Current and future implications of basic and translational research on amyloid-ß peptide production and removal pathways.

Inherited variants in multiple different genes are associated with increased risk for Alzheimer's disease (AD). In many of these genes, the inherited variants alter some aspect of the production or clearance of the neurotoxic amyloid ß-peptide (Aß). Thus missense, splice site or duplication mutants in the presenilin 1 (PS1), presenilin 2 (PS2) or the amyloid precursor protein (APP) genes, which alter the levels or shift the balance of Aß produced, are associated with rare, highly penetrant autosomal dominant forms of Familial Alzheimer's Disease (FAD). Similarly, the more prevalent late-onset forms of AD are associated with both coding and non-coding variants in genes such as SORL1, PICALM and ABCA7 that affect the production and clearance of Aß. This review summarises some of the recent molecular and structural work on the role of these genes and the proteins coded by them in the biology of Aß. We also briefly outline how the emerging knowledge about the pathways involved in Aß generation and clearance can be potentially targeted therapeutically. This article is part of Special Issue entitled "Neuronal Protein".

Proliferation, differentiation and amyloid-ß production in neural progenitor cells isolated from TgCRND8 mice.

The amyloid precursor protein (APP) and amyloid-ß (Aß) peptide play central roles in the pathology and etiology of Alzheimer's disease. Amyloid-induced impairments in neurogenesis have been investigated in several transgenic mouse models but the mechanism of action remains to be conclusively demonstrated. The changes in neurogenesis during this transition of increasing Aß levels and plaque formation were investigated in the present study. We found that the proliferation of newborn cell in the dentate gyrus was enhanced prior to elevations in soluble Aß production as well as amyloid deposition in 5-week-old TgCRND8 mice, which are well-established Alzheimer's disease models, compared to non-transgenic (Non-Tg) mice. The number of BrdU-positive cells remained higher in TgCRND8 vs Non-Tg mice for a period of 8weeks. The numbers of BrdU/NeuN-positive cells were not significantly different in TgCRND8 compared to Non-Tg mice. A significant decrease in BrdU/GFAP but not in BrdU/S100ß was found in Tg vs Non-Tg at 6-weeks of age. In addition, a unique observation was made using isolated neuroprogenitor cells from TgCRND8 mice which were found to be less viable in culture and produced substantial amounts of secreted Aß peptides. This suggests that the proliferation of neural progenitors in vivo may be modulated by high levels of APP expression and the resulting Aß generated directly by the progenitor cells. These findings indicate that cell proliferation is increased prior to Aß deposition and that cell viability is decreased in TgCRND8 mice over time.

Independent and epistatic effects of variants in VPS10-d receptors on Alzheimer disease risk and processing of the amyloid precursor protein (APP).

Genetic variants in the sortilin-related receptor (SORL1) and the sortilin-related vacuolar protein sorting 10 (VPS10) domain-containing receptor 1 (SORCS1) are associated with increased risk of Alzheimer's disease (AD), declining cognitive function and altered amyloid precursor protein (APP) processing. We explored whether other members of the (VPS10) domain-containing receptor protein family (the sortilin-related VPS10 domain-containing receptors 2 and 3 (SORCS2 and SORCS3) and sortilin (SORT1)) would have similar effects either independently or together. We conducted the analyses in a large Caucasian case control data set (n=11,840 cases, 10,931 controls) to determine the associations between single nucleotide polymorphisms (SNPs) in all the five homologous genes and AD risk. Evidence for interactions between SNPs in the five VPS10 domain receptor family genes was determined in epistatic statistical models. We also compared expression levels of SORCS2, SORCS3 and SORT1 in AD and control brains using microarray gene expression analyses and assessed the effects of these genes on γ-secretase processing of APP. Several SNPs in SORL1, SORCS1, SORCS2 and SORCS3 were associated with AD. In addition, four specific linkage disequilibrium blocks in SORCS1, SORCS2 and SORCS3 showed additive epistatic effects on the risk of AD (P≤0.0006). SORCS3, but not SORCS2 or SORT1, showed reduced expression in AD compared with control brains, but knockdown of all the three genes using short hairpin RNAs in HEK293 cells caused a significant threefold increase in APP processing (from P<0.001 to P<0.05). These findings indicate that in addition to SORL1 and SORCS1, variants in other members of the VPS10 domain receptor family (that is, SORCS1, SORCS2, SORCS3) are associated with AD risk and alter APP processing. More importantly, the results indicate that variants within these genes have epistatic effects on AD risk.

Latrepirdine improves cognition and arrests progression of neuropathology in an Alzheimer's mouse model.

Latrepirdine (Dimebon) is a pro-neurogenic, antihistaminic compound that has yielded mixed results in clinical trials of mild to moderate Alzheimer's disease, with a dramatically positive outcome in a Russian clinical trial that was unconfirmed in a replication trial in the United States. We sought to determine whether latrepirdine (LAT)-stimulated amyloid precursor protein (APP) catabolism is at least partially attributable to regulation of macroautophagy, a highly conserved protein catabolism pathway that is known to be impaired in brains of patients with Alzheimer's disease (AD). We utilized several mammalian cellular models to determine whether LAT regulates mammalian target of rapamycin (mTOR) and Atg5-dependent autophagy. Male TgCRND8 mice were chronically administered LAT prior to behavior analysis in the cued and contextual fear conditioning paradigm, as well as immunohistological and biochemical analysis of AD-related neuropathology. Treatment of cultured mammalian cells with LAT led to enhanced mTOR- and Atg5-dependent autophagy. Latrepirdine treatment of TgCRND8 transgenic mice was associated with improved learning behavior and with a reduction in accumulation of Aß42 and α-synuclein. We conclude that LAT possesses pro-autophagic properties in addition to the previously reported pro-neurogenic properties, both of which are potentially relevant to the treatment and/or prevention of neurodegenerative diseases. We suggest that elucidation of the molecular mechanism(s) underlying LAT effects on neurogenesis, autophagy and behavior might warranty the further study of LAT as a potentially viable lead compound that might yield more consistent clinical benefit following the optimization of its pro-neurogenic, pro-autophagic and/or pro-cognitive activities.

Assembly of the presenilin γ-/ε-secretase complex.

The presenilin complex is composed of four core proteins (presenilin 1 or presenilin 2, APH1, nicastrin, and PEN2). Several endogenous proteins have been reported to selectively modulate the function of the presenilin complexes; these include transmembrane trafficking protein, 21-KD (TMP21), CD147 antigen (basigin), the γ-secretase-activating protein (gSAP), and the orphan G-protein-coupled receptor 3. Because the structure and assembly of these complexes underlies their activity, this review will discuss current work on the assembly of the complex and on presenilin-interacting proteins that regulate secretase activity.

Worldwide distribution of PSEN1 Met146Leu mutation: a large variability for a founder mutation.

OBJECTIVE: Large kindreds segregating familial Alzheimer disease (FAD) offer the opportunity of studying clinical variability as observed for presenilin 1 (PSEN1) mutations. Two early-onset FAD (EOFAD) Calabrian families with PSEN1 Met146Leu (ATG/CTG) mutation constitute a unique population descending from a remote common ancestor. Recently, several other EOFAD families with the same mutation have been described worldwide. METHODS: We searched for a common founder of the PSEN1 Met146Leu mutation in families with different geographic origins by genealogic and molecular analyses. We also investigated the phenotypic variability at onset in a group of 50 patients (mean age at onset 40.0 +/- 4.8 years) by clinical, neuropsychological, and molecular methodologies. RESULTS: EOFAD Met146Leu families from around the world resulted to be related and constitute a single kindred originating from Southern Italy before the 17th century. Phenotypic variability at onset is broad: 4 different clinical presentations may be recognized, 2 classic for AD (memory deficits and spatial and temporal disorientation), whereas the others are expressions of frontal impairment. The apathetic and dysexecutive subgroups could be related to orbital-medial prefrontal cortex and dorsolateral prefrontal cortex dysfunction. CONCLUSIONS: Genealogic and molecular findings provided evidence that the PSEN1 Met146Leu families from around the world analyzed in this study are related and represent a single kindred originating from Southern Italy. The marked phenotypic variability might reflect early involvement by the pathologic process of different cortical areas. Although the clinical phenotype is quite variable, the neuropathologic and biochemical characteristics of the lesions account for neurodegenerative processes unmistakably of Alzheimer nature.

A novel presenilin 2 mutation (V393M) in early-onset dementia with profound language impairment.

BACKGROUND: Mutations in the Presenilin 2 gene (PSEN2) are rare causes of Alzheimer's disease (AD). Pathogenic mutations in the genes associated with autosomal dominant inherited AD have been shown to alter processing of the amyloid precursor protein (APP) resulting in a relative increase of the amount of Abeta42 peptide. METHODS AND RESULTS: We present a patient with neuropathologically confirmed early-onset AD characterized by profound language impairment. The patient was heterozygous for a novel missense mutation in exon 11 of the PSEN2 gene leading to a predicted amino acid substitution from valine to methionine in position 393, a conserved residue. However, in vitro expression of PSEN2 V393M cDNA did not result in detectable increase of the secreted Abeta42/40 peptide ratio. The mutation was not found in 384 control individuals tested. CONCLUSIONS: The possible pathogenic nature of the mutation is not clarified. We discuss the limitations of functional PSEN2 studies and the challenges associated with genetic counselling of family members at risk.

Heterogeneity within a large kindred with frontotemporal dementia: a novel progranulin mutation.

BACKGROUND: Frontotemporal dementia (FTD) in several 17q21-linked families was recently explained by truncating mutations in the progranulin gene (GRN). OBJECTIVE: To determine the frequency of GRN mutations in a cohort of Caucasian patients with FTD without mutations in known FTD genes. METHODS: GRN was sequenced in a series of 78 independent FTD patients including 23 familial subjects. A different Calabrian dataset (109 normal control subjects and 96 FTD patients) was used to establish the frequency of the GRN mutation. RESULTS: A novel truncating GRN mutation (c.1145insA) was detected in a proband of an extended consanguineous Calabrian kindred. Segregation analysis of 70 family members revealed 19 heterozygous mutation carriers including 9 patients affected by FTD. The absence of homozygous carriers in a highly consanguineous kindred may indicate that the loss of both GRN alleles might lead to embryonic lethality. An extremely variable age at onset in the mutation carriers (more than five decades apart) is not explained by APOE genotypes or the H1/H2 MAPT haplotypes. Intriguingly, the mutation was excluded in four FTD patients belonging to branches with an autosomal dominant mode of inheritance of FTD, suggesting that another novel FTD gene accounts for the disease in the phenocopies. It is difficult to clinically distinguish phenocopies from GRN mutation carriers, except that language in mutation carriers was more severely compromised. CONCLUSION: The current results imply further genetic heterogeneity of frontotemporal dementia, as we detected only one GRN-linked family (about 1%). The value of discovering large kindred includes the possibility of a longitudinal study of GRN mutation carriers.

Intron 2 (T/C) CYP46 polymorphism is associated with Alzheimer's disease in Chinese patients.

BACKGROUND: Cholesterol metabolism has been implicated in the pathophysiology of Alzheimer's disease (AD), and cholesterol-related genes are plausible candidate genes for AD. Genetic association of CYP46A1 polymorphisms with AD had been under extensive investigations; however, observations on intron 2 T-->C (rs754203) generated inconclusive results. OBJECTIVE: To analyse an independent data set in a Chinese population to see whether the polymorphic site rs754203 of the CYP46A1 gene is associated with AD. METHODS: We analysed 130 sporadic AD patients and 110 healthy controls of the Southern Chinese origin. RESULTS: An association between the genotype frequency and AD was suggested in the general population (p = 0.047, odds ratio, OR = 1. 61, 95% confidence interval, CI = 0.96-2.70), while the association was most significant in the apolipoprotein E (ApoE) epsilon4-negative group (p = 0.004, OR = 2.54, 95% CI =1.31-4.95). Linkage disequilibrium block prediction results also favoured this association. Consistent with previous reports, intron 3 C-->T (rs4900442) polymorphism did not show any evidence of association; in our data set ApoEepsilon4 was confirmed to be a genetic risk factor for AD (p = 0.0016, OR = 2.76, 95% CI = 1.50-5.11).

Neprilysin activity and expression are controlled by nicastrin.

We recently demonstrated that the presenilin-dependent gamma-secretase complex regulates the expression and activity of neprilysin, one of the main enzymes that degrade the amyloid beta-peptide (Abeta) which accumulates in Alzheimer's disease. Here, we examined the influence of endogenous nicastrin (NCT), a member of the gamma-secretase complex, on neprilysin physiology. We show that nicastrin deficiency drastically lowers neprilysin expression, membrane-bound activity and mRNA levels, but it did not modulate the expression of two other putative Abeta-cleaving enzymes, endothelin-converting enzyme and insulin-degrading enzyme. Furthermore, we show that nicastrin restores neprilysin activity and expression in nicastrin-deficient, but not presenilin-deficient fibroblasts, indicating that the control of neprilysin necessitates the complete gamma-secretase complex harbouring its four reported components. Finally, we show that NCT expression peaked 24 h after NCT cDNA transfection of wild-type and NCT-/- fibroblasts, while neprilysin expression drastically increased only after 36 h and was maximal at 48 h. This delayed effect on neprilysin expression correlates well with our demonstration of an indirect gamma-secretase-dependent modulation of neprilysin at its transcriptional level.

Functional characterization of novel presenilin-2 variants identified in human breast cancers.

We identified in breast cancer cases two germline alterations, R62H and R71W, in presenilin-2 (PS-2), a gene involved in familial Alzheimer's disease (FAD). The role of these alleles in FAD is unclear, but neither allele affected Abeta(42)/Abeta(40) ratio. However, both R62H and R71W alterations compromised PS-2 function in Notch signaling in Caenorhabditis elegans and cell growth inhibition in mouse embryonic fibroblasts, and these effects were dependent on gene dosage. We found that both alterations enhanced the degradation of the PS-2 full-length protein, indicating that they may have a loss-of function effect. The effect of the R71W alteration was noticeably stronger, and we observed an almost threefold higher frequency of this allele in breast cancer cases versus controls, but this difference did not reach statistical significance. Nonetheless, these results collectively suggest that the novel PS-2 alleles described here, especially R71W, affect PS-2 function and may potentially confer a moderate risk of susceptibility to breast cancer.

The effects of APOE and tau gene variability on risk of frontotemporal dementia.

Frontotemporal dementia (FTD) is a complex dementing syndrome whose genetic/non genetic risk factors are mostly unknown. Aim of the present work was to investigate whether APOE and/or tau gene variability does affect the risk of FTD. A sample of FTD cases (sporadic: n = 54; familial: n = 46, one subject per family) was collected in a genetically homogeneous population (Calabria, southern Italy) and analyzed in comparison with an age- and sex-matched control group (n = 180) extracted from the same population. Logistic regression analysis showed that APOE gene variability affects the probability of disease, with allele epsilon4 increasing (exp(beta1) = 2.68 with [1.51-4.76] 95% confidence interval; p = 0.001) and allele epsilon2 decreasing (exp(beta1) = 0.28 with [0.12-0.66] 95% confidence interval; p = 0.003) the risk of FTD. On the contrary, tau gene variability was ineffectual (exp(beta1) non significantly different from 1 for either H1 or H2 haplotypes), although a small effect was observed by the H1 haplotype in increasing the protective effect of the epsilon2 allele (p = 0.007).

LRRK2 gene in Parkinson disease: mutation analysis and case control association study.

BACKGROUND: In addition to the four well-confirmed genes linked to early-onset Parkinson disease (PD) (SNCA, PARKIN, DJ-1, and PINK1), mutations in the leucine-rich repeat kinase 2 gene (LRRK2) have recently been identified in families with autosomal dominant late-onset PD. OBJECTIVE: To perform mutation analysis of LRRK2 in probands of families showing dominant inheritance of PD and to conduct a case control association study to test the hypothesis that common coding variations might be associated with increased susceptibility to PD. METHODS: All 51 LRRK2 coding exons were sequenced in 23 probands and the mutation frequencies were evaluated in 180 neurologically normal control subjects. For the association study the authors genotyped four coding LRRK2 polymorphisms in 250 normal control subjects and 121 patients with PD (predominantly white patients of Canadian origin), 84% of whom had age at onset before 50 years and 42% had a positive family history. RESULTS: The authors identified three probands with heterozygous LRRK2 mutations: two of them have the known G2019S substitution and one proband has a novel I1371V substitution. Mutation analysis of a large family demonstrated complete segregation of the G2019S with PD. However, there was no association between PD and any of the four polymorphisms at the allelic or genotypic levels (p > 0.17). Furthermore, the authors did not detect a modifying effect for any genotype or of APOE genotypes upon the age at onset in the PD group (p > 0.20). CONCLUSIONS: The results support the prior suggestion that LRRK2 mutations cause PD. The disease in the families reported here presents a phenotype indistinguishable from typical PD. All three families demonstrate a very variable age at onset that is not explained by APOE genotypes. The common coding variations in the LRRK2 gene neither constitute strong PD risk factors nor modify the age at onset; however, the possibility of a modest risk effect remains to be assessed in large datasets.

Familial Alzheimer disease: decreases in CSF Abeta42 levels precede cognitive decline.

CSF amyloid beta-peptide 42 (Abeta42) levels in presymptomatic subjects with pathogenic mutations in the PS1 gene are significantly lower than in an age-matched control group. Consequently, in these subjects, there is a window of opportunity estimated as at least 4 to 12 years to evaluate the ability of any putative prophylactic therapy to decrease, arrest, or reverse abnormalities in Abeta42 metabolism many years before clinical symptoms of Alzheimer disease are otherwise likely to occur.

An African American family with early-onset Alzheimer disease and an APP (T714I) mutation.

The occurrence of an APP T174I mutation is described in a large American family of African descent with Alzheimer disease. The clinical characteristics were an unusually early onset of disease (early 30s), similar to a previously reported age at onset of this mutation in an Austrian family. Distinct from that family, seizures and myoclonus were prominent features of the disease in this kindred.

[Familial fronto-temporal dementia with brain stem ubiquitin-positive neuronal inclusions]. / Démence fronto-temporale familiale avec inclusions marquées par l'anti-ubiquitine dans le tronc cérébral.

INTRODUCTION: Fronto-temporal dementias (FTD) were described a century ago on the macroscopic basis of frontal and/or temporal lobe atrophy. Progress in neuropathology, immunohistochemistry, biochemistry and genetics has since shown that they are heterogeneous entities, encompassing many different diseases with similar clinical presentations. A few, such as tauopathies due to mutations of the gene coding for tau protein (MAPtau form a well-defined group. Definition and grouping of other types of FTD is still problematic. MATERIAL AND METHOD: We studied a family where the mother and 4/8 children were affected with FTD. Clinical presentation was typical of FTD. Onset was ill-defined with early (at age 40 years or less) personality changes. The clinical course was protracted (about 30 years). For a long period, the patients were able to live in the community in spite of obvious signs such as hyperorality and loss of verbal initiative; operative orientation as to place was preserved for a long time: a mute patient was still able to drive. Signs of extrapyramidal or motoneuron involvement were not observed. RESULTS: The genetic study failed to detect any mutation in MAPtau; the lod score for flanking markers was positive but not significant. Biochemical study showed no qualitative abnormality in tau protein. Neuropathological study of one affected subject showed brain atrophy (962 g), with elective frontal lobe involvement. Cortical nerve cell loss was more marked in superficial layers and in frontal areas; glia was inconspicuous; pseudolaminar spongiosis was present in the more severely affected zones. No argentophilic "Pick bodies" were seen; ubiquitin-positive, tau-negative round inclusions were present in the cytoplasm of fascia dentata neurones. "Tangles" were mostly restricted to the entorhinal cortex, partly correlated with tau immunoreactivity, but better with ubiquitin immunoreactivity. Large, ovoid or reniform, moderately dense, spongy, granular or filamentous argentophilic cytoplasmic nerve cell inclusions were observed. They were ubiquitin-positive, but did not react with other antibodies, particularly anti-tau. They were present in swollen nerve cells in the deeper cortical layers but were most conspicuous in the brain stem: in the magnocellular reticular nuclei (e.g. nucleus centralis pontis), in the pes pontis, in the inferior olive and in motor nuclei, especially in the trigeminal motor nucleus. They were not associated with nerve cell loss, atrophy nor pycnosis. Cerebellar relay nuclei neurones were swollen, and their cytoplasm contained argentophilic filaments. CONCLUSION: In our opinion, "ubiquitinopathy" would be non-specific and "Motor Neuron Disease-Inclusion Dementia" (MNDID) would not be satisfactory as a diagnosis for the present cases of FTD. Hopefully, progress in genetics may allow a causal, and thence definitive, classification.

Fine mapping of 10q and 18q for familial Alzheimer's disease in Caribbean Hispanics.

Familial Alzheimer's disease (AD [MIM 104300]) has been a focus of intense investigation, primarily in Caucasian families from Europe and North America families. Although the late-onset form of familial AD, beginning after age 65 years, has been linked to regions on chromosomes 10q and 12p, the specific genetic variants have not yet been consistently identified. Using a unique cohort of families of Caribbean Hispanics ancestry, we screened the genome using 340 markers on 490 family members from 96 families with predominantly late-onset AD. We observed the strongest support for linkage on 18q (LOD=3.14). However, 17 additional markers (chromosomes 1-6, 8, 10, 12, and 14) exceeded a two-point LOD score of 1.0 under the affecteds-only autosomal dominant model or affected sibpair model. As we previously reported the fine-mapping effort on 12p showing modest evidence of linkage, we focused our fine-mapping efforts on two other candidate regions in the current report, namely 10q and 18q. We added 31 family members and eight additional Caribbean Hispanic families to fine map 10q and 18q. With additional microsatellite markers, the evidence for linkage for 18q strengthened near 112 cM, where the two-point LOD score for D18S541 was 3.37 and the highest NPL score in that region was 3.65 (P=0.000177). This narrow region contains a small number of genes expressed in the brain. However, at 10q (134-138 cM), the NPL score decreased from 3.15 (P=0.000486) to 2.1 (P=0.0218), but two broad peaks remained overlapping with previously reported peaks. Our results provide modest support for linkage on 10q and 12p in this cohort of Caribbean Hispanic families with familial Alzheimer's disease, and strong evidence for a new locus on 18q.

A new SPG4 mutation in a variant form of spastic paraplegia with congenital arachnoid cysts.

The clinical and genetic findings are described for 16 patients from a large Italian family with a variant form of hereditary spastic paraplegia and congenital arachnoid cysts inherited as an autosomal dominant trait. A molecular study has revealed a novel missense mutation, T614I, in exon 17 of SPG4, which may play a role in both focal cortical dysgenesis and neurodegeneration of the motor neurons in the corticospinal tract.

Baculoviruses expressing the human familial Alzheimer's disease presenilin 1 mutation lacking exon 9 increase levels of an amyloid beta-like protein in Sf9 cells.

Presenilin 1 (PS1) plays a pivotal role in the production of the amyloid-beta protein (Abeta) that is central to the pathogenesis of Alzheimer's disease. PS1 regulates the intramembranous proteolysis of a 99-amino-acid C-terminal fragment of the amyloid precursor protein (APP-C99), a cleavage event that releases Abeta following a reaction catalyzed by an enzyme termed 'gamma-secretase'. The molecular mechanism of PS1-mediated, gamma-secretase cleavage remains largely unresolved. In particular, controversy surrounds whether PS1 includes the catalytic site of the gamma-secretase protease or whether instead PS1 mediates gamma-secretase activity indirectly, perhaps by regulating the trafficking or presentation of substrates to the 'authentic' protease, which may be a molecule distinct from PS1. To address this issue, the baculovirus expression system was used to co-express: (i) APP-C99; (ii) a pathogenic, constitutively active mutant form of PS1 lacking exon 9 (PS1DeltaE9); (iii) nicastrin and (iv) tropomyosin in Spodoptera frugiperda (Sf9) cells. Cells infected with APP-C99 alone produced an Abeta-like species, and levels of this species were enhanced by the addition of baculoviruses bearing the PS1DeltaE9 mutation. The addition to APP-C99-infected cells of baculoviruses bearing nicastrin, also a transmembrane protein, had a neutral or inhibitory effect on the reaction; tropomyosin viruses had the same effect as nicastrin viruses. These results suggest that PS1DeltaE9 molecules expressed in Sf9 cells retain the ability to modulate Abeta levels. Baculoviral-expressed PS1DeltaE9 provides a source of microgram quantities of bioactive molecules for use as starting material for purifying and reconstituting gamma-secretase activity from its individual purified component parts.

PS1 Alzheimer's disease family with spastic paraplegia: the search for a gene modifier.

PS1 mutations are associated with classic Alzheimer's disease (AD); however, some families develop AD and spastic paraplegia (SP) with brain pathology characterized by Abeta cotton wool plaques. The authors report a variant AD family with the E280Q PS1 mutation. The fact that the same PS1 mutation can be found in patients with either variant or classic AD argues in favor of the presence of a genetic modifier. The authors have excluded that this modifier effect originates from coding sequence variations in three SP genes or from a second mutation in the other AD genes.