Health and Medicine

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

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 , collaborators of the CNR-MAJ project
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Summary

Background Amyloid 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 findings We 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. Conclusions Our 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.

Key Points

  • 1Mutations were identified in fifty-three new AD-EOAD families and eighteen sporadic cases
  • 2A notable fraction of PSEN1 mutations were found to occur de novo
  • 3The study highlights the need for continued classification of genetic variants for improved genetic counseling
  • 477% of mutations identified have reached sufficient evidence of pathogenicity for clinical use
  • 5A potential benefit is suggested for screening nonfamilial AD cases for these mutations.

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