rs63751287 — PSEN1 M233V

PSEN1 M233V — The Most Common Gene Causing Familial Early-Onset Alzheimer's Disease

Presenilin-1¹¹ Presenilin-1
Encoded by PSEN1 on chromosome 14; forms the catalytic subunit of the gamma-secretase complex, responsible for cleaving the amyloid precursor protein (APP) within neuronal membranes mutations account for the largest share of familial early-onset Alzheimer's disease (EOFAD), with over 300 pathogenic variants identified across the gene. The M233V substitution is among the most severe — documented cases develop cognitive symptoms as early as age 25, with a typical onset range of 25–32 years. No population-wide allele frequency exists because the variant is so rare it was absent in both the PAGE Study (78,692 samples) and the ALFA cohort (660 samples); it is detected only in affected families and sporadic de novo cases.

The Mechanism

PSEN1 encodes the catalytic aspartyl protease component of the gamma-secretase complex²² gamma-secretase complex
A four-protein membrane complex that cleaves type I transmembrane proteins within the lipid bilayer; substrates include APP and Notch. In normal processing, gamma-secretase cleaves APP at multiple sites to produce a mixture of Aβ peptides, predominantly the shorter Aβ40 form. The M233V substitution (c.697A>G) disrupts cleavage geometry at the active site of PSEN1, shifting the Aβ42/Aβ40 ratio markedly upward. Aβ42 is far more prone to aggregation than Aβ40 and seeds amyloid plaques preferentially. Functional studies confirm M233V alters both the Aβ49→40 and Aβ48→38 cleavage lines, simultaneously increasing Aβ42 production and decreasing Aβ38 production in a pattern consistent with the most aggressive PSEN1 mutations.

Methionine 233 sits within transmembrane domain 5 of PSEN1. The homologous position in PSEN2 (M239V) is also pathogenic, indicating that this methionine residue is conserved and structurally critical for correct APP positioning at the active site. The result is a relentless, lifelong increase in Aβ42 generation beginning from conception — explaining why carriers accumulate amyloid decades before symptoms appear and decades earlier than in sporadic Alzheimer's disease.

The Evidence

Houlden et al. 2001³³ Houlden et al. 2001
First description of PSEN1 M233V: onset approximately age 30, pathological examination showed extensive cortical Lewy bodies alongside plaques and tangles — an unusual combined neuropathology. Neurosci Lett. established that the M233V mutation produces "exceptionally early onset" disease, distinguishing it from typical sporadic AD (mean onset ~73 years) and even from most PSEN1 mutations (mean onset typically 40–55 years). Subsequent case reports have consistently replicated onset in the 25–32 year range.

A Chinese case series from Liu et al. 2019⁴⁴ Liu et al. 2019
Two de novo PSEN1 EOFAD cases; M233V patient had cognitive decline, parkinsonism, and epilepsy beginning at age 25. J Alzheimers Dis. documented a negative family history — confirming that de novo M233V mutations occur and must be considered in diagnostically challenging young-onset dementia even without a family history of the disease.

A 2021 multimodal neuroimaging study by Aghakhanyan et al.⁵⁵ Aghakhanyan et al.
31-year-old male with de novo M233V; simultaneous PET/MRI revealed widespread cortical amyloid (Thal stage III), hippocampal tau (Braak III/IV), structural atrophy, and disrupted functional connectivity. Curr Alzheimers Res. demonstrated that even in a single affected individual, the molecular, structural, and functional signatures of advanced Alzheimer's disease are fully established by age 31 — more than four decades earlier than typical disease.

Appel-Cresswell et al. 2018⁶⁶ Appel-Cresswell et al. 2018
Canadian-Vietnamese family with M233V; ataxia, parkinsonism, spasticity, dystonia, myoclonus, hallucinations, and behavioral changes alongside dementia. J Mov Disord. and Seliverstov et al. 2020⁷⁷ Seliverstov et al. 2020
M233V presenting as spinocerebellar ataxia-like syndrome at age 26; confirmed via Sanger sequencing after SCA panel was negative. Cerebellum. together demonstrate that M233V frequently presents with prominent extrapyramidal and cerebellar features — a phenotype broad enough that clinicians may pursue SCA or Parkinson workups before the correct diagnosis is reached.

Practical Actions

Because M233V is fully penetrant and autosomal dominant, every carrier will develop Alzheimer's disease if they live long enough. The genetic finding demands three parallel tracks of action:

Cognitive monitoring: Pre-symptomatic carriers should establish a baseline neuropsychological profile and consider enrolment in prevention trials. Longitudinal assessments allow detection of earliest cognitive changes (typically working memory and executive function decline) years before clinical diagnosis.

Family communication: First-degree relatives (parents, siblings, children) each have a 50% probability of carrying the same mutation. Cascade genetic testing — offered through clinical genetics or a specialist memory clinic — allows relatives to make informed decisions about whether they want to know their own status.

Prevention trial access: PSEN1 mutations are the most actively studied genetic target in Alzheimer's disease prevention research. Carriers should be informed about and offered referral to trials in the Dominantly Inherited Alzheimer Network (DIAN-TU) or similar prevention programmes that enrol pre-symptomatic carriers.

Interactions

PSEN1 interacts functionally with APOE genotype — APOE4 (rs7412/rs429358) accelerates amyloid deposition in PSEN1 mutation carriers. While the M233V mutation is dominant and fully penetrant regardless of APOE status, co-inheritance of APOE4 may influence the age of onset and the rate of amyloid accumulation. PSEN1 also interacts with APP (Amyloid Precursor Protein) variants — pathogenic APP mutations at the gamma-secretase cleavage site (e.g. rs63750447, rs63749810) may compound the Aβ42-elevating effect. These interactions should be assessed through comprehensive familial AD gene panels rather than single-SNP testing.