A Novel Missense Variant in SORBS2 Is Causative With Familial Alzheimer's Disease

Abstract

Background: Alzheimer's disease (AD) is a common neurodegenerative disorder with a substantial genetic component. Despite advances in elucidating the genetic underpinnings of AD, much of its heritability remains unexplained. Discovering novel genetic variants and understanding their pathogenic roles are crucial challenges in AD research.

Objective: This study aimed to identify pathogenic genes and elucidate their role in familial early-onset AD (EOAD).

Methods: Blood samples from an EOAD pedigree and Sorbin and SH3 Domain-Containing Protein 2 (SORBS2) T189M transgenic mice were analyzed. Cognitive function was assessed via the Morris water maze (MWM). Protein expression was evaluated by western blotting, while amyloid-β (Aβ) levels were quantified via immunohistochemistry and enzyme-linked immunosorbent assay. Inflammatory markers were measured using immunofluorescence and quantitative reverse transcription polymerase chain reaction (PCR). Neuronal morphology, including dendritic and spine alterations, was examined using Golgi staining.

Results: We identified a novel SORBS2 variant (c. 566C>T, p. T189M) in a Han Chinese family, segregating with AD in a Mendelian fashion. SORBS2 T189M transgenic mice exhibited cognitive deficits, cortical Aβ accumulation, and an increased Aβ42/Aβ40 ratio. Additionally, elevated levels of interleukin (IL)-1β, IL-6, tumor necrosis factor α (TNF-α), and ionized calcium-binding adaptor molecule 1 (Iba1)-positive microglia, along with neuronal loss, were observed in the brains of T189M mice.

Conclusion: Our study suggest that the SORBS2 T189M variant is a novel candidate causal mutation associated with familial AD in a Chinese pedigree, contributing to AD pathogenesis by promoting neuroinflammation and neuronal injury. Notably, this study is the first to establish a link between SORBS2 mutations and AD.

Keywords: SORBS2; Alzheimer's disease; neuroinflammation; pathological mechanism.

FIGURE 1

Identification of SORBS2 T189M variant in a Han Chinese family with early‐onset Alzheimer's disease (EOAD). (A) A multigenerational pedigree showing individuals affected by EOAD. Affected members (indicated by ) underwent whole genome sequencing. The proband is indicated by an arrow; black symbols represent affected members, white symbols represent unaffected members, squares indicate males, circles indicate females, and slashes denote deceased members. Age1/age2 in the top left corner of symbols indicates current age or age at death/age at onset. (+) indicates mutation carriers, and (−) indicates non‐carriers. NA indicates data not available. (B)¹¹C‐PIB PET imaging of the proband confirms amyloid deposition, consistent with AD pathology. (C) Sequencing chromatogram showing the heterozygous SORBS2 c.566C>T mutation in affected family members.

FIGURE 2

SORBS2 T189M mutation decreased learning and memory abilities in transgenic mice. (A) Representative sequencing results for the SORBS2 gene in transgenic mice. (B) Representative qRT‐PCR analysis showing SORBS2 mRNA level in the brains of 6‐month‐old transgenic mice. (C) Representative western blotting analysis showing SORBS2 protein level in 6‐month‐old transgenic mice. (D–G) Morris water maze (MWM) test results showing latency to a platform (D), number of platform crosses (E), time spent in the target quadrant (F), and swim traces (G) in T189M mice. n = 4 or 6 mice/genotype; error bars SEM; one‐way ANOVA; *p < 0.05; **p < 0.01; ***p < 0.001 for (A–C); n = 17 mice/genotype; error bars SEM; two‐way ANOVA; *p < 0.05; **p < 0.01; ***p < 0.001 for (D–G). hSORBS2, Human SORBS2 wild‐type overexpression; T189M, SORBS2 T189M mutation; WT, wild type.

FIGURE 3

SORBS2 T189M mutation increased Aβ in the cerebral cortex of transgenic mice. (A) Representative immunohistochemistry images of Aβ deposition in the cortex of 9‐month‐old WT, hSORBS2, and T189M mice. (B) Quantification of Aβ‐positive cells in the cortex. (C) ELISA results showing Aβ40, Aβ42, and Aβ42/Aβ40 levels in the cortex of 9‐month‐old WT, hSORBS2, and T189M mice. n = 4 or 6 mice/genotype; error bars SEM; 3 fields from 4 mice per group (B); one‐way ANOVA; *p < 0.05; **p < 0.01; ***p < 0.001. hSORBS2, Human SORBS2 wild‐type overexpression; T189M, SORBS2 T189M mutation; WT, wild type.

FIGURE 4

Inflammatory markers and microglia activation in the hippocampal tissues of SORBS2 T189M transgenic mice. (A–D) Immunofluorescence of IL‐1β, IL‐6, TNF‐α, and Iba1 in the CA1, CA3, and dentate gyrus (DG) regions. (E) Quantification of IL‐1β, IL‐6, TNF‐α, and Iba1‐positive cells in the hippocampus. (F) qRT‐PCR analysis of inflammatory factor expression levels in the three groups. Scale bar, 100 μm as indicated. n = 4 mice/genotype; error bars SEM; one‐way ANOVA; *p < 0.05; **p < 0.01; ***p < 0.001. hSORBS2, Human SORBS2 wild‐type overexpression; T189M, SORBS2 T189M mutation; WT, wild type.

FIGURE 5

SORBS2 T189 mutation caused neuronal loss in transgenic mice. (A) Representative Golgi‐stained images of hippocampal sections from 9‐month‐old WT, hSORBS2, and T189M mice. (B) Quantification of total dendritic length from the soma across the three groups. (C) Quantification of dendritic branching as a function of distance from the soma. (D) Representative images of Golgi‐stained hippocampal pyramidal neurons. (E) Quantification of dendritic spine density in the images shown in D. Scale bars, 1 mm (A: Above), 200 μm (A: Below), 20 μm (D). n = 4 mice/genotype; error bars SEM; 8 somas from 4 mice per group (B); 3 somas from 4 mice per group (E); one‐way ANOVA; two‐way ANOVA; *p < 0.05; **p < 0.01; ***p < 0.001. hSORBS2, Human SORBS2 wild‐type overexpression; T189M, SORBS2 T189M mutation; WT, wild type.