Joanne Norton

Association of missense and 5 '-splice-site mutations in tau with the inherited dementia FTDP-17

Thirteen families have been described with an autosomal dominantly inherited dementia named frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17), historically termed Picks disease. Most FTDP-17 cases show neuronal and/or glial inclusions that stain positively with antibodies raised against the microtubule-associated protein Tau, although the Tau pathology varies considerably in both its quantity (or severity) and characteristics,. Previous studies have mapped the FTDP-17 locus to a 2-centimorgan region on chromosome 17q21.11; the tau gene also lies within this region. We have now sequenced tau in FTDP-17 families and identified three missense mutations (G272V, P301L and R406W) and three mutations in the 5′ splice site of exon 10. The splice-site mutations all destabilize a potential stem–loop structure which is probably involved in regulating the alternative splicing of exon10 (ref. 13). This causes more frequent usage of the 5′ splice site and an increased proportion of tau transcripts that include exon 10. The increase in exon 10+ messenger RNA will increase the proportion of Tau containing four microtubule-binding repeats, which is consistent with the neuropathology described in several families with FTDP-17 (refs 12, 14).

TDP-43 A315T mutation in familial motor neuron disease

To identify novel causes of familial neurodegenerative diseases, we extended our previous studies of TAR DNA‐binding protein 43 (TDP‐43) proteinopathies to investigate TDP‐43 as a candidate gene in familial cases of motor neuron disease. Sequencing of the TDP‐43 gene led to the identification of a novel missense mutation, Ala‐315‐Thr, which segregates with all affected members of an autosomal dominant motor neuron disease family. The mutation was not found in 1,505 healthy control subjects. The discovery of a missense mutation in TDP‐43 in a family with dominantly inherited motor neuron disease provides evidence of a direct link between altered TDP‐43 function and neurodegeneration. Ann Neurol 2008

Rare coding variants in the phospholipase D3 gene confer risk for Alzheimer's disease

Genome-wide association studies (GWAS) have identified several risk variants for late-onset Alzheimers disease (LOAD). These common variants have replicable but small effects on LOAD risk and generally do not have obvious functional effects. Low-frequency coding variants, not detected by GWAS, are predicted to include functional variants with larger effects on risk. To identify low-frequency coding variants with large effects on LOAD risk, we carried out whole-exome sequencing (WES) in 14 large LOAD families and follow-up analyses of the candidate variants in several large LOAD case–control data sets. A rare variant in PLD3 (phospholipase D3; Val232Met) segregated with disease status in two independent families and doubled risk for Alzheimer’s disease in seven independent case–control series with a total of more than 11,000 cases and controls of European descent. Gene-based burden analyses in 4,387 cases and controls of European descent and 302 African American cases and controls, with complete sequence data for PLD3, reveal that several variants in this gene increase risk for Alzheimer’s disease in both populations. PLD3 is highly expressed in brain regions that are vulnerable to Alzheimer’s disease pathology, including hippocampus and cortex, and is expressed at significantly lower levels in neurons from Alzheimer’s disease brains compared to control brains. Overexpression of PLD3 leads to a significant decrease in intracellular amyloid-β precursor protein (APP) and extracellular Aβ42 and Aβ40 (the 42- and 40-residue isoforms of the amyloid-β peptide), and knockdown of PLD3 leads to a significant increase in extracellular Aβ42 and Aβ40. Together, our genetic and functional data indicate that carriers of PLD3 coding variants have a twofold increased risk for LOAD and that PLD3 influences APP processing. This study provides an example of how densely affected families may help to identify rare variants with large effects on risk for disease or other complex traits.

HDDD2 is a familial frontotemporal lobar degeneration with ubiquitin‐positive, tau‐negative inclusions caused by a missense mutation in the signal peptide of progranulin

Familial autosomal dominant frontotemporal dementia with ubiquitin‐positive, tau‐negative inclusions in the brain linked to 17q21‐22 recently has been reported to carry null mutations in the progranulin gene (PGRN). Hereditary dysphasic disinhibition dementia (HDDD) is a frontotemporal dementia with prominent changes in behavior and language deficits. A previous study found significant linkage to chromosome 17 in a HDDD family (HDDD2), but no mutation in the MAPT gene. Longitudinal follow‐up has enabled us to identify new cases and to further characterize the dementia in this family. The goals of this study were to develop research criteria to classify the different clinical expressions of dementia observed in this large kindred, to identify the causal mutation in affected individuals and correlate this with phenotypic characteristics in this pedigree, and to assess the neuropathological characteristics using immunohistochemical techniques.

Hereditary dysphasic disinhibition dementia: a frontotemporal dementia linked to 17q21-22

Objective The clinical and pathologic features of hereditary dysphasic disinhibition dementia (HDDD) are described to determine whether it is a variant of known dementias. Background Several dementing disorders have clinical and pathologic similarities with AD, Picks disease, and the “nonspecific” dementias. A detailed description of clinical and pathologic presentation will aid classification, but ultimately the discovery of causative gene(s) will define these disorders. Methods The authors performed a clinical assessment gross and microscopic pathologic evaluation of brain tissue, genetic linkage studies, and sequence analyses. Results HDDD is an autosomal-dominant frontotemporal dementia with many similarities to Picks disease. Salient clinical features are global dementia with disproportionate dysphasia and “frontotemporal” symptoms. A linkage between HDDD and 17q 21–22 was shown, with a maximum lod score of 3.68 at zero recombination. Conclusions Several dementias have been linked to the same region and have been termed frontotemporal dementia with parkinsonism linked to chromosome 17. These disorders may represent phenotypic variants arising from mutations within a common gene.

Apolipoprotein Eε4 modifies Alzheimer's disease onset in an E280A PS1 kindred

We previously have identified a large kindred from Colombia in which Alzheimers disease (AD) is caused by the E280A presenilin 1 (PS1) mutation. The objective of this study was to examine whether environmental and genetic factors are responsible for variation in the phenotypic expression of the E280A PS1 mutation. We genotyped coding and promoter polymorphisms of the APOE gene in carriers of the E280A PS1 mutation. Kaplan–Meier product‐limit and Cox proportional hazard models were used in the statistical analyses. DNA was available from 114 carriers of the E280A PS1 mutation, including 52 subjects with AD. APOE ε4 allele carriers were more likely to develop AD at an earlier age than subjects without the ε4 allele (hazard ratio, 2.07; 95% confidence interval, 1.07–3.99; p = 0.030). Subjects with low education were more likely to develop AD later than those with higher education (hazard ratio, 0.476; 95% confidence interval, 0.26–0.87). Low educational level was associated with rural residence (p < 0.001). Promoter APOE variants did not influence either the onset or the duration of the disease. This study is the first to our knowledge to demonstrate that genetic and environmental factors influence age of onset in a kindred with a familial AD mutation. Ann Neurol 2003

E280A PS-1 mutation causes Alzheimer's disease but age of onset is not modified by ApoE alleles

A single base substitution of a glutamic acid to an alanine codon 280 was found in the presenilin‐1 (PS‐1) gene on chromosome 14 in affected individuals in each of seven Colombian early‐onset Alzheimers disease (AD) kindreds. The mutation segregated with disease in kindreds tested. In the largest kindred (C2), the maximum two‐point lod score between the mutation and AD was Z = 8.14 at θ = 0. The presence of a single mutation and the common geographic origin, with all families from the state of Antioquia, suggest a founder effect in this population. This finding is supported by the observation of a rare haplotype inherited with AD in all kindreds. These kindreds form the largest collection of AD cases with the same PS‐1 mutation and the same educational, environmental, and ethnic background in which to study the phenotypic effect of putative risk factors, such as the ϵ4 allele of apolipoprotein E (ApoE) or head trauma. Of the few AD cases having a history of head trauma, the age of onset was not lowered. No effect of ApoE genotype on the age of onset was detected. Previous investigations of the effect of ApoE genotype on the age of onset were confounded by small patient numbers, familial clustering of ApoE genotypes, and combining data from unrelated families with different mutations. Hum Mutat 10:186–195, 1997.

Exome-Sequencing Confirms DNAJC5 Mutations as Cause of Adult Neuronal Ceroid-Lipofuscinosis

We performed whole-exome sequencing in two autopsy-confirmed cases and an elderly unaffected control from a multigenerational family with autosomal dominant neuronal ceroid lipofuscinosis (ANCL). A novel single-nucleotide variation (c.344T>G) in the DNAJC5 gene was identified. Mutational screening in an independent family with autosomal dominant ANCL found an in-frame single codon deletion (c.346_348 delCTC) resulting in a deletion of p.Leu116del. These variants fulfill all genetic criteria for disease-causing mutations: they are found in unrelated families with the same disease, exhibit complete segregation between the mutation and the disease, and are absent in healthy controls. In addition, the associated amino acid substitutions are located in evolutionarily highly conserved residues and are predicted to functionally affect the encoded protein (CSPα). The mutations are located in a cysteine-string domain, which is required for membrane targeting/binding, palmitoylation, and oligomerization of CSPα. We performed a comprehensive in silico analysis of the functional and structural impact of both mutations on CSPα. We found that these mutations dramatically decrease the affinity of CSPα for the membrane. We did not identify any significant effect on palmitoylation status of CSPα. However, a reduction of CSPα membrane affinity may change its palmitoylation and affect proper intracellular sorting. We confirm that CSPα has a strong intrinsic aggregation propensity; however, it is not modified by the mutations. A complementary disease-network analysis suggests a potential interaction with other NCLs genes/pathways. This is the first replication study of the identification of DNAJC5 as the disease-causing gene for autosomal dominant ANCL. The identification of the novel gene in ANCL will allow us to gain a better understanding of the pathological mechanism of ANCLs and constitutes a great advance toward the development of new molecular diagnostic tests and may lead to the development of potential therapies.

Molecular characterization of novel progranulin (GRN) mutations in frontotemporal dementia

Frontotemporal dementia (FTD) is a clinical term encompassing dementia characterized by the presence of two major phenotypes: 1) behavioral and personality disorder, and 2) language disorder, which includes primary progressive aphasia and semantic dementia. Recently, the gene for familial frontotemporal lobar degeneration (FTLD) with ubiquitin‐positive, tau‐negative inclusions (FTLD‐U) linked to chromosome 17 was cloned. In the present study, 62 unrelated patients from the Washington University Alzheimers Disease Research Center and the Midwest Consortium for FTD with clinically diagnosed FTD and/or neuropathologically characterized cases of FTLD‐U with or without motor neuron disease (MND) were screened for mutations in the progranulin gene (GRN; also PGRN). We discovered two pathogenic mutations in four families: 1) a single‐base substitution within the 3′ splice acceptor site of intron 6/exon 7 (g.5913A>G [IVS6–2A>G]) causing skipping of exon 7 and premature termination of the coding sequence (PTC); and 2) a missense mutation in exon 1 (g.4068C>A) introducing a charged amino acid in the hydrophobic core of the signal peptide at residue 9 (p.A9D). Functional analysis in mutation carriers for the splice acceptor site mutation revealed a 50% decrease in GRN mRNA and protein levels, supporting haploinsufficiency. In contrast, there was no significant difference in the total GRN mRNA between cases and controls carrying the p.A9D mutation. Further, subcellular fractionation and confocal microscopy indicate that although the mutant protein is expressed, it is not secreted, and appears to be trapped within an intracellular compartment, possibly resulting in a functional haploinsufficiency. Hum Mutat 29(4), 512–521, 2008.

Novel haplotypes in 17q21 are associated with progressive supranuclear palsy.

Progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD) are sporadic neurodegenerative diseases presenting as atypical parkinsonian disorders, characterized by the presence of tau‐positive neurofibrillary tangles. Recently, an extended haplotype (H1E) of 787.6kb that comprises several genes including MAPT showed increased association with PSP. The objective of this study was to determine the size of the H1E haplotype associated with PSP and CBD in different populations and to identify specific subhaplotypes in the background of H1E haplotype. Nineteen single nucleotide polymorphisms (SNPs) in the 17q21 region were genotyped in two case–control samples. The SNPs that were associated with higher risk for the disease in the homozygous state delimit a region of more that 1Mb. Haplotype analyses in the Spanish sample showed that the most frequent haplotype found among the patients (H1E′), which extends 1.04Mb and contains several genes such as MAPT, CRHR1, IMP5, Saitohin, WTN3, and NSF. A specific subhaplotype (H1E′A) was present in 16% of PSP patients but was not observed in the controls. Furthermore, the H2E′A haplotype, was rarely present in the disease group suggesting that it plays a protective role. The identification of these specific subhaplotypes that modify risk for PSP/CBD supports the hypothesis that a pathogenic allele exists in a subgroup of PSP patients. Ann Neurol 2004;56:249–258