Genetic report abstractC9orf72 hexanucleotide repeat expansions in Chinese sporadic amyotrophic lateral sclerosis
Introduction
Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterized by loss of upper and lower motor neurons. The disorder occurs as sporadic ALS (sALS) in most cases, while 5%–10% of cases are familial (fALS).
Because SOD1 mutations were linked to fALS in 1993 (Rosen et al., 1993), a number of causative genes have been identified. A large hexanucleotide (GGGGCC) repeat expansion (HRE) in the first intron of the C9orf72 gene has been identified as the most common mutation detected in ALS and frontotemporal dementia patients in Caucasian populations (DeJesus-Hernandez et al., 2011, Millecamps et al., 2012, Renton et al., 2011). Mutations in these genes may also be found in sALS, but other than C9orf72 HRE each accounts for <1% of cases (Renton et al., 2014).
The C9orf72 HRE mutation has been comprehensively screened and assessed in Caucasian populations. The HRE frequency amongst sALS cases ranges from 3.2% to 21% in different populations (Beck et al., 2013, Galimberti et al., 2014, Majounie et al., 2012, Ratti et al., 2012, Renton et al., 2011). However, in Asian sALS, it was found to be rare in Japanese cohorts (Ishiura et al., 2012, Konno et al., 2012, Majounie et al., 2012, Ogaki et al., 2012) and absent in cases from mainland China (Jiao et al., 2014, Liu et al., 2013, Majounie et al., 2012, Zou et al., 2013). Additionally, genotypes consistent with a common founder risk haplotype have been reported in most C9orf72 HRE-carrying ALS cases in diverse populations, and it has been proposed that the HRE arose in a single common founder ∼1500 years ago (Majounie et al., 2012).
The function of the protein encoded by C9orf72 remains unclear; however, there are several hypotheses about functional consequences of the HRE including toxicity of the transcribed repeat, toxicity of protein dipeptides translated from the transcribed repeat, or loss of function (DeJesus-Hernandez et al., 2011, Donnelly et al., 2013, Gijselinck et al., 2012, Mori et al., 2013). Abnormal methylation of the CpG island upstream of the repeat was found in 73% of HRE carriers and was associated with downregulation of C9orf72 transcripts (Xi et al., 2013).
To date, large cohorts of Caucasian ALS cases have been assessed for the C9orf72 HRE, however, comparatively few studies of small cohorts of Asian cases have been conducted. We aimed to more accurately determine the frequency of the C9orf72 HRE in a large cohort of Chinese sALS subjects and investigate its relationship with single-nucleotide polymorphism (SNP) haplotypes and nearby CpG methylation.
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Participants
Patients attending the ALS specialty clinic at the Department of Neurology of the Peking University Third Hospital, Beijing, China, from 2003–2013 were recruited. Patients in the case cohort were diagnosed with ALS according to El Escorial revised criteria (Brooks et al., 2000) by a neurologist specializing in ALS. In the present analysis, only sALS cases were included based on self-report and clinical interview. The cohort consisted of 65% males, while mean age of onset of sALS was 49.7 ±
C9orf72 genotypes in Chinese ALS cases and controls
The HRE was identified in 3 Chinese patients with sALS (Ch_MND_11,134; Ch_MND_8446; Ch_MND_9059) of the 1092 (0.3%) cases genotyped and was not found in controls (p = 0.25, Fisher's exact test). The average repeat number in the 1089 ALS patients without the HRE was 3.79 ± 2.59 (range 2–25), whereas the repeat number in the 1062 control subjects was 3.84 ± 2.60 (range 2–23). The repeat number distribution of the alleles is shown in (Fig. 1). Principal component analysis showed minimal evidence
Discussion
This study confirms that C9orf72 repeat expansions are less prevalent in patients of Chinese compared with Caucasian ethnicity. The expansion was identified in 3 of 1092 cases compared with 0 of 1062 controls. Given the low prevalence, this difference does not achieve statistical significance (Fisher's exact test, p = 0.25). The HRE has previously been identified in Caucasian control subjects with an observed frequency of 0%–0.4% (Beck et al., 2013, Galimberti et al., 2014, Majounie et al., 2012
Disclosure statement
The authors have no actual or potential conflicts of interest.
Acknowledgements
This work was supported by the National Natural Sciences Foundation of China [81030019] [Dongsheng Fan]; The Australian Research Council Linkage Grant [LPl10200926]; National Health and Medical Research Council grants [APP1048853, APP1046880 to Peter M. Visscher], Senior Research Fellowship [613602 to Naomi R. Wray], Senior Principal Research Fellowships to Matthew A. Brown, Peter M. Visscher, The Ross Maclean Senior Research Fellowship and the Peter Goodenough Bequest [Marie Mangelsdorf] and
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