APP A673T (Icelandic)

Other Names: Icelandic


Pathogenicity: Alzheimer's Disease : Protective
Clinical Phenotype: None
Reference Assembly: GRCh37/hg19
Position: Chr21:27269932 G>A
dbSNP ID: rs63750847
Coding/Non-Coding: Coding
DNA Change: Substitution
Expected RNA Consequence: Substitution
Expected Protein Consequence: Missense
Codon Change: GCA to ACA
Reference Isoform: APP Isoform APP770 (770 aa)
Genomic Region: Exon 16
Research Models: 1


A673T is the first variant in APP associated with protection against amyloid pathology and Alzheimer's disease. This variant appears to be restricted primarily to Icelandic and Scandinavian populations.

The A673T variant was first reported in a Caucasian individual who died at the age of 65 with a history of ischemic stroke, but in good cognitive health. Postmortem examination revealed negligible cerebral amyloid in the parenchyma and vessels (Peacock et al., 1993). Two decades later, this rare variant was reported to be more prevalent in non-demented elderly Icelandic individuals than in those with Alzheimer’s disease. Specifically, the A673T variant was five times more common in dementia-free elderly individuals than in people with AD. The A673T variant was also found to protect against age-associated cognitive decline (Jonsson et al., 2012).

The A673T variant was subsequently identified in one out of 515 Finnish individuals over age 85. Although the woman who carried the A673T variant had dementia at the age of 104, her impairment was attributed to hippocampal sclerosis rather than to AD. Overall, her brain contained very little amyloid pathology (CERAD=0), although some vascular amyloid was observed in her meningeal arteries. The minimal amyloid pathology in this A673T carrier, along with her longevity, is regarded as further evidence supporting a protective effect of A673T against amyloid pathology and the development of AD (Kero et al., 2013).

In 2012, the A673T variant was reported in an American woman with a family history of late-onset AD. The proband was originally reported as having late-onset AD, but a later study corrected the classification to unaffected at age 84. Of the 17 family members genotyped, only the proband's mother carried the variant. Like her daughter, she was cognitively healthy into advanced age. Although residing the United States, this family had Scandinavian ancestry (Cruchaga et al., 2012; Wang et al., 2015).

The A673T variant may be restricted primarily to Icelandic and Scandinavian populations. In Icelandic individuals the frequency of the A673T variant was reported as 0.13 percent in AD cases and 0.45 to 0.79 percent in controls. In other control cohorts, frequencies were reported as Norwegian (0.21 percent), Finnish (0.51 percent), and Swedish (0.42 percent) (Jonsson et al., 2012). Recent studies have shown that A673T is extremely rare in Caucasian individuals from the United States and may not play a substantial role in risk of AD in this population. Specifically, it was absent in 1,674 cases with late-onset AD and 2,644 elderly control subjects (Bamne et al., 2014). Another large genotyping study found just three heterozygous Caucasian individuals in North America. Of these, two were cognitively healthy at the ages of 77 and 82, and the third carrier, who was of Russian ancestry, developed AD at the age of 89. Therefore, the study reported a carrier frequency of 0.011 percent in American individuals with AD and 0.018 percent in cognitively normal controls. The same study also identified three heterozygous individuals in a Swedish cohort. All were cognitively healthy at the ages of 55, 59, and 72, respectively. Like Jonsson et al., 2012, this study also found a carrier frequency of 0.42 percent for Swedish controls (Wang et al., 2015).

In contrast to other Nordic populations, the A673T variant may be rare in Denmark. It was present in only 1 of 3,487 Danish individuals (0.014 percent), precluding an assessment of association with longevity or cognitive functioning (Mengel-From et al., 2015). 

The A673T variant appears to be extremely rare in Asian populations. It was absent in a screen of 8,721 Asian individuals (Ting et al., 2013), as well as in 1,237 long-lived Chinese individuals (mean age 96.9 years) (Liu et al., 2013).


This variant is associated with minimal amyloid deposition and is thought to protect against amyloid pathology.

Cortical biopsies from three Finnish carriers with idiopathic normal pressure hydrocephalus (iNPH)—a condition whose treatment involves biopsy collection—showed no detectable Aβ, phospho-tau, or p62 pathology (Wittrahm et al., 2023). Importantly, approximately 50 percent of iNPH patients have AD-related brain pathology. Moreover, the three carriers had decreased levels of soluble APPβ (sAPPβ) and Aβ42 in cerebrospinal fluid (CSF) compared with those of three matched non-carriers. Although decreased Aβ42 levels in CSF can be associated with AD, in this case, they likely reflect reduced production of Aβ peptides (see Biological Effect below).

Biological Effect

The A673 residue of APP lies very near the primary β-secretase site, and after cleavage the residue becomes part of the Aβ peptide. In vitro studies suggest that the A673T mutation may reduce amyloid accumulation via effects on both APP and Aβ. Although results vary, several studies indicate this variant shifts APP processing towards the non-amyloidogenic pathway, making APP a less-favorable substrate for β-secretase, and thus resulting in less Aβ production overall. In addition, the Aβ peptides that are generated are less prone to aggregation.

When overexpressed in HEK293 cells, APP with the A673T mutation produced about 40 percent less Aβ40 and Aβ42 than wild-type APP (Jonsson et al., 2012). Additional β-secretase cleavage products, such as sAPP-β and β-CTF, were likewise reduced. Similar effects were subsequently observed in primary mouse neurons expressing human APP (isoform 695) with A673T (Benilova et al., 2014; Maloney et al., 2014) and in iPSC-derived human neurons (Maloney et al., 2014, Wittrahm et al., 2023).

However, the mutation’s effects on APP processing appear to be more complex and vary between different model systems. In Chinese hamster ovary cells overexpressing APP A673T, for example, Aβ and sAPPβ peptides were reduced, but not CTFβ (Kokawa et al., 2015). The authors proposed the mutation caused a reduction in γ-secretase-mediated cleavage. Reduced levels of total Aβ with no change in total β-CTF was also observed in iPSC neurons (Kwart et al., 2019). These authors noted that, although not reaching statistical significance, C89 β-CTF—a non-amyloidogenic fragment—was increased. A673T expression did not result in endosomal enlargement, an alteration tied to elevated β-CTF levels.

Yet another  study suggested increased cleavage by BACE1 at the β′-site, rather than the β-site, based on increased Aβ (11-40) levels observed  in N2a cells expressing human A673T (Kimura et al., 2016). Moreover, in HEK cells and human neural cells expressing the mutant protein, sAPPβ was decreased, while sAPPα was increased, with no detectable changes in Aβ levels or the Aβ42/40 ratio (Wittrahm et al., 2023).

Two studies that examined the effects of A673T on cells also expressing pathogenic APP mutations yielded mixed results. When A673T was co-expressed with other APP mutations associated with AD in SH-SY5Y neuroblastoma cells, Aβ40 production decreased in 10, and Aβ42 decreased in 14, mutant-expressing cell populations (Guyon et al., 2020). Although less common, opposite effects were also observed: for three pathogenic mutations, Aβ40 increased, and for four mutations Aβ42 increased.  Moreover, in 2D and 3D cultures of human neuronal cells expressing pathogenic APP mutations K670N/M671L (Swedish) and V717I (London), APP A673T had no effect on Aβ levels, but the levels of sAPPα were increased while those of sAPPβ were decreased (Wittrahm et al., 2023). In Guyon and colleagues’ study, A673T decreased Aβ40 and Aβ42 when co-expressed with the London mutation but had no effect on Aβ production in the presence of the Swedish mutation. Wittrahm and co-workers suggested that the levels of sAPPα and sAPPβ might be more reliable measures of A673T’s protective effects.

The A673T mutation also appears to lower Aβ aggregation. The mutant Aβ, referred to as A2T because it corresponds to position 2 in Aβ, is less aggregation-prone than wild-type Aβ. It is not yet clear whether this reduced aggregation is primarily due to effects on Aβ40, Aβ42, or both (see Benilova et al., 2014; Maloney et al., 2014). A study using ion mobility-mass spectrometry showed that Aβ42 with the A2T mutation formed dimers, tetramers, and hexamers, but dodecamer formation was inhibited. In contrast no significant effects on Aβ40 assembly were observed (Zheng et al., 2015). Other studies have found that Aβ40 A2T, as well as short  N-terminal fragments of the peptide, can delay aggregation of both A2T and wildtype Aβ40 (Benilova et al., 2014; Lin et al., 2017). In one of these studies, Aβ42 A2T had little effect on aggregation (Benilova et al., 2014).

A2T’s effects on Aβ toxicity remain uncertain.  One study found that across a range of concentrations, the neuronal toxicity associated with mutant Aβ40 and Aβ42 was comparable to wild-type Aβ peptides (Maloney et al., 2014). However, there is some evidence that the A673T variant may reduce Aβ-independent neurotoxicity. Neurons expressing APP with the A673T mutation were resistant to TGFβ2-induced cell death (Hashimoto et al., 2014). Also, plasma membrane binding and internalization of mutant Aβ40 in mouse primary cortical neurons were reduced compared with wildtype Aβ40 (Zhang et al., 2018), and mutant Aβ oligomers had substantially lower affinity for synapses in primary rat hippocampal/cortical neurons than wildtype oligomers (Limegrover et al., 2021). In primary cortical mouse neurons, mutant Aβ40 was reported to inhibit retrograde, and enhance anterograde, axonal transport of mitochondria, however, these effects were modest (Zhang et al., 2018).

Proteomic analyses of plasma and cerebrospinal fluid of three heterozygote carriers revealed differential expression of proteins involved in protein phosphorylation (particularly tau dephosphorylation), inflammation, and mitochondrial function (Wittrahm et al., 2023).

Note: The residue A673T in isoform 770 corresponds to A598T in isoform 695, and the mutation is sometimes referred to by the name A598T (e.g., Hashimoto et al., 2014).

Research Models

Several cell and rodent models carrying the A673T mutation have been generated. For example researchers have created induced pluripotent stem cell lines, including an isogenic line with the A673T mutation knocked in at both alleles (Kwart et al., 2017, Paquet et al., 2016) and a line derived from a skin biopsy of a healthy heterozygous Finnish carrier (Rolova et al., 2020). Moreover, the APP A673T variant was introduced into 2D and 3D cell culture models of human neural cells expressing either the Swedish K670N/M671L or London V717I pathogenic mutations (Wittrahm et al., 2023). A rat knockin model with a humanized Aβ sequence and carrying the A673T mutation has also been generated (Tambini et al., 2020).

Last Updated: 30 Jun 2023


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Mutations Citations

  1. APP K670_M671delinsNL (Swedish)
  2. APP V717I (London)

Paper Citations

  1. . Precise and efficient scarless genome editing in stem cells using CORRECT. Nat Protoc. 2017 Feb;12(2):329-354. Epub 2017 Jan 19 PubMed.
  2. . Efficient introduction of specific homozygous and heterozygous mutations using CRISPR/Cas9. Nature. 2016 May 5;533(7601):125-9. Epub 2016 Apr 27 PubMed.
  3. . Generation of a human induced pluripotent stem cell line (UEFi003-A) carrying heterozygous A673T variant in amyloid precursor protein associated with a reduced risk of Alzheimer's disease. Stem Cell Res. 2020 Oct;48:101968. Epub 2020 Sep 2 PubMed.
  4. . Protective Alzheimer's disease-associated APP A673T variant predominantly decreases sAPPβ levels in cerebrospinal fluid and 2D/3D cell culture models. Neurobiol Dis. 2023 Jun 15;182:106140. Epub 2023 Apr 28 PubMed.
  5. . Opposite changes in APP processing and human Aβ levels in rats carrying either a protective or a pathogenic APP mutation. Elife. 2020 Feb 5;9 PubMed.
  6. . Novel polymorphism in the A4 region of the amyloid precursor protein gene in a patient without Alzheimer's disease. Neurology. 1993 Jun;43(6):1254-6. PubMed.
  7. . A mutation in APP protects against Alzheimer's disease and age-related cognitive decline. Nature. 2012 Aug 2;488(7409):96-9. PubMed.
  8. . Amyloid precursor protein (APP) A673T mutation in the elderly Finnish population. Neurobiol Aging. 2013 May;34(5):1518.e1-3. PubMed.
  9. . Rare variants in APP, PSEN1 and PSEN2 increase risk for AD in late-onset Alzheimer's disease families. PLoS One. 2012;7(2):e31039. Epub 2012 Feb 1 PubMed.
  10. . Rarity of the Alzheimer disease-protective APP A673T variant in the United States. JAMA Neurol. 2015 Feb;72(2):209-16. PubMed.
  11. . Investigation of an amyloid precursor protein protective mutation (A673T) in a North American case-control sample of late-onset Alzheimer's disease. Neurobiol Aging. 2014 Jul;35(7):1779.e15-6. Epub 2014 Jan 23 PubMed.
  12. . The APP A673T frequency differs between Nordic countries. Neurobiol Aging. 2015 Oct;36(10):2909.e1-4. Epub 2015 Jul 11 PubMed.
  13. . Absence of A673T amyloid-β precursor protein variant in Alzheimer's disease and other neurological diseases. Neurobiol Aging. 2013 Oct;34(10):2441.e7-8. PubMed.
  14. . Absence of A673T variant in APP gene indicates an alternative protective mechanism contributing to longevity in Chinese individuals. Neurobiol Aging. 2013 Oct 12; PubMed.
  15. . The Alzheimer disease protective mutation A2T modulates kinetic and thermodynamic properties of amyloid-β (Aβ) aggregation. J Biol Chem. 2014 Nov 7;289(45):30977-89. Epub 2014 Sep 24 PubMed.
  16. . Molecular mechanisms of Alzheimer disease protection by the A673T allele of amyloid precursor protein. J Biol Chem. 2014 Nov 7;289(45):30990-1000. Epub 2014 Sep 24 PubMed.
  17. . The A673T mutation in the amyloid precursor protein reduces the production of β-amyloid protein from its β-carboxyl terminal fragment in cells. Acta Neuropathol Commun. 2015 Nov 4;3:66. PubMed.
  18. . A Large Panel of Isogenic APP and PSEN1 Mutant Human iPSC Neurons Reveals Shared Endosomal Abnormalities Mediated by APP β-CTFs, Not Aβ. Neuron. 2019 Oct 23;104(2):256-270.e5. Epub 2019 Aug 12 PubMed.
  19. . Alternative Selection of β-Site APP-Cleaving Enzyme 1 (BACE1) Cleavage Sites in Amyloid β-Protein Precursor (APP) Harboring Protective and Pathogenic Mutations within the Aβ Sequence. J Biol Chem. 2016 Nov 11;291(46):24041-24053. Epub 2016 Sep 29 PubMed.
  20. . The protective mutation A673T in amyloid precursor protein gene decreases Aβ peptides production for 14 forms of Familial Alzheimer's Disease in SH-SY5Y cells. PLoS One. 2020;15(12):e0237122. Epub 2020 Dec 28 PubMed.
  21. . Amyloid β-Protein Assembly: Differential Effects of the Protective A2T Mutation and Recessive A2V Familial Alzheimer's Disease Mutation. ACS Chem Neurosci. 2015 Oct 21;6(10):1732-40. Epub 2015 Aug 12 PubMed.
  22. . Alzheimer's amyloid-β A2T variant and its N-terminal peptides inhibit amyloid-β fibrillization and rescue the induced cytotoxicity. PLoS One. 2017;12(3):e0174561. Epub 2017 Mar 31 PubMed.
  23. . A mutation protective against Alzheimer's disease renders amyloid β precursor protein incapable of mediating neurotoxicity. J Neurochem. 2014 Jul;130(2):291-300. Epub 2014 Apr 10 PubMed.
  24. . Differential effect of amyloid beta peptides on mitochondrial axonal trafficking depends on their state of aggregation and binding to the plasma membrane. Neurobiol Dis. 2018 Jun;114:1-16. Epub 2018 Mar 2 PubMed.
  25. . Alzheimer's protection effect of A673T mutation may be driven by lower Aβ oligomer binding affinity. J Neurochem. 2021 May;157(4):1316-1330. Epub 2020 Oct 25 PubMed.

Further Reading

Protein Diagram

Primary Papers

  1. . Novel polymorphism in the A4 region of the amyloid precursor protein gene in a patient without Alzheimer's disease. Neurology. 1993 Jun;43(6):1254-6. PubMed.
  2. . A mutation in APP protects against Alzheimer's disease and age-related cognitive decline. Nature. 2012 Aug 2;488(7409):96-9. PubMed.

Other mutations at this position


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