rs63750066 — APP A713T (Calabrian)

APP A713T — The Calabrian Mutation at the Amyloid Cleavage Site

The amyloid precursor protein (APP) is processed by a series of secretase enzymes that determine whether it generates toxic amyloid-beta fragments or harmless non-amyloidogenic peptides. The A713T variant (rs63750066) introduces an alanine-to-threonine substitution at position 713 of APP — a residue lying directly at the gamma-secretase cleavage site¹¹ directly at the gamma-secretase cleavage site
the molecular scissor that cuts APP to produce amyloid-beta peptides of different lengths, with Aβ42 being the most aggregation-prone. Disrupting this cleavage site is one of the most direct mechanisms by which an APP mutation can cause familial Alzheimer's disease (FAD).

This variant is one of the rarest known APP mutations worldwide — present at a global allele frequency of roughly 1 in 25,000 alleles in gnomAD exomes²² 1 in 25,000 alleles in gnomAD exomes
gnomAD v4 exomes: AC=61/1,401,454; AF≈0.000044; near-zero in all non-European populations. Phylogenomic analysis in Abondio et al. 2021³³ Abondio et al. 2021 traced five of seven studied carriers to a shared 1.7-megabase haplotype in the Calabria region of southern Italy, estimating a common ancestor from over 1,000 years ago. All confirmed carriers had roots in Calabria or the Italian diaspora, giving it the informal name "the Calabrian mutation." Despite this ancient shared ancestry, carriers are not more closely related to each other than to unaffected Calabrian controls, indicating the variant has remained rare even in its region of origin.

The Mechanism

The A713T substitution falls at the epsilon/gamma-secretase cleavage boundary of APP, where [gamma-secretase | a multiprotein complex containing presenilin-1 or presenilin-2 as its catalytic subunit] cuts the transmembrane domain of APP to release the Aβ C-terminus. Mutations at this site typically alter the Aβ42:Aβ40 ratio or total Aβ production. The A713T variant is mechanistically distinct from the Swedish mutation (which boosts total Aβ production upstream) and from the Dutch/Iowa mutations (which change Aβ sequence to favor cerebral deposition). For A713T, Rossi et al.⁴⁴ Rossi et al. proposed that altered gamma-secretase cleavage increases the proportion of longer, aggregation-prone Aβ species that preferentially deposit in vessel walls rather than parenchymal plaques — explaining the prominent cerebral amyloid angiopathy (CAA) phenotype seen across multiple families. Armstrong et al. noted the mutation may not primarily affect Aβ production, suggesting additional mechanisms including altered APP trafficking or tau hyperphosphorylation may contribute.

The Evidence

Clinical reports converge on a consistent phenotype. Rossi et al. 2004⁵⁵ Rossi et al. 2004 described an Italian family in which three A713T carriers developed dementia with recurrent strokes; neuropathological examination of the proband confirmed "Alzheimer's disease with severe cerebral amyloid angiopathy and multiple infarcts." Bernardi et al. 2009⁶⁶ Bernardi et al. 2009 identified A713T in 3 of 59 late-onset AD patients selected for cerebrovascular lesions and family dementia history — all three showed MRI subcortical ischemic lesions and neuropathological confirmation of CAA and stroke pathology.

The most comprehensive genetic study is Conidi et al. 2015⁷⁷ Conidi et al. 2015, which followed 21 members of a six-generation Italian pedigree, including three homozygous carriers and eight heterozygous carriers. A critical finding: homozygous individuals showed no substantially greater disease severity or earlier onset than heterozygotes, confirming that a single copy of A713T is sufficient to cause disease — a hallmark of autosomal dominant inheritance rather than the additive dose-response seen in, for example, APOE4. Age of onset showed a wide span not explained by APOE, TOMM40, or TREM2 genotype, indicating that currently unknown environmental or genetic modifiers influence penetrance.

One case report (Lombardi et al. 2017, PMID 28304299⁸⁸ Lombardi et al. 2017, PMID 28304299) documented an A713T carrier with unexpectedly low amyloid PET uptake and normal CSF Aβ1-42, raising the possibility that standard biomarkers may underestimate or misdirect diagnosis in some A713T carriers — a clinically important caveat for neurologists evaluating patients from Calabrian families.

Practical Implications

For carriers of one copy (CT genotype), familial Alzheimer's disease is a serious but manageable risk in the context of neurological surveillance. Because A713T is autosomal dominant, each first-degree relative has a 50% probability of also carrying the mutation; cascade family testing has immediate clinical value. Neuropsychological testing beginning in the mid-40s can establish cognitive baselines and detect subtle early decline, enabling earlier access to clinical trials for disease-modifying therapies, which are being tested at progressively earlier stages of disease. The CAA component of this mutation means that stroke-like presentations and cerebrovascular events may occur, and management of cardiovascular risk factors should be specifically targeted.

Anti-amyloid immunotherapies (lecanemab, donanemab) are currently indicated for early symptomatic AD and MCI due to AD confirmed by amyloid biomarker testing. However, the atypical biomarker profile described in one A713T case warrants caution: carriers should undergo comprehensive biomarker evaluation (amyloid PET, tau PET, and CSF Aβ42/tau ratio) rather than relying on any single test, and discussion of anti-amyloid therapy should involve a specialist aware of the CAA risk, as these therapies carry an increased risk of amyloid-related imaging abnormalities (ARIA) in individuals with significant CAA.

Interactions

The most important interaction is with APOE4 (rs7412, rs429358). In late-onset TREM2 and other AD-risk variants, APOE4 co-carrier status substantially amplifies risk, but for A713T, the Conidi et al. 2015 study found that APOE genotype did not systematically alter age of onset — suggesting this mutation's dominant mechanism may be partially APOE-independent. Nonetheless, APOE4 co-carriers may still face compounding risks through parallel amyloid clearance impairment, and APOE status should be communicated alongside A713T results in a clinical genetics context.

Other APP mutations affecting the same gamma-secretase cleavage region include the London mutation (rs63750671, V717I) and the Austrian mutation — carriers of a second APP pathogenic variant in the same gene would have compound heterozygosity, though this has not been reported for A713T given its extreme rarity.