Research Article: APP, PSEN1, and PSEN2 mutations in early-onset Alzheimer disease: A genetic screening study of familial and sporadic cases

Date Published: March 28, 2017

Publisher: Public Library of Science

Author(s): Hélène-Marie Lanoiselée, Gaël Nicolas, David Wallon, Anne Rovelet-Lecrux, Morgane Lacour, Stéphane Rousseau, Anne-Claire Richard, Florence Pasquier, Adeline Rollin-Sillaire, Olivier Martinaud, Muriel Quillard-Muraine, Vincent de la Sayette, Claire Boutoleau-Bretonniere, Frédérique Etcharry-Bouyx, Valérie Chauviré, Marie Sarazin, Isabelle le Ber, Stéphane Epelbaum, Thérèse Jonveaux, Olivier Rouaud, Mathieu Ceccaldi, Olivier Félician, Olivier Godefroy, Maite Formaglio, Bernard Croisile, Sophie Auriacombe, Ludivine Chamard, Jean-Louis Vincent, Mathilde Sauvée, Cecilia Marelli-Tosi, Audrey Gabelle, Canan Ozsancak, Jérémie Pariente, Claire Paquet, Didier Hannequin, Dominique Campion, Bruce L Miller

Abstract: BackgroundAmyloid protein precursor (APP), presenilin-1 (PSEN1), and presenilin-2 (PSEN2) mutations cause autosomal dominant forms of early-onset Alzheimer disease (AD-EOAD). Although these genes were identified in the 1990s, variant classification remains a challenge, highlighting the need to colligate mutations from large series.Methods and findingsWe report here a novel update (2012–2016) of the genetic screening of the large AD-EOAD series ascertained across 28 French hospitals from 1993 onwards, bringing the total number of families with identified mutations to n = 170. Families were included when at least two first-degree relatives suffered from early-onset Alzheimer disease (EOAD) with an age of onset (AOO) ≤65 y in two generations. Furthermore, we also screened 129 sporadic cases of Alzheimer disease with an AOO below age 51 (44% males, mean AOO = 45 ± 2 y). APP, PSEN1, or PSEN2 mutations were identified in 53 novel AD-EOAD families. Of the 129 sporadic cases screened, 17 carried a PSEN1 mutation and 1 carried an APP duplication (13%). Parental DNA was available for 10 sporadic mutation carriers, allowing us to show that the mutation had occurred de novo in each case. Thirteen mutations (12 in PSEN1 and 1 in PSEN2) identified either in familial or in sporadic cases were previously unreported. Of the 53 mutation carriers with available cerebrospinal fluid (CSF) biomarkers, 46 (87%) had all three CSF biomarkers—total tau protein (Tau), phospho-tau protein (P-Tau), and amyloid β (Aβ)42—in abnormal ranges. No mutation carrier had the three biomarkers in normal ranges. One limitation of this study is the absence of functional assessment of the possibly and probably pathogenic variants, which should help their classification.ConclusionsOur findings suggest that a nonnegligible fraction of PSEN1 mutations occurs de novo, which is of high importance for genetic counseling, as PSEN1 mutational screening is currently performed in familial cases only. Among the 90 distinct mutations found in the whole sample of families and isolated cases, definite pathogenicity is currently established for only 77%, emphasizing the need to pursue the effort to classify variants.

Partial Text: Alzheimer disease (AD) (MIM #104300) is the most common form of dementia. However, early-onset AD (EOAD) constitutes a minority of patients, with an estimated prevalence of 41.2 per 100,000 persons at risk [1]. Among these forms, presenilin-1 (PSEN1) (MIM #104311), presenilin-2 (PSEN2) (MIM #600759) [2–5], and amyloid protein precursor (APP) (MIM #104760) mutations [6–8] and duplications [9] cause autosomal-dominant EOAD (AD-EOAD), the prevalence of which is estimated to be 5.3 per 100,000 persons at risk [1]. PSEN1 is the most commonly involved gene, with 221 mutations reported as pathogenic in the Alzforum database (www.alzforum.org/mutations). The second most commonly involved gene is APP, with 32 pathogenic mutations described, while 19 different PSEN2 pathogenic mutations have been reported. APP encodes the amyloid-β precursor protein, the processing of which by the β-secretase and the γ-secretase complex leads to the production of the amyloid β (Aβ) peptide, a key event in AD pathogeny. The aggregation of the Aβ peptide in the brain’s parenchyma indeed triggers a cascade of events leading to AD. Its aggregation in cerebromeningeal vessels leads to cerebral amyloid angiopathy (CAA), a condition frequently associated with AD and responsible for recurrent haemorrhagic strokes and white matter lesions. PSEN1 and PSEN2 encode the presenilins, which constitute the catalytic subunit of the γ-secretase complex (for review, see [10,11]). AD-EOAD causative mutations are thought to be responsible for the increased aggregation of the Aβ peptide in the brain’s parenchyma through one of the two following mechanisms: increased overall production of all Aβ species (e.g., APP duplications or APP mutations located around the β cleavage site) or production of a more aggregation-prone form of the Aβ peptide.

The study was approved by the Paris Ile de France II ethics committee.

We have studied two samples of EOAD patients and identified 10 novel missense mutations, 1 novel indel, and 1 novel genomic deletion in PSEN1 and 1 novel missense mutation in PSEN2. According to the Guerreiro’s algorithm [12], pathogenicity was considered as definite for 1 mutation, probable for 9, and possible for 3. Considering the whole French EOAD series, 90 distinct mutations (including the APP duplication) are now reported, and pathogenicity is considered definite for 69 mutations (77%), probable for 16 (18%), and possible for 3 (5%).

Source:

http://doi.org/10.1371/journal.pmed.1002270

 

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