Identification and description of three families with familial Alzheimer disease that segregate variants in the SORL1 gene

Abstract

Alzheimer disease (AD) is a progressive neurodegenerative disorder and the most common form of dementia. The majority of AD cases are sporadic, while up to 5% are families with an early onset AD (EOAD). Mutations in one of the three genes: amyloid beta precursor protein (APP), presenilin 1 (PSEN1) or presenilin 2 (PSEN2) can be disease causing. However, most EOAD families do not carry mutations in any of these three genes, and candidate genes, such as the sortilin-related receptor 1 (SORL1), have been suggested to be potentially causative. To identify AD causative variants, we performed whole-exome sequencing on five individuals from a family with EOAD and a missense variant, p.Arg1303Cys (c.3907C > T) was identified in SORL1 which segregated with disease and was further characterized with immunohistochemistry on two post mortem autopsy cases from the same family. In a targeted re-sequencing effort on independent index patients from 35 EOAD-families, a second SORL1 variant, c.3050-2A > G, was found which segregated with the disease in 3 affected and was absent in one unaffected family member. The c.3050-2A > G variant is located two nucleotides upstream of exon 22 and was shown to cause exon 22 skipping, resulting in a deletion of amino acids Gly1017- Glu1074 of SORL1. Furthermore, a third SORL1 variant, c.5195G > C, recently identified in a Swedish case control cohort included in the European Early-Onset Dementia (EU EOD) consortium study, was detected in two affected siblings in a third family with familial EOAD. The finding of three SORL1-variants that segregate with disease in three separate families with EOAD supports the involvement of SORL1 in AD pathology. The cause of these rare monogenic forms of EOAD has proven difficult to find and the use of exome and genome sequencing may be a successful route to target them.

Keywords: Alzheimer disease; Familial; Inherited; SORL1; Whole-exome sequencing.

Fig. 1

Pedigree of family PED.25. The family segregates the SORL1 variant c.3907C > T (p.Arg1303Cys) in two generations. Individuals included in the whole-exome sequencing (WES) are marked with an *. The genetic status for variant c.3907C > T is indicated “carrier” for heterozygotes, “wildtype” when absent, and “unknown” if DNA was unavailable. Diagnosis refers to the clinical diagnosis. Onset refers to first observation of dementia symptoms, and APOE indicates the ε-alleles. Neuropath indicates the post mortem neuropathological diagnosis. The age at last known affection status of individuals still alive is indicated in parenthesis

Fig. 2

Pedigree of family PED.27. The family segregates the SORL1 variant c.3050-2A > G (p.Gly1017-Glu1074) in two generations. Index patient (III:1) included in the targeted gene sequencing is marked with an *. The numbers inside the symbol for I:4–7 (4) and in II:4–6 (3) indicates multiple individuals. The genetic status for variant c.3050-2A > G is indicated “carrier” for heterozygotes, “wildtype” when absent, and “unknown” if DNA was unavailable. Diagnosis refers to the clinical diagnosis. Onset refers to first observation of dementia symptoms, and APOE indicates the ε-alleles. Diagnosis refers to the clinical diagnosis

Fig. 3

Sequencing of exons 21 to 23 of SORL1 performed on cDNA. a Agarose gel of PCR products from a patient (PED.27 II:1) with the c3050-2A > G variant and a control (wt) showing the existence of two band in the patient, sized 205 bp and 379 bp, and only the larger 379 bp-band in the control, wild type individual. b Sequencing of the lower band in the c3050-2A > G carrier shows skipping of exon 22 in SORL1

Fig. 4

Pedigree of family PED.1499. The family segregates the SORL1 variant c.5195G > C (p.Gly1732Ala) in one generation. Individual (II:2) included in the EU EOD case-control study is marked with an *. The genetic status for variant c.5195G > C is indicated “carrier” for heterozygotes, “wildtype” when absent, and “unknown” if DNA was unavailable. NPH = normal pressure hydrocephalus. Onset refers to first observation of dementia symptoms, and APOE indicates the ε-alleles. The age at last known affection status of individuals still alive is indicated in parenthesis

Fig. 5

Immunohistochemical localization of SORL1 in postmortem brain material from controls, sporadic AD and PED.25. Representative pictures from control (a-c), sporadic AD (d-f) and PED.25 (g-i) in the CA1 region of hippocampus (a-b, d-e, g-h) and subcortical white matter in frontal cortex (c, f, i) using two different SORL1 antibodies, AF5699 (a, d, g) and MAB5699 (b-c, e-f, h-i). The AF5699 SORL1 antibody showed an intense immunoreactivity of extracellular SORL1 aggregates in PED.25 (arrows in g). Arrowheads indicate strong SORL1 immunoreactivity (MAB5699) in glial cells in grey (h) and white matter (i) in the affected member from PED.25. Scalebar: 50 μm. Ctrl = control, sAD = sporadic AD, PED.25 = affected family member II:6

Fig. 6

Co-localization between SORL1 and APP in postmortem brain as detected by in situ PLA. a-c shows representative sections of the three groups; a Ctrl = controls, b sAD = sporadic AD, and c PED.25 = affected family member from PED.25. d The mean number of PLA dots/neuron, represented as horizontal bars (−), was quantified in pyramidal neurons in frontal cortex from controls (n = 4), sporadic AD (n = 4) and PED.25 (n = 2). A minimum of 100 neurons was quantified from every individual. The individual values are represented as filled squares (■)

Fig. 7

Schematic picture of the SORL1 protein adapted from Verheijen et al., presenting the location of SORL1 variants that are “likely pathogenic” (in bold) and variants of “uncertain significance”, also see Table 2. Functional domains are based on Uniprot information (Q92673), and numbering of variations are based on NM_003105.5 (cDNA) and NP_003096 (protein). Below are lines corresponding to the epitopes used to generate the respective antibodies (MAB5699, AF5699, 612633 and ab190684) applied in the study. Vps10p: vacuolar protein sorting 10 domain; LDLR class B: low-density lipoprotein-receptor class B repeats; EGF: epidermal growth factor-like domain; LDLR class A: low-density lipoprotein-receptor class A domain; Fibronectin III: Fibronectin type-III domain; TM: transmembrane domain