Clinical and Neuropathological Features of the Arctic APP Gene Mutation Causing Early-Onset Alzheimer Disease

Background  A majority of mutations within the β-amyloid region of the amyloid precursor protein (APP) gene cause inherited forms of intracerebral hemorrhage. Most of these mutations may also cause cognitive impairment, but the Arctic APP mutation is the only known intra–β-amyloid mutation to date causing the more typical clinical picture of Alzheimer disease.

Objective  To describe features of 1 Swedish and 1 American family with the previously reported Arctic APP mutation.

Design, Setting, and Participants  Affected and nonaffected carriers of the Arctic APP mutation from the Swedish and American families were investigated clinically. In addition, 1 brain from each family was investigated neuropathologically.

Results  The clinical picture, with age at disease onset in the sixth to seventh decade of life and dysfunction in multiple cognitive areas, is indicative of Alzheimer disease and similar to the phenotype for other Alzheimer disease APP mutations. Several affected mutation carriers displayed general brain atrophy and reduced blood flow of the parietal lobe as demonstrated by magnetic resonance imaging and single-photon emission computed tomography. One Swedish case and 1 American case with the Arctic APP mutation came to autopsy, and both showed no signs of hemorrhage but revealed severe congophilic angiopathy, region-specific neurofibrillary tangle pathological findings, and abundant amyloid plaques. Intriguingly, most plaques from both of these cases had a characteristic ringlike character.

Conclusions  Overall, our findings corroborate that the Arctic APP mutation causes a clinical and neuropathological picture compatible with Alzheimer disease.

Alzheimer disease (AD), the most common neurodegenerative disorder, is neuropathologically characterized by extracellular deposition of β-amyloid peptide (Aβ) into plaques and intraneuronal accumulation of abnormal tau protein as neurofibrillary tangles.

Autosomal dominant forms of AD may explain approximately 5% of all disease cases. Mutations causing AD have been identified in 3 genes: the amyloid precursor protein (APP) gene (GenBank NM_201414) on chromosome 21,^1-3 the presenilin (PS) 1 gene (GenBank NM_000021) on chromosome 14, and the PS2 gene (GenBank NM_012486) on chromosome 1.⁴ To date, close to 20 APP mutations, about 150 PS1 mutations, and 11 PS2 mutations have been described (for an overview, see http://www.molgen.ua.ac.be/ADMutations/). Most of the known APP mutations are located in the vicinity of the cleavage sites for β- and γ-secretases and are believed to cause increased levels of Aβ by affecting its enzymatic cleavage from APP. Also, PS1 and PS2 mutations result in elevated Aβ levels as has been demonstrated both in vivo and in vitro.^5,6 The convergence of AD mutations toward APP metabolism has been explained by the identification of presenilin as an essential component of the complex that mediates γ-secretase cleavage.⁷

Several APP mutations have been identified within the Aβ sequence. Massive amyloid accumulation in brain vessel walls, cerebral hemorrhages, and parenchymal amyloid plaques can be demonstrated in carriers of the Flemish mutation (APP mutation A692G).⁸ Typically, patients with the Flemish mutation clinically present in their 40s either with symptoms related to cerebrovascular events or with cognitive dysfunction.^9-11 Some mutation carriers even develop a progressive dementia compatible with AD both clinically and neuropathologically.^8,11

Carriers of the Dutch mutation (APP mutation E693Q) also develop cerebral amyloid angiopathy, but only rarely amyloid plaque pathological findings, in their 40s and 50s. Clinically, the Dutch mutation is mainly characterized by focal symptoms related to recurrent strokes.¹² Most mutation carriers also develop a clinical picture of dementia, which sometimes precedes but more often follows the initial cerebrovascular event.

A third intra-Aβ APP mutation has been described in an American family. The Iowa mutation (APP mutation D694N) affects carriers in their 50s or 60s, causing a neuropathological picture of discrete brain atrophy, severe cerebral amyloid angiopathy with numerous small infarcts and hemorrhages of the brain parenchyma, and extensive deposition of amyloid plaques and neurofibrillary tangles.¹³ The clinical picture is one of a progressive dementia with speech impairment without any apparent focal symptoms of cerebrovascular events.¹³

Finally, another APP mutation located within the Aβ sequence was identified in a Swedish family with onset of AD in their 50s.¹⁴ The Arctic mutation (APP mutation E693G) had originally been identified in an American family,¹⁵ but the significance of the mutation was unclear at the time. Interestingly, the Arctic mutation has been found to cause diminished instead of increased Aβ₄₀ and Aβ₄₂ levels in conditioned media from transfected cells and in plasma from mutation carriers.¹⁶ As recently shown, this paradoxical feature of the Arctic mutation can be explained by increased formation of Aβ protofibrils. Such intermediate Aβ species are promoted by the Arctic APP mutation and were found to not be appropriately recognized by the Aβ antibodies under the nondenaturing conditions in commonly used enzyme-linked immunosorbent assays.¹⁷ Based on the Arctic mutation and other findings, it has been suggested that the neurotoxic effects in the AD brain are exerted mainly by oligomeric and protofibrillar forms of Aβ,¹⁸ whereas the mature Aβ fibrils in plaques may be less noxious remnants of the disease process.¹⁹

The identification of a Swedish and an American family with the Arctic mutation in the APP gene has been previously reported,^14,15 and here we describe a more detailed account on the clinical and neuropathological picture of affected members in these same families.

The Swedish family with the Arctic mutation originates from a small village in northern Sweden and the pedigree extends over 5 generations (Figure 1A). Information on affected individuals was gained through hospital records, interviews of family members (many of whom had remained in the same geographical area), and collection of information from parish registers and historical archives.

Affected (n = 6) and unaffected (n = 1) mutation carriers as well as mutation noncarriers (n = 5) were investigated with physical examination, routine blood tests, neuropsychological assessment, magnetic resonance imaging, electroencephalography, and single-photon emission computed tomography.

In the American family, there had been 6 persons affected with dementia over 4 generations. The family is descended from Swedish immigrants and is thus likely to be related to the Swedish family with the same mutation. One family member (Figure 1B, subject III:2) had autopsy-proven AD but not the Arctic mutation and is therefore not included in the analyses. This individual most likely had a sporadic form of AD.

For brain imaging (magnetic resonance imaging and single-photon emission computed tomography) and electrophysiological investigation (electroencephalography), only individuals of the Swedish family have been evaluated.

The respective institutional review boards approved this study. Moreover, informed consent for the genetic analyses was obtained from all of the participants and for the neuropathological work from close relatives of the 2 subjects autopsied.

Genetic analyses were carried out on subjects from whom informed consent had been obtained. On these, sequencing of exon 17 of the APP gene was performed according to methods previously described.¹⁴

On subjects from the Swedish family, a comprehensive set of neuropsychological tests was performed by an experienced neuropsychologist (O.A.). The tests assessed global cognitive function (full-scale IQ),²⁰ verbal abstraction (similarities),²⁰ verbal fluency (FAS),²¹ visuospatial construction (block design),²⁰ copying of geometrical designs,²¹ verbal (Rey Auditory Verbal Learning Test)²¹ and visuospatial episodic memory,²¹ executive function (digit symbol),²⁰ and cognitive speed (Trail-Making Test A).²¹

For the American family, only 1 individual underwent neuropsychological evaluation. Apart from the Mini-Mental State Examination, the tests performed were the Mattis Dementia Rating Scale, Trail-Making Test A, Trail-Making Test B, Clinical Dementia Rating, Hamilton Depression Scale, New York University Paragraph Recall Test, and Lawton-Brody Activities of Daily Living.

Magnetic resonance imaging of the brain was performed using a Vision 1.5-T system (Siemens AG, Munich, Germany). Five-mm transaxial slices were acquired and processed to generate T2-weighted and proton density–weighted images as well as fluid-attenuated inversion recovery sequence images. Degrees of atrophy and white matter changes were evaluated visually from hard copies of the images.

Single-photon emission computed tomography was performed using a 3-headed Picker gamma camera (Marconi Medical Systems, Coventry, England). The regional cerebral blood flow was measured using the tracer compound hexylmethylpropylene amineoxine marked with technetium Tc 99m. The assessment followed a standardized protocol in which the patient was scanned for 20 minutes. The reduction in regional cerebral blood flow was evaluated visually from hard copies of the images.

Quantitative electroencephalography was performed in the morning with the subject awake and with eyes closed. Data were recorded using the Nervus system (Tangagreining HF, Reykjavik, Iceland), according to the 10/20 system. Frequency analysis was performed using fast Fourier transformation. A global rating of the pathological changes was undertaken.

To date, 1 subject from the Swedish family and 1 subject from the American family have been autopsied. The Swedish subject (Figure 1A, subject IV:10) was aged 62 years and died after 6 years of disease duration. The American subject (Figure 1B, subject III:1) was aged 72 years and died after 16 years of disease duration. To harmonize the staining conditions, brain tissues from both subjects were stained simultaneously.

For both cases, sections from various cortical, subcortical, and brainstem regions were prepared from paraffin-embedded blocks and fixed in buffered 4% formaldehyde. The sections were stained with hematoxylin-eosin, Nissl, Bielschowsky silver, Gallyas silver impregnation, and Congo red–thioflavine S. In addition, immunohistochemical staining was performed according to a regular protocol for immunohistochemistry²² with a panel of monoclonal and polyclonal antibodies against Aβ as well as against nonphosphorylated and phosphorylated forms of tau (AT8; Innogenetics, Ghent, Belgium).

Sections from the Swedish case were immunostained by the Aβ monoclonal antibody 6E10 (Signet, Dedham, Massachusetts) and with polyclonal antibodies specific for Aβ₄₀ and Aβ₄₂.²³ In addition, sections from this case were stained by the phosphorylated tau monoclonal antibody AT8 (Innogenetics). Tissue from the American case was stained with the Aβ monoclonal antibody 5D10 (Athena Neuroscience, San Francisco, California), with Aβ₄₀ and Aβ₄₂ as well as with the tau monoclonal antibody tau-2 (Sigma Immunochemicals, St Louis, Missouri).

Semiquantitative assessment of silver-positive neurofibrillary tangles, neuritic plaques, and thioflavine S–positive blood vessels was done following the Consortium to Establish a Registry for Alzheimer's Disease protocol.²⁴

The Swedish family descended from ancestors I:1 and I:2 (Figure 1), who were born in 1853 and 1862, respectively. The mode of inheritance was compatible with an autosomal dominant disorder with absolute penetrance as the rate of affected offspring was close to 50%.

In the Swedish family, age at disease onset ranged from 52 years to 62 years with a mean (SD) age of 56.9 (1.1) years. No sex difference in age at onset was observed. The mean (SD) disease duration was 5.7 (0.3) years for the deceased subjects with mutation (n = 6) and 8.8 (1.3) years for mutation-bearing affected individuals still alive (n = 6). An insidious loss of memory for recently acquired information was the presenting symptom in 9 of the 10 affected cases, whereas headache and fatigue were the first symptoms for 1 individual (Table 1). Furthermore, spatial and temporal disorientation, dysphasia, and dyspraxia occurred early in the course of the disease. Psychiatric symptoms such as anxiety, paranoia, and hallucinations were observed in some cases. In several patients, myoclonus and rigidity were late symptoms (Table 1).

In the American family, 4 affected mutation carriers (subjects I:1, II:3, II:4, and III:1) had a mean age at onset of 58.8 years and a mean age at death of 71.8 years. The index case (Figure 1B, subject III:1) presented with disorientation and insomnia at age 56 years. During the disease course, there were no psychiatric symptoms, rigidity, or myoclonus but the patient had a grand mal seizure at age 65 years before dying at age 72 years. This individual's offspring (Figure 1B, subject IV:1) developed disorientation and disrupted sleep at age 59 years and subsequently developed dementia.

No affected cases in the Swedish and American families had a history of cerebrovascular events, and focal neurological signs were not observed in any of the examined subjects.

From the Swedish family, 6 affected subjects, 1 nonsymptomatic mutation carrier, and 5 mutation noncarriers were examined. All of the 6 affected subjects demonstrated a clear reduction in multiple cognitive domains, eg, in global cognitive function, verbal abilities, visuospatial performance, episodic memory, and attention or cognitive speed, whereas simple motor performance was unremarkable (Table 2). The mean degree of cognitive decline vs the only examined healthy mutation noncarrier in the same family amounted to 1 to 2 SDs or more.

In the Swedish family, magnetic resonance imaging and single-photon emission computed tomography were performed on 6 of the affected mutation carriers and 5 of the healthy mutation noncarriers. No cerebral infarcts were seen, but slight to moderate white matter changes could be demonstrated for 3 of the affected cases (Table 3). One of the affected individuals demonstrated a pronounced general brain atrophy and a marked reduction of temporoparietal blood flow but no changes in the white matter. Another subject (Figure 1A, subject IV:10) had a moderate parietal lobe atrophy but no atrophies in other brain regions. In this subject, extensive white matter changes were seen.

The Swedish patient who came to autopsy (Figure 1A, subject IV:10) presented with memory impairment at age 62 years. Subsequent symptoms included spatial disorientation and apathy. He responded temporarily to treatment with acetylcholinesterase inhibitors (tacrine hydrochloride and rivastigmine tartrate) and died at age 68 years.

The brain weighed 1385 g. A gross examination revealed a focal moderate atrophy of the parietal superior lobulus with no signs of infarcts or hemorrhages. Microscopically, there was a general widening of the perivascular spaces and severe congophilic angiopathy of parenchymal, leptomeningeal, and hippocampal vessels (Figure 2A). Moreover, ring-formed plaques without amyloid core pathological findings were abundant in the neocortex and stained strongly for the C-terminal Aβ₄₀ and Aβ₄₂ monoclonal antibodies (Figure 2A) but only weakly against the N-terminal antibodies 6E10 (not shown). In addition, the ring-formed plaques stained positive with silver impregnation techniques but negative with Congo red (Figure 3A). Silver-positive tangles could be demonstrated in the hippocampus and to a lesser extent in the neocortex. Tangles positive for the phosphorylated tau–specific AT8 monoclonal antibody were present in neocortical pyramidal neurons in low to moderate numbers. Neuronal loss was mainly seen in the limbic areas and to a lesser extent in the neocortex.

The American patient who came to autopsy (Figure 1B, subject III:1) presented with disorientation and insomnia at age 57 years. No psychiatric symptoms, rigidity, or myoclonus were present during the disease course. The patient died at age 72 years.

The brain of the American case weighed 822 g. Macroscopically, there was marked atrophy of the frontal, temporal, and parietal lobes. Numerous lacunae in the range of 0.1 to 0.3 cm were seen throughout the white matter and ventral to the left putamen. On the microscopic level, a moderate diffuse neuronal loss was present throughout the cerebral cortex, with a more severe cell loss accompanied by reactive gliosis in certain regions, such as amygdala, parahippocampal gyrus, and hippocampus. A patchy spongiosis of the external granule cell layer was evident. Further, both Aβ-immunopositive (10D5) and thioflavine S–positive amyloid angiopathy were observed, most notably in the hippocampus and parahippocampal gyrus (not shown). Also for the American case, there were ring-formed amyloid plaques mainly in the neocortex that were immunopositive for the Aβ₄₀ and Aβ₄₂ monoclonal antibodies (Figure 2B). These plaques consisted of a crown of delicate neurites surrounding pale brown fibrillary material corresponding to neuritic plaques without amyloid cores (Figure 3B). Finally, frequent neurofibrillary tangles, neuritic plaques, and dystrophic neurites were mainly seen in the various neocortical layers (not shown). The neuropathological description has been somewhat restricted owing to the clinical nature of this account.

We describe a Swedish family and an American family with an inherited form of dementia caused by the Arctic mutation in the APP gene. Affected individuals typically present with memory impairment between ages 52 and 65 years, after which their cognitive status slowly deteriorates with additional symptoms such as disorientation, dysphasia, and dyspraxia. No remarkable motor symptoms except late-stage myoclonus and rigidity can be seen, and because no signs or symptoms indicating cerebrovascular events are evident, the clinical features can be considered indistinguishable from AD. Moreover, the AD phenotype in carriers of other APP or PS mutations is generally characterized by a typical pattern of multiple cognitive dysfunction during the early disease stages. The neuropsychological test results for affected carriers of the Arctic mutation show that the same pattern of cognitive impairments can be seen also for this APP mutation. In addition to profound disturbances in episodic memory and deficits in attention and cognitive speed, we could also demonstrate marked impairments of verbal and visuospatial functions among the subjects with disease. Thus, the clinical picture in affected carriers of the Arctic mutation appears to be in good correspondence with the cognitive phenotype of AD, familial as well as sporadic.

The structural and functional imaging of members of the Swedish family with the Arctic mutation showed that white matter changes were present in most mutation carriers but not in noncarriers of similar ages. The most evident imaging pattern in cases with the Arctic mutation revealed atrophy and reduction of blood flow in parietal lobes without any corresponding changes in temporal and frontal lobes (Figure 2A).

In addition to a severe amyloid angiopathy in subarachnoidal and parenchymal vessels, both the Swedish (Figure 1A, subject IV:10) and American (Figure 1B, subject III:1) autopsy cases were neuropathologically characterized by the presence of severe congophilic angiopathy, amyloid plaques, and neurofibrillary pathological findings. For both cases, the plaques displayed a specific ringlike form lacking a core and being strongly positive for C-terminal Aβ antibodies. In addition, these plaques could be stained by silver impregnation techniques, which indicates that they have assumed a neuriticlike pathology. This unusual plaque feature highlights the possibility that the amyloid core pathology is not obligate for the neurodegeneration in AD and that other forms of Aβ may be more significant in terms of neurotoxicity. Conflicting results exist as to how well soluble Aβ serves as a clinicopathological marker,^23,25 but Aβ intermediates such as oligomers or protofibrils may exert the neurotoxic effects seen in the AD brain. Interestingly, in mice, Aβ oligomers have been demonstrated to disrupt synaptic plasticity¹⁹ and cause neurotoxicity¹⁸ at concentrations comparable to those found in human brains.

The presence of plaques without an amyloid core and negative for Congo red is not unique to brains with the Arctic APP mutation. A similar plaque variant is a striking neuropathological feature in brains from individuals with a genomic deletion of exon 9 of PS1.²⁶ This so-called cotton-wool plaque is characterized by its large size (up to 120 μm in diameter) and its lack of a thioflavine S–positive core.²⁷ Finally, the intra-Aβ Dutch APP mutation has been described to cause a similar noncored Congo red–negative type of amyloid plaques.¹²

Because a thorough neuropsychological evaluation had been performed on the Swedish subject who eventually came to autopsy (Figure 1A, subject IV:10), we had the opportunity to relate the neuropathological picture with the cognitive aberrations caused by the Arctic mutation. Notably, the neuropsychological test results of this individual were compatible with the more advanced pathological findings in the right vs left hemisphere. Moreover, the negative influence on speed of performance (z score < −3) in this subject is compatible with knowledge-based rather than personality-based difficulties, which is in accordance with the finding of more aggravated pathological findings in posterior brain regions as compared with anterior brain regions. By and large, the observations on this subject are in agreement with the clinical and neuropathological criteria for a moderate stage of AD, albeit with some asymmetry in cognition. This overall evaluation was further strengthened by the patient's clear response to treatment with an acetylcholinesterase inhibitor.

The Arctic APP mutation leads to a severe amyloidosis of the brain, as do other mutations within the Aβ region. However, in carriers of the Arctic mutation, no brain hemorrhage has yet been observed, clinically or by adopting modern imaging techniques. This is in stark contrast to both the Dutch and Flemish APP mutations, for which a conspicuous amyloid angiopathy causes cerebral hemorrhage probably owing to a local weakening of the vessel wall. It is not understood why the Arctic and Dutch mutations, both affecting APP amino acid 693, lead to disparate phenotypes. However, it could be speculated that an exchange of the electrically charged glutamic acid to the polar and hydrophilic glutamine renders the Dutch Aβ peptide more prone to adhere to and disrupt the intima of the vessel wall as compared with wild-type Aβ or Arctic Aβ (in which the glutamic acid is replaced by the nonpolar and hydrophobic glycine instead).

In conclusion, our clinical and neuropathological findings suggest that the Arctic APP mutation causes a picture of AD with brain deposition of a particular ring-formed type of amyloid plaque. These findings indicate marked phenotypical differences between this and other various intra-Aβ APP mutations.

Correspondence: Hans Basun, MD, PhD, Division of Molecular Geriatrics, Department of Public Health, Uppsala University, Uppsala University Hospital, Box 609, S-751 25 Uppsala, Sweden (hans.basun@pubcare.uu.se).

Accepted for Publication: April 24, 2007.

Author Contributions:Study concept and design: Basun, Ingelsson, Schellenberg, and Lannfelt. Acquisition of data: Basun, Ingelsson, Almkvist, Näslund, Axelman, Bird, Nochlin, Schellenberg, Wahlund, and Lannfelt. Analysis and interpretation of data: Basun, Bogdanovic, Ingelsson, Almkvist, Nochlin, Wahlund, and Lannfelt. Drafting of the manuscript: Basun, Ingelsson, Almkvist, Axelman, Nochlin, and Lannfelt. Critical revision of the manuscript for important intellectual content: Basun, Bogdanovic, Ingelsson, Näslund, Bird, Schellenberg, and Wahlund. Statistical analysis: Almkvist. Obtained funding: Bogdanovic, Schellenberg, and Lannfelt. Administrative, technical, and material support: Ingelsson, Näslund, Axelman, Bird, Schellenberg, Wahlund, and Lannfelt. Study supervision: Basun, Schellenberg, and Lannfelt.

Financial Disclosure: None reported.