Gerard D. Schellenberg

Secreted amyloid β-protein similar to that in the senile plaques of Alzheimer's disease is increased in vivo by the presenilin 1 and 2 and APP mutations linked to familial Alzheimer's disease

To determine whether the presenilin 1 (PS1), presenilin 2 (PS2) and amyloid β-protein precursor (APP) mutations linked to familial Alzheimers disease (FAD) increase the extracellular concentration of amyloid β–protein (Aβ) ending at Aβ42(43) in vivo, we performed a blinded comparison of plasma Aβ levels in carriers of these mutations and controls. Aβ1 –42(43) was elevated in plasma from subjects with FAD–linked PS1 (P < 0.0001), PS2N141I (P = 0.009), APPK670N,M671L (P < 0.0001), and APPV717I (one subject) mutations. Aβ ending at Aβ42(43) was also significantly elevated in fibroblast media from subjects with PS1 (P < 0.0001) or P52 (P = 0.03) mutations. These findings indicate that the FAD–linked mutations may all cause Alzheimers disease by increasing the extracellular concentration of Aβ42(43), thereby fostering cerebral deposition of this highly amyloidogenic peptide.

Positional Cloning of the Werner's Syndrome Gene

Werners syndrome (WS) is an inherited disease with clinical symptoms resembling premature aging. Early susceptibility to a number of major age-related diseases is a key feature of this disorder. The gene responsible for WS (known as WRN) was identified by positional cloning. The predicted protein is 1432 amino acids in length and shows significant similarity to DNA helicases. Four mutations in WS patients were identified. Two of the mutations are splice-junction mutations, with the predicted result being the exclusion of exons from the final messenger RNA. One of these mutations, which results in a frameshift and a predicted truncated protein, was found in the homozygous state in 60 percent of Japanese WS patients examined. The other two mutations are nonsense mutations. The identification of a mutated putative helicase as the gene product of the WS gene suggests that defective DNA metabolism is involved in the complex process of aging in WS patients.

Patterns and rates of exonic de novo mutations in autism spectrum disorders

Autism spectrum disorders (ASD) are believed to have genetic and environmental origins, yet in only a modest fraction of individuals can specific causes be identified. To identify further genetic risk factors, here we assess the role of de novo mutations in ASD by sequencing the exomes of ASD cases and their parents (n = 175 trios). Fewer than half of the cases (46.3%) carry a missense or nonsense de novo variant, and the overall rate of mutation is only modestly higher than the expected rate. In contrast, the proteins encoded by genes that harboured de novo missense or nonsense mutations showed a higher degree of connectivity among themselves and to previous ASD genes as indexed by protein-protein interaction screens. The small increase in the rate of de novo events, when taken together with the protein interaction results, are consistent with an important but limited role for de novo point mutations in ASD, similar to that documented for de novo copy number variants. Genetic models incorporating these data indicate that most of the observed de novo events are unconnected to ASD; those that do confer risk are distributed across many genes and are incompletely penetrant (that is, not necessarily sufficient for disease). Our results support polygenic models in which spontaneous coding mutations in any of a large number of genes increases risk by 5- to 20-fold. Despite the challenge posed by such models, results from de novo events and a large parallel case–control study provide strong evidence in favour of CHD8 and KATNAL2 as genuine autism risk factors.

Autism genome-wide copy number variation reveals ubiquitin and neuronal genes

Autism spectrum disorders (ASDs) are childhood neurodevelopmental disorders with complex genetic origins. Previous studies focusing on candidate genes or genomic regions have identified several copy number variations (CNVs) that are associated with an increased risk of ASDs. Here we present the results from a whole-genome CNV study on a cohort of 859 ASD cases and 1,409 healthy children of European ancestry who were genotyped with ∼550,000 single nucleotide polymorphism markers, in an attempt to comprehensively identify CNVs conferring susceptibility to ASDs. Positive findings were evaluated in an independent cohort of 1,336 ASD cases and 1,110 controls of European ancestry. Besides previously reported ASD candidate genes, such as NRXN1 (ref. 10) and CNTN4 (refs 11, 12), several new susceptibility genes encoding neuronal cell-adhesion molecules, including NLGN1 and ASTN2, were enriched with CNVs in ASD cases compared to controls (P = 9.5 × 10-3). Furthermore, CNVs within or surrounding genes involved in the ubiquitin pathways, including UBE3A, PARK2, RFWD2 and FBXO40, were affected by CNVs not observed in controls (P = 3.3 × 10-3). We also identified duplications 55 kilobases upstream of complementary DNA AK123120 (P = 3.6 × 10-6). Although these variants may be individually rare, they target genes involved in neuronal cell-adhesion or ubiquitin degradation, indicating that these two important gene networks expressed within the central nervous system may contribute to the genetic susceptibility of ASD.

Common genetic variants on 5p14.1 associate with autism spectrum disorders

Autism spectrum disorders (ASDs) represent a group of childhood neurodevelopmental and neuropsychiatric disorders characterized by deficits in verbal communication, impairment of social interaction, and restricted and repetitive patterns of interests and behaviour. To identify common genetic risk factors underlying ASDs, here we present the results of genome-wide association studies on a cohort of 780 families (3,101 subjects) with affected children, and a second cohort of 1,204 affected subjects and 6,491 control subjects, all of whom were of European ancestry. Six single nucleotide polymorphisms between cadherin 10 (CDH10) and cadherin 9 (CDH9)—two genes encoding neuronal cell-adhesion molecules—revealed strong association signals, with the most significant SNP being rs4307059 (P = 3.4 × 10-8, odds ratio = 1.19). These signals were replicated in two independent cohorts, with combined P values ranging from 7.4 × 10-8 to 2.1 × 10-10. Our results implicate neuronal cell-adhesion molecules in the pathogenesis of ASDs, and represent, to our knowledge, the first demonstration of genome-wide significant association of common variants with susceptibility to ASDs.

A familial Alzheimer's disease locus on chromosome 1

The Volga German kindreds are a group of seven related families with autosomal dominant early-onset Alzheimers disease (AD). Linkage to known AD-related loci on chromosomes 21 and 14 has been excluded. Significant evidence for linkage to AD in these families was obtained with D1S479 and there was also positive evidence for linkage with other markers in the region. A 112-base pair allele of D1S479 co-segregated with the disease in five of seven families, which is consistent with a common genetic founder. This study demonstrates the presence of an AD locus on chromosome 1q31-42.

Frequency of the C9orf72 hexanucleotide repeat expansion in patients with amyotrophic lateral sclerosis and frontotemporal dementia: A cross-sectional study

Summary Background We aimed to accurately estimate the frequency of a hexanucleotide repeat expansion in C9orf72 that has been associated with a large proportion of cases of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Methods We screened 4448 patients diagnosed with ALS (El Escorial criteria) and 1425 patients with FTD (Lund-Manchester criteria) from 17 regions worldwide for the GGGGCC hexanucleotide expansion using a repeat-primed PCR assay. We assessed familial disease status on the basis of self-reported family history of similar neurodegenerative diseases at the time of sample collection. We compared haplotype data for 262 patients carrying the expansion with the known Finnish founder risk haplotype across the chromosomal locus. We calculated age-related penetrance using the Kaplan-Meier method with data for 603 individuals with the expansion. Findings In patients with sporadic ALS, we identified the repeat expansion in 236 (7·0%) of 3377 white individuals from the USA, Europe, and Australia, two (4·1%) of 49 black individuals from the USA, and six (8·3%) of 72 Hispanic individuals from the USA. The mutation was present in 217 (39·3%) of 552 white individuals with familial ALS from Europe and the USA. 59 (6·0%) of 981 white Europeans with sporadic FTD had the mutation, as did 99 (24·8%) of 400 white Europeans with familial FTD. Data for other ethnic groups were sparse, but we identified one Asian patient with familial ALS (from 20 assessed) and two with familial FTD (from three assessed) who carried the mutation. The mutation was not carried by the three Native Americans or 360 patients from Asia or the Pacific Islands with sporadic ALS who were tested, or by 41 Asian patients with sporadic FTD. All patients with the repeat expansion had (partly or fully) the founder haplotype, suggesting a one-off expansion occurring about 1500 years ago. The pathogenic expansion was non-penetrant in individuals younger than 35 years, 50% penetrant by 58 years, and almost fully penetrant by 80 years. Interpretation A common Mendelian genetic lesion in C9orf72 is implicated in many cases of sporadic and familial ALS and FTD. Testing for this pathogenic expansion should be considered in the management and genetic counselling of patients with these fatal neurodegenerative diseases. Funding Full funding sources listed at end of paper (see Acknowledgments).

Cerebrospinal fluid and plasma insulin levels in Alzheimer's disease Relationship to severity of dementia and apolipoprotein E genotype

Patients with Alzheimers disease (AD) have elevations of fasting plasma insulin that are hypothesized to be associated with disrupted brain insulin metabolism. We examined paired fasted plasma and CSF insulin levels in 25 patients with AD and 14 healthy age-matched adults and determined whether insulin levels were related to severity of dementia and apolipoprotein E-ϵ4 homozygosity, a known genetic risk factor for AD. The AD patients had lower CSF insulin, higher plasma insulin, and a reduced CSF-to-plasma insulin ratio when compared with healthy adults. The differences were greater for patients with more advanced AD. Patients who were not apolipoprotein E-ϵ4 homozygotes had higher plasma insulin levels and reduced CSF-to-plasma ratios, whereas ϵ4 homozygotes with AD had normal values. Both plasma and CSF insulin levels are abnormal in AD, and there are metabolic differences among apolipoprotein E genotypes.

Effects of intranasal insulin on cognition in memory-impaired older adults: modulation by APOE genotype.

Raising insulin acutely in the periphery and in brain improves verbal memory. Intranasal insulin administration, which raises insulin acutely in the CNS without raising plasma insulin levels, provides an opportunity to determine whether these effects are mediated by central insulin or peripheral processes. Based on prior research with intravenous insulin, we predicted that the treatment response would differ between subjects with (epsilon4+) and without (epsilon4-) the APOE-epsilon4 allele. On separate mornings, 26 memory-impaired subjects (13 with early Alzheimers disease and 13 with amnestic mild cognitive impairment) and 35 normal controls each underwent three intranasal treatment conditions consisting of saline (placebo) or insulin (20 or 40 IU). Cognition was tested 15 min post-treatment, and blood was acquired at baseline and 45 min after treatment. Intranasal insulin treatment did not change plasma insulin or glucose levels. Insulin treatment facilitated recall on two measures of verbal memory in memory-impaired epsilon4- adults. These effects were stronger for memory-impaired epsilon4- subjects than for memory-impaired epsilon4+ subjects and normal adults. Unexpectedly, memory-impaired epsilon4+ subjects showed poorer recall following insulin administration on one test of memory. These findings suggest that intranasal insulin administration may have therapeutic benefit without the risk of peripheral hypoglycemia and provide further evidence for apolipoprotein E (APOE) related differences in insulin metabolism.

A framework for the interpretation of de novo mutation in human disease

Spontaneously arising (de novo) mutations have an important role in medical genetics. For diseases with extensive locus heterogeneity, such as autism spectrum disorders (ASDs), the signal from de novo mutations is distributed across many genes, making it difficult to distinguish disease-relevant mutations from background variation. Here we provide a statistical framework for the analysis of excesses in de novo mutation per gene and gene set by calibrating a model of de novo mutation. We applied this framework to de novo mutations collected from 1,078 ASD family trios, and, whereas we affirmed a significant role for loss-of-function mutations, we found no excess of de novo loss-of-function mutations in cases with IQ above 100, suggesting that the role of de novo mutations in ASDs might reside in fundamental neurodevelopmental processes. We also used our model to identify ∼1,000 genes that are significantly lacking in functional coding variation in non-ASD samples and are enriched for de novo loss-of-function mutations identified in ASD cases.