Overview

Pathogenicity: Cerebral Amyloid Angiopathy : Pathogenic
ACMG/AMP Pathogenicity Criteria: PS3, PS4, PM1, PM2, PP3
Clinical Phenotype: Cerebral Amyloid Angiopathy, Dementia, Vascular Dementia
Position: (GRCh38/hg38):Chr21:25891853 G>A
Position: (GRCh37/hg19):Chr21:27264165 G>A
dbSNP ID: rs63749810
Coding/Non-Coding: Coding
DNA Change: Substitution
Expected RNA Consequence: Substitution
Expected Protein Consequence: Missense
Codon Change: GAT to AAT
Reference Isoform: APP Isoform APP770 (770 aa)
Genomic Region: Exon 17
Research Models: 2

Findings

This mutation was first documented in a large, three-generation kindred from Iowa (Grabowski et al., 2001). The family, who were of German descent, included 10 individuals affected by autosomal-dominant dementia beginning in the sixth or seventh decade of life. Progressive cognitive decline was the defining clinical feature. Postmortem examination was notable for microscopic hemorrhagic lesions.

The "Iowa mutation" was later found in a Spanish pedigree (Greenberg et al., 2003). Affected individuals presented with a hereditary syndrome involving hemorrhagic stroke, dementia, leukoencephalopathy, and occipital calcifications. Age of onset in this family was 58 to 66 years. Symptomatic intracerebral hemorrhage (ICH) occurred in three of the four reported Spanish patients. Although the original report of the Iowa family did not identify symptomatic ICH, 15 of 20 carriers eventually developed lobar ICH (Sellal et al., 2016).

This mutation was also observed in two Irishwomen with early onset ICH who shared a common great-grandfather (Mok et al., 2014). In contrast to the Iowan and Spanish families, both of whom had fairly uniform familial presentations, the clinical course of the two Irishwomen varied. One patient, LM, had a history of chronic migraine headaches, but was cognitively normal at age 53 when she died of a brain hemorrhage. The second patient, DG, developed cognitive decline at age 51, and two years later experienced symptomatic ICH. A CT scan showed a right subcortical occipital lobe hemorrhage. Her father died in his 70s with dementia, and a paternal aunt and grandmother suffered brain hemorrhages in their sixth and seventh decades. Additional relatives suffered from various neurologic conditions, including stroke, seizures, and a brain tumor, with three infants affected by fatal anencephaly.

Three members of a Polish family (Iwanowski et al,. 2015) and two French siblings (Sellal et al., 2016) were also found to carry the Iowa mutation. The Polish pedigree included seven individuals affected by ICH across three generations with an autosomal-dominant pattern of  inheritance. The proband presented at the age of 43 with neurological symptoms related to an ICH in the left occipital lobe. Her mother had her first stroke at age 40 and died at age 60 from ICH. Three of the proband’s brothers developed symptoms in middle age. Overall, symptom onset in this family ranged from 38 to 47 years of age.

This mutation was absent from the gnomAD variant database (v2.1.1, Oct 2021).

Neuropathology

Carriers of this mutation typically have severe cerebral amyloid angiopathy (CAA) with calcifications observed in the cortex, primarily the occipital cortex, and AD pathology (Sellal et al., 2016). Neuropathological examination of the original Iowa proband revealed severe CAA, widespread neurofibrillary tangles, and abundant Aβ40 in plaques. The proband and an affected brother also had microscopic hemorrhagic lesions and cortical calcifications in the occipital lobe (Grabowski et al., 2001).

Neuropathological examination in the Irish patient, LM, confirmed a large occipital hemorrhage with severe amyloid angiopathy of meningeal, cerebro-cortical, and cerebellar parenchymal arteries and veins. Calcification was observed in brain vessels, including those with and without amyloid. Some neuritic plaques and neurofibrillary tangles were seen, along with tau-positive neuropil threads in the hippocampus and frontal and temporal neocortices. Her relative, DG, had a right subcortical occipital lobe hemorrhage visible by CT, which extended anteriorly into the right temporal lobe, along with prominent calcifications (Mok et al., 2014). Calcifications, particularly in the cortex, were also observed in the French carriers (Sellal et al., 2016).

Interestingly, abundant fibrin deposits and fibrinogen/Aβ co-deposits were found in the occipital cortex of an Irish mutation carrier (Cajamarca et al., 2020). The mutation appears to increase Aβ’s affinity for fibrinogen which may exacerbate CAA.

Also of note, MRI scans revealed nine of 11 carriers had leukoencephalopathy (Sellal et al., 2016).

One non-demented Iowa carrier with CAA had decreased levels of Aβ40 and Aβ42 in cerebrospinal fluid, with slightly increased levels of tau and phospho-tau (Verbeek et al., 2009).

Biological Effect

In cultured human cerebrovascular smooth muscle cells, Aβ40 peptides derived from D694N—corresponding to position 23 in Aβ (D23N)—induced elevated levels of cell-associated APP, actin breakdown, and cell death (Van Nostrand et al., 2002; Van Nostrand et al., 2001). Also, in PC12 cells, both D23N Aβ40 and Aβ42 peptides were more toxic than their wildtype counterparts (Murakami et al., 2003). Interestingly, APP Iowa was increased in early endosomes of transfected HeLa and HEK cells, although localization to the cis-Golgi and cell surface was similar to that of wildtype APP (Schilling et al., 2023). 

In vitro, the peptides readily formed fibrils. Initial findings showed more rapid fibrillization of mutant Aβ40 than the wildtype peptide (Van Nostrand et al., 2001, Bitan et al., 2003). Although one study reported slightly slower aggregation of Aβ42 compared with the wildtype peptide (Murakami et al., 2003), subsequent studies indicated both mutant Aβ40 and Aβ42 peptides aggregate rapidly, without a detectable lag phase (Tycko et al., 2009, Illes-Toth et al., 2017). One study suggested this acceleration stems primarily from the enhancement of secondary nucleation on the surface of existing fibrils (Yang et al., 2018).

D23N peptides aggregate into structures with diverse morphologies, including long and thin fibrils, some bundled into dense clusters, as well as amorphous structures (Tycko et al., 2009, Hatami et al., 2017, Yang et al., 2018). Interestingly, in addition to D23N peptides adopting the parallel β-sheet structure commonly found in Aβ aggregates, an anti-parallel β-sheet structure has also been observed (Tycko et al., 2009, Qiang et al., 2012, Mar 2012 news, Amyloid Atlas). In both parallel and anti-parallel structures, hydrophobic residues are close to similar residues in neighboring strands. However, in the anti-parallel configuration, fewer amino acids hold the strands together. This could explain anti-parallel aggregates’ reduced stability (Qiang et al., 2012, Alred et al., 2014). Anti-parallel structures also propagate less efficiently in seeded fibril growth (Qiang et al., 2012). The authors suggested anti-parallel fibrils may be detectable because they nucleate more efficiently than parallel fibrils and may represent an intermediate toxic form. In neuronal cell cultures, both parallel and antiparallel aggregates were toxic. Parallel Iowa Aβ fibrils have been modeled at high-resolution using solid-state NMR (Sgourakis et al., 2015, Amyloid Atlas).

The D23N substitution results in one less negative charge, without substantially affecting the size of the residue’s side-chain (Yang et al., 2018). Simulations have suggested it affects monomer Aβ folding (Krone et al., 2008), destabilizing the hydrogen bonding required for helical structure (Lin and Pande, 2012, Davidson et al., 2022). This in turn may lead to the adoption of an extended, unfolded structure with mostly short-range contacts (Davidson et al., 2022). The increased exposure of residues in the hydrophobic C-terminal region may contribute to fibril formation. Of note, D694 lies within a cholesterol-binding site as determined by NMR resonance spectroscopy and site-directed mutagenesis (Barrett et al., 2012).

The Iowa mutation also appears to affect APP processing. D694 is close to the α-secretase cleavage site and APP Iowa has been reported to decrease proteolysis at this site (Schilling et al., 2023). This may explain its enrichment in early endosomes. Moreover, although the mutation does not affect either Aβ40 or Aβ42 production (Van Nostrand et al., 2001), a detailed survey of Aβ peptides revealed increased production of N-terminally truncated peptides starting at position 5: Aβ5-29 and Aβ5-33 (Schilling et al., 2023). This is in contrast to wildtype APP, where mainly Aβ5-40 was identified. Also, Aβ1-19 and Aβ1-33 were significantly increased compared with wildtype APP, suggesting the mutation facilitates BACE2 and neprilysin cleavage. The authors noted the mutant C-terminally shortened peptides might contribute to AD progression. 

Pathogenicity

Cerebral Amyloid Angiopathy : Pathogenic*

*Although not AD, the cerebrovascular conditions associated with this variant appear to be inherited in an autosomal dominant manner, so its pathogenicity was classified using the ACMG-AMP guidelines.

This variant fulfilled the following criteria based on the ACMG/AMP guidelines. See a full list of the criteria in the Methods page.

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References

News Citations

1. Anti-parallel Universe—Rare Amyloid Peptides in Cylinders, Sheets 9 Mar 2012

Paper Citations

1. Grabowski TJ, Cho HS, Vonsattel JP, Rebeck GW, Greenberg SM. Novel amyloid precursor protein mutation in an Iowa family with dementia and severe cerebral amyloid angiopathy. Ann Neurol. 2001 Jun;49(6):697-705. PubMed.
2. Greenberg SM, Shin Y, Grabowski TJ, Cooper GE, Rebeck GW, Iglesias S, Chapon F, Tournier-Lasserve E, Baron JC. Hemorrhagic stroke associated with the Iowa amyloid precursor protein mutation. Neurology. 2003 Mar 25;60(6):1020-2. PubMed.
3. Sellal F, Wallon D, Martinez-Almoyna L, Marelli C, Dhar A, Oesterlé H, Rovelet-Lecrux A, Rousseau S, Kourkoulis CE, Rosand J, DiPucchio ZY, Frosch M, Gombert C, Audoin B, Miné M, Riant F, Frebourg T, Hannequin D, Campion D, Greenberg SM, Tournier-Lasserve E, Nicolas G. APP Mutations in Cerebral Amyloid Angiopathy with or without Cortical Calcifications: Report of Three Families and a Literature Review. J Alzheimers Dis. 2017;56(1):37-46. PubMed.
4. Mok T, Chalissery AJ, Byrne S, Costelloe L, Galvin L, Vinters HV, Farrell MA, Brett FM, Moroney JT. Familial cerebral amyloid angiopathy due to the iowa mutation in an irish family. Can J Neurol Sci. 2014 Jul;41(4):512-7. PubMed.
5. Iwanowski P, Kozubski W, Losy J. Iowa-type hereditary cerebral amyloid angiopathy in a Polish family. J Neurol Sci. 2015 Sep 15;356(1-2):202-4. Epub 2015 Jun 14 PubMed.
6. Cajamarca SA, Norris EH, van der Weerd L, Strickland S, Ahn HJ. Cerebral amyloid angiopathy-linked β-amyloid mutations promote cerebral fibrin deposits via increased binding affinity for fibrinogen. Proc Natl Acad Sci U S A. 2020 Jun 23;117(25):14482-14492. Epub 2020 Jun 9 PubMed.
7. Verbeek MM, Kremer BP, Rikkert MO, Van Domburg PH, Skehan ME, Greenberg SM. Cerebrospinal fluid amyloid beta(40) is decreased in cerebral amyloid angiopathy. Ann Neurol. 2009 Aug;66(2):245-9. PubMed.
8. Van Nostrand WE, Melchor JP, Romanov G, Zeigler K, Davis J. Pathogenic effects of cerebral amyloid angiopathy mutations in the amyloid beta-protein precursor. Ann N Y Acad Sci. 2002 Nov;977:258-65. PubMed.
9. Van Nostrand WE, Melchor JP, Cho HS, Greenberg SM, Rebeck GW. Pathogenic effects of D23N Iowa mutant amyloid beta -protein. J Biol Chem. 2001 Aug 31;276(35):32860-6. Epub 2001 Jul 5 PubMed.
10. Murakami K, Irie K, Morimoto A, Ohigashi H, Shindo M, Nagao M, Shimizu T, Shirasawa T. Neurotoxicity and physicochemical properties of Abeta mutant peptides from cerebral amyloid angiopathy: implication for the pathogenesis of cerebral amyloid angiopathy and Alzheimer's disease. J Biol Chem. 2003 Nov 14;278(46):46179-87. Epub 2003 Aug 27 PubMed.
11. Schilling S, Pradhan A, Heesch A, Helbig A, Blennow K, Koch C, Bertgen L, Koo EH, Brinkmalm G, Zetterberg H, Kins S, Eggert S. Differential effects of familial Alzheimer's disease-causing mutations on amyloid precursor protein (APP) trafficking, proteolytic conversion, and synaptogenic activity. Acta Neuropathol Commun. 2023 Jun 1;11(1):87. PubMed.
12. Bitan G, Vollers SS, Teplow DB. Elucidation of primary structure elements controlling early amyloid beta-protein oligomerization. J Biol Chem. 2003 Sep 12;278(37):34882-9. Epub 2003 Jul 2 PubMed.
13. Tycko R, Sciarretta KL, Orgel JP, Meredith SC. Evidence for novel beta-sheet structures in Iowa mutant beta-amyloid fibrils. Biochemistry. 2009 Jul 7;48(26):6072-84. PubMed.
14. Illes-Toth E, Meisl G, Rempel DL, Knowles TP, Gross ML. Pulsed Hydrogen-Deuterium Exchange Reveals Altered Structures and Mechanisms in the Aggregation of Familial Alzheimer's Disease Mutants. ACS Chem Neurosci. 2021 Jun 2;12(11):1972-1982. Epub 2021 May 14 PubMed.
15. Yang X, Meisl G, Frohm B, Thulin E, Knowles TP, Linse S. On the role of sidechain size and charge in the aggregation of Aβ42 with familial mutations. Proc Natl Acad Sci U S A. 2018 Jun 26;115(26):E5849-E5858. Epub 2018 Jun 12 PubMed.
16. Hatami A, Monjazeb S, Milton S, Glabe CG. Familial Alzheimer's Disease Mutations within the Amyloid Precursor Protein Alter the Aggregation and Conformation of the Amyloid-β Peptide. J Biol Chem. 2017 Feb 24;292(8):3172-3185. Epub 2017 Jan 3 PubMed.
17. Qiang W, Yau WM, Luo Y, Mattson MP, Tycko R. Antiparallel β-sheet architecture in Iowa-mutant β-amyloid fibrils. Proc Natl Acad Sci U S A. 2012 Mar 20;109(12):4443-8. Epub 2012 Mar 8 PubMed.
18. Alred EJ, Scheele EG, Berhanu WM, Hansmann UH. Stability of Iowa mutant and wild type Aβ-peptide aggregates. J Chem Phys. 2014 Nov 7;141(17):175101. PubMed.
19. Sgourakis NG, Yau WM, Qiang W. Modeling an in-register, parallel "iowa" aβ fibril structure using solid-state NMR data from labeled samples with rosetta. Structure. 2015 Jan 6;23(1):216-227. Epub 2014 Dec 24 PubMed.
20. Krone MG, Baumketner A, Bernstein SL, Wyttenbach T, Lazo ND, Teplow DB, Bowers MT, Shea JE. Effects of familial Alzheimer's disease mutations on the folding nucleation of the amyloid beta-protein. J Mol Biol. 2008 Aug 1;381(1):221-8. PubMed.
21. Lin YS, Pande VS. Effects of familial mutations on the monomer structure of Aβ₄₂. Biophys J. 2012 Dec 19;103(12):L47-9. Epub 2012 Dec 18 PubMed.
22. Davidson DS, Kraus JA, Montgomery JM, Lemkul JA. Effects of Familial Alzheimer's Disease Mutations on the Folding Free Energy and Dipole-Dipole Interactions of the Amyloid β-Peptide. J Phys Chem B. 2022 Oct 6;126(39):7552-7566. Epub 2022 Sep 23 PubMed.
23. Barrett PJ, Song Y, Van Horn WD, Hustedt EJ, Schafer JM, Hadziselimovic A, Beel AJ, Sanders CR. The amyloid precursor protein has a flexible transmembrane domain and binds cholesterol. Science. 2012 Jun 1;336(6085):1168-71. PubMed.

External Citations

1. Amyloid Atlas

Further Reading