Elsevier

Neurobiology of Aging

Volume 34, Issue 6, June 2013, Pages 1712.e1-1712.e7
Neurobiology of Aging

Genetic reports abstract
C9orf72 G4C2 repeat expansions in Alzheimer's disease and mild cognitive impairment

https://doi.org/10.1016/j.neurobiolaging.2012.12.019Get rights and content

Abstract

C9orf72 G4C2 repeat expansion is a major cause of amyotrophic lateral sclerosis and frontotemporal lobar degeneration. Its role in Alzheimer's disease (AD) is less clear. We assessed the prevalence of G4C2 pathogenic repeat expansions in Flanders-Belgian patients with clinical AD or mild cognitive impairment (MCI). In addition, we studied the effect of non-pathogenic G4C2 repeat length variability on susceptibility to AD, and on AD cerebrospinal fluid (CSF) biomarker levels. A pathogenic repeat expansion was identified in 5 of 1217 AD patients (frequency <1%). No pathogenic expansions were observed in patients with MCI (n = 200) or control individuals (n = 1119). Nonpathogenic repeat length variability was not associated with AD, risk of conversion to AD in MCI individuals, or CSF biomarker levels. We conclude that pathogenic C9orf72 G4C2 repeat expansions can be detected in clinical AD patients and could act as a contributor to AD pathogenesis. Non-pathogenic repeat length variability did not affect risk of AD or MCI, nor AD biomarker levels in CSF, indicating that C9orf72 is not a direct AD risk factor.

Introduction

A pathogenic expansion of a hexanucleotide (G4C2) repeat in the regulatory region of C9orf72 (Fig. 1) was recently identified as a major cause of disease in the amyotrophic lateral sclerosis (ALS)–frontotemporal lobar degeneration (FTLD) spectrum of neurodegeneration (DeJesus-Hernandez et al., 2011, Gijselinck et al., 2012, Renton et al., 2011). C9orf72 repeat expansions exceeding 30 G4C2 units have been suggested to be pathological (Renton et al., 2011), although a much higher number of G4C2 repeats have been detected, depending on the sensitivity of the molecular genetics technology used to size the G4C2 repeat itself (i.e. fluorescence in situ hybridization [FISH] and Southern blot assay) (DeJesus-Hernandez et al., 2011, Renton et al., 2011). A heterozygous pathogenic G4C2 repeat expansion in C9orf72 explains 4% to 8% of patients with isolated ALS or FTLD, and 25% to 40% of familial ALS or FTLD worldwide (Majounie et al., 2012b), but is particularly common in patients or families with a mixed phenotype of both FTLD and ALS, explaining up to 85% of the occurrence of disease (Gijselinck et al., 2012). In line with this, detailed clinical investigations on cohorts of pathogenic repeat expansion carriers demonstrate a relatively high prevalence of cognitive dysfunction and frontal symptoms in mutation carriers with ALS, and of motor neuron disease in mutation carriers with FTLD (Chio et al., 2012, Mahoney et al., 2012). Nevertheless, substantial clinical heterogeneity is observed between mutation carriers and within families, providing evidence for a wider phenotypic spectrum associated with this genetic defect (Mahoney et al., 2012, Stewart et al., 2012). Psychosis or other psychiatric symptoms appear to be prominent features (Arighi et al., 2012, Mahoney et al., 2012, Snowden et al., 2012). Pathogenic repeat expansions have been identified in patients with atypical clinical presentation, i.e. clinically diagnosed with olivopontocerebellar degeneration, atypical Parkinsonian syndrome, corticobasal syndrome (Lindquist et al., 2012) or slowly progressive behavioral variant FTD (Khan et al., 2012). Also, a family-based study detected a pathogenic repeat expansion in 6 patients clinically diagnosed with probable AD in 3 of 342 families (Majounie et al., 2012a). In a second series of 114 AD patients with early age at onset and cerebrospinal fluid (CSF) biomarker profile typical of AD, 3 carried a G4C2 expansion (Wallon et al., 2012). In 2 additional series of 568 and 424 AD patients, no pathogenic repeat expansions were detected (Rollinson et al., 2012, Xi et al., 2012). An additional cohort of 1184 AD patients was recently investigated by Kohli et al. and resulted in the identification of 9 patients carrying the G4C2 expansion of which 3 were autopsy confirmed (Kohli et al., 2012). These studies suggest that, albeit rare, C9orf72 repeat expansions may underlie a spectrum of neurodegenerative brain disease phenotypes, such as Alzheimer's disease (AD), with repercussions for clinical decision making. A general role of C9orf72 in different neurodegenerative diseases was recently proposed (Satoh et al., 2012), because of co-expression of C9orf72 and UBQLN1 in dystrophic neurites distributed in the CA1 region and the molecular layer in the hippocampus of AD and non-AD brains, suggesting that C9orf72 in dystrophic neurites is involved in the homeostasis of protein degradation by acting together with UBQLN1 (Satoh et al., 2012).

In this study, we assess the prevalence of pathogenic C9orf72 repeat expansions in a large prospective cohort of Flanders-Belgian patients with clinical diagnosis of AD (n = 1217) or mild cognitive impairment (MCI; n = 200), an intermediate stage of cognitive decline that may convert to AD. We extend these analyses to explore whether repeat expansions of intermediate length might convey risk to develop AD and/or could influence the CSF biomarker levels of amyloid-β peptide (Aβ1-42), total tau (T-tau), and tau phosphorylated at threonine 181 (P-tau181P).

Section snippets

Patient/control cohort

The Flanders-Belgian study cohort consisted of 1417 patients, 1217 AD patients (mean age at onset, 74.5 ± 8.9 years; 65.2% women) and 200 MCI individuals (mean age at onset, 72.4 ± 8.8 years; 50.7% women), and another group of 1119 healthy control individuals (mean age at inclusion, 65.1 ± 13.6 years; 56.5% women).

The majority of the AD and MCI cohort was ascertained at the Memory Clinic of the ZNA Middelheim and Hoge Beuken, Antwerp, Belgium (P.P.D.D. and S.E.) as part of a prospective study

C9orf72 pathogenic G4C2 repeat expansions

In the AD cohort of Flanders-Belgian origin we detected a pathological G4C2 repeat expansion in 5 out of 1217 patients, hence a mutation frequency of 0.41%. All pathogenic G4C2 expansion carriers had an expansion of >60 units. In the MCI cohort, no pathological G4C2 repeat expansions were observed. The age at disease onset of the 5 pathogenic G4C2 expansion carriers ranged from 56 to 80 years (Table 1). The 5 patients were clinically diagnosed with AD. Patient DR212.1 showed signs of frontal

Discussion

C9orf72 hexanucleotide repeat expansion has recently been identified as a major cause of FTLD and ALS (DeJesus-Hernandez et al., 2011, Gijselinck et al., 2012, Renton et al., 2011). In the present study we investigated the frequency of the pathological C9orf72 G4C2 expansion in a prospective Flanders-Belgian cohort of patients with a clinical diagnosis of AD (n = 1217) or MCI (n = 200), and we assessed the potential role of intermediate C9orf72 repeat expansion in conferring risk for developing

Conclusion

In conclusion, our data underscore that C9orf72 G4C2 expansion could lead to a spectrum of neurodegenerative brain disease phenotypes and, in a small percentage of cases, could act as a contributor to AD pathogenesis. The genetic screening of C9orf72 G4C2 expansion in AD patients, both with typical clinical presentation and differential diagnosis with atypical symptoms (i.e., frontal dysfunction, paranoid delusion), could be an additional tool to establish an even more cautious clinical

Disclosure statement

The authors declare that they have no conflicts of interest.

Acknowledgements

The authors acknowledge the important participation of patients and their relatives as well as control individuals. Research in the authors' group is funded in part by the Interuniversity Attraction Poles program P7/16 of the Belgian Science Policy Office (BELSPO, http://www.belspo.be/), the Foundation for Alzheimer Research (SAO/FRMA, http://alzh.org/), the Europe Initiative on Centers of Excellence in Neurodegeneration (CoEN), a Methusalem Excellence Grant of the Flemish Government (EWI, //www.ewi-vlaanderen.be/

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