Confirmation of high frequency of C9orf72 mutations in patients with frontotemporal dementia from Sweden.

Frontotemporal dementia (FTD) is the second most common early-onset dementia. Up to half of the cases are familial, and several mutations have been identified as pathogenic. Repeat expansion mutations in C9orf72 are the most common genetic cause of FTD and are particularly frequent in Sweden and Finland. We aimed to determine the mutation frequency in patients with FTD ascertained at a memory clinic in Sweden and assess the inheritance pattern in the families. We screened 132 patients with FTD for mutations in C9orf72, GRN, and MAPT, and the frequency was 34.1%. Two novel variations, not previously published, were found; a pathogenic GRN mutation and a MAPT variation in intron 9 that we report as VUS. The likelihood of finding a mutation was highest in patients with a clear family history of dementia or motor neuron disease (76%), but mutations were also found in apparent sporadic cases. This confirms that FTD cohorts from Sweden have a relatively higher risk of an underlying mutation in all risk categories compared with other reported cohorts.

Identification and description of three families with familial Alzheimer disease that segregate variants in the SORL1 gene.

Alzheimer disease (AD) is a progressive neurodegenerative disorder and the most common form of dementia. The majority of AD cases are sporadic, while up to 5% are families with an early onset AD (EOAD). Mutations in one of the three genes: amyloid beta precursor protein (APP), presenilin 1 (PSEN1) or presenilin 2 (PSEN2) can be disease causing. However, most EOAD families do not carry mutations in any of these three genes, and candidate genes, such as the sortilin-related receptor 1 (SORL1), have been suggested to be potentially causative. To identify AD causative variants, we performed whole-exome sequencing on five individuals from a family with EOAD and a missense variant, p.Arg1303Cys (c.3907C > T) was identified in SORL1 which segregated with disease and was further characterized with immunohistochemistry on two post mortem autopsy cases from the same family. In a targeted re-sequencing effort on independent index patients from 35 EOAD-families, a second SORL1 variant, c.3050-2A > G, was found which segregated with the disease in 3 affected and was absent in one unaffected family member. The c.3050-2A > G variant is located two nucleotides upstream of exon 22 and was shown to cause exon 22 skipping, resulting in a deletion of amino acids Gly1017- Glu1074 of SORL1. Furthermore, a third SORL1 variant, c.5195G > C, recently identified in a Swedish case control cohort included in the European Early-Onset Dementia (EU EOD) consortium study, was detected in two affected siblings in a third family with familial EOAD. The finding of three SORL1-variants that segregate with disease in three separate families with EOAD supports the involvement of SORL1 in AD pathology. The cause of these rare monogenic forms of EOAD has proven difficult to find and the use of exome and genome sequencing may be a successful route to target them.

Mutation screening of patients with Alzheimer disease identifies APP locus duplication in a Swedish patient.

BACKGROUND: Missense mutations in three different genes encoding amyloid-ß precursor protein, presenilin 1 and presenilin 2 are recognized to cause familial early-onset Alzheimer disease. Also duplications of the amyloid precursor protein gene have been shown to cause the disease. At the Dept. of Geriatric Medicine, Karolinska University Hospital, Sweden, patients are referred for mutation screening for the identification of nucleotide variations and for determining copy-number of the APP locus. METHODS: We combined the method of microsatellite marker genotyping with a quantitative real-time PCR analysis to detect duplications in patients with Alzheimer disease. RESULTS: In 22 DNA samples from individuals diagnosed with clinical Alzheimer disease, we identified one patient carrying a duplication on chromosome 21 which included the APP locus. Further mapping of the chromosomal region by array-comparative genome hybridization showed that the duplication spanned a maximal region of 1.09 Mb. CONCLUSIONS: This is the first report of an APP duplication in a Swedish Alzheimer patient and describes the use of quantitative real-time PCR as a tool for determining copy-number of the APP locus.