and chromatin remodeling.

Genetics of Alzheimer's disease: an update

It is clear that late-onset Alzheimer's disease (AD), the most common form of dementia in western societies, has a significant genetic component. The recent technological advances in high-throughput genome technologies have enabled the identification of more than 20 novel susceptibility loci. These findings have significantly advanced the understanding of the molecular mechanisms potentially underlying AD etiology, and have therefore provided valuable information for the development of targets for genetic testing, prevention and treatment. This article reviews these recent findings in AD genomics and discusses their implications for understanding the molecular underpinnings of the disease.

Preservation of cell-survival mechanisms by the presenilin-1 K239N mutation may cause its milder clinical phenotype

Presenilin 1 (PSEN1) mutations are the main cause of monogenic Alzheimer's disease. We studied the functional effects of the mutation K239N, which shows incomplete penetrance at the age of 65 years and compared it with the more aggressive mutation E120G. We engineered stable cell lines expressing human PSEN1 wild type or with K239N or E120G mutations. Both mutations induced dysfunction of γ-secretase in the processing of amyloid-β protein precursor, leading to an increase in the amyloid β42/amyloid β40 ratio. Analysis of homeostatic mechanisms showed that K239N induced lower basal and hydrogen peroxide induced intracellular levels of reactive oxygen species than E120G. Similarly, K239N induced lower vulnerability to apoptosis by hydrogen peroxide injury than E120G. Accordingly, the proapoptotic signaling pathways c-Jun NH2-terminal kinase and p38 mitogen-activated protein kinase maintained PSEN1-mediated negative regulation in K239N but not in E120G-bearing cells. Furthermore, the activation of the prosurvival signaling pathways mitogen-activated protein kinase/extracellular signal-regulated kinase and phosphoinositide 3-kinase/Akt was lower in E120G-bearing cells. Therefore, preservation of mechanisms regulating cell responses independent of amyloid-β protein precursor processing may account for the milder phenotype induced by the PSEN1 K239N mutation.