Introduction

Human prion diseases are neurodegenerative disorders characterized by the accumulation of an abnormal protease-resistant isoform of the prion protein PrPSc (Prusiner 1996). The prion-like protein gene (PRNP) plays an important role in conferring susceptibility to prion disease. A number of point and insert mutations of the PRNP have been linked to familial Creutzfeldt-Jakob disease (CJD), Gerstmann-Straussler-Scheinker disease (GSS), and fatal familial insomnia (FFI) (McKintosh et al. 2003). Morever, polymorphisms of PRNP appear able to influence expression of prion disease in sporadic and iatrogenic CJD (Collinge et al. 1991; Palmer et al. 1991; Shibuya et al. 1998).

The term Doppel (Dpl) is derived from downstream prion protein-like gene. The PrP-like protein gene (PRND) is located 27 kb downstream of the human PRNP. Its open reading frame (ORF) has been shown to contain polymorphic codons, but studies on a correlation between the PRND polymorphisms and neurodegenerative disorders in various populations have shown contradictory results (Peoc’h et al. 2000; Mead et al. 2000; Schroder et al. 2001; Infante et al. 2002; Croes et al. 2004; Golanska et al. 2004). Therefore, the role of the PRND in CJD and other diseases still remains unexplained. The major PRND polymorphism in these studies was substitution from threonine (Thr) to methionine (Met) at codon 174 (T174 M). This polymorphism in British, French, and Dutch populations was not associated with sporadic CJD or variant CJD (Peoc’h et al. 2000; Mead et al. 2000; Croes et al. 2004). In the Spanish population, the polymorphism was not associated with sporadic Alzheimer’s disease (AD) (Infante et al. 2002). In contrast with these studies, this polymorphism in German populations was associated with sporadic CJD and/or AD (Schroder et al. 2001). Other polymorphisms were observed at codons 26 (T26 M) and 56 (P56L) (Mead et al. 2000; Peoc’h et al. 2000; Schroder et al. 2001; Infante et al. 2002). These polymorphisms were not associated with susceptibility to sporadic CJD (Mead et al. 2000; Peoc’h et al. 2000). The genotype frequencies of PRNP polymorphisms in Asian populations, including Koreans, are considerably different from those in Europeans (Palmer et al. 1991; Jeong et al. 2004). Although the polymorphisms of the PRND have been studied in several European countries (Mead et al. 2000; Peoc’h et al. 2000; Schroder et al. 2001; Infante et al. 2002; Croes et al. 2004; Golanska et al. 2004), there have been no reports of PRND from other parts of the world.

The purpose of this study was to examine the genotype frequencies of PRND polymorphisms in Koreans and to investigate whether the polymorphisms of the PRND can be associated with sporadic CJD in a Korean population.

Materials and methods

CJD patient population

We used previously established criteria (Zerr et al. 2000) for diagnosis of sporadic CJD in Korea. Neuropathologically confirmed patients and/or patients with immunochemical detection of PrPSc in the brain were classified as definite CJD. Patients were classified as probable CJD if they exhibited a rapidly progressive dementia, periodical sharp wave complexes (PSWC) on electroencephalography (EEG), 14-3-3 protein in CSF, a duration of dementia <2 years, and two of the following: myoclonus, visual and cerebellar symptoms or both, pyramidal and extrapyramidal signs or both, or akinetic mutism. Those fulfilling the above clinical criteria and the duration of <2 years but without PSWC and 14-3-3 detection in CSF were classified as possible CJD. The sporadic CJD cases, which were diagnosed by immunohistochemistry and DNA sequencing, have been reported previously (Jeong et al. 1998). To provide a reliable assessment of CJD in Korea, we carried out molecular experiments, including Western blots as well as immunohistochemistry. Of the 188 suspected CJD cases, 17 were classified as definite CJD, and 93 were classified as probable CJD.

Blood samples

Blood samples were collected from 102 healthy Korean volunteers and 110 sporadic CJD patients, which were collected between May 1996 and October 2004. The study was approved by the ethical committee of Chunchon Sacred Heart Hospital, and all volunteers and sporadic CJD patients gave informed consent. All blood samples were frozen at −70°C prior to analysis.

Polymerase chain reaction

Genomic DNA was extracted from 200 μl blood using the QIAamp DNA blood mini kit (Qiagen, USA) following the supplier’s instructions. Polymerase chain reaction (PCR) was performed with Hdpl-1 (5′-GTTTCTCTGGCAGGTTCTG-3′) and Hdpl-2 (5′-GAGAAGAGCTGGGTCACTT-3′) primers. These primers were designed to amplify a 623-bp product, including the complete human Dpl ORF (528-bp) and adjacent sequences. The PCR reagents contained 50 pmol of each primer, 5 μl of ten times Taq DNA polymerase buffer, 1.5 mM MgCl2, 0.2 mM of each dNTP mixture, and 2.5 units of Taq DNA polymerase (Promega, USA). The PCR conditions were as follows: an initial denaturation step of 95°C for 2 min, 30 cycles of 94°C for 40 s, 56°C for 45 s, 72°C for 90 s, and final extension at 72°C for 10 min.

Nucleotide sequencing analysis

The purification of PCR products for sequencing was done using a QIA quick gel extraction kit (Qiagen, USA). The PCR products were directly sequenced on an ABI 377 automatic DNA sequencer using a Taq dideoxy terminator cycle sequencing kit (ABI, USA) and the same primers as above in standard conditions. Nucleic acid sequences were assembled, and edited was done using a combination of the ABI 377 DNA Sequencer Data Analysis Program and Sequence Navigator Software.

Statistical analysis

Statistical analysis was performed using the SAS 8.1 Software (SAS Institute Inc., USA). Differences in genotype frequencies between healthy Koreans and patients with sporadic CJD were analyzed by χ2 tests.

Results

Direct sequencing of the PCR products of the PRND amplified in 102 Korean normal individuals and 110 sporadic CJD patients identified two SNPs in the ORF of the gene. The minor PRND polymorphism found in the Korean population was P56L (Table 1). No significant difference was found in genotype distribution or allele frequency between sporadic CJD and the normal population at this codon. Furthermore, this polymorphism, examined in France, Britain, and Korea, was not associated with susceptibility to sporadic CJD. The major PRND polymorphism was T174 M (Table 1). The frequency of the T174 allele in the normal Korean population is slightly higher than that in the German population and substantially greater than French, British, and Dutch populations. In order to examine correlation between this polymorphism and susceptibility of sporadic CJD in Koreans, we also analyzed the genotype and allele frequencies in 110 sporadic CJD cases. No significant difference was found between sporadic CJD and normal controls in genotype distribution or allele frequency. Furthermore, neither was there any association between age at onset or duration of disease in sporadic CJD and genotype (Table 2). Also, there was no significant association between PrPSc type and this polymorphism. These results suggest that the T174 M polymorphism does not increase susceptibility to sporadic CJD.

Table 1 Genotype and allele frequencies of the polymorphisms at codons 56 and 174 of the prion-like protein gene (PRND) in normal population and sporadic Creutzfeldt-Jakob disease (CJD) patients from France, Britain, Germany, Holland, and Korea
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Table 2 Genotypes at codon 174 of the prion-like protein gene (PRND), age of onset, duration of illness, and PrPSc type in Korean normal population and sporadic Creutzfeldt-Jakob disease (CJD) patients
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In addition, we looked for the presence of T26 M PRND polymorphism and elsewhere in the ORF of the PRND, but we found none in our sampling of the Korean population.

Discussion

In this study, we studied polymorphisms at codons 56 and 174 of the ORF of PRND in a sampling of the Korean population. We have not found evidence that polymorphisms of the ORF of PRND are associated with susceptibility to sporadic CJD. To our knowledge, ours is the first genetic association study of the ORF of PRND with CJD in an Asian population.

In previous studies, the P56L PRND polymorphism has been reported in the French population but not in the British population (Mead et al. 2000; Peoc’h et al. 2000). In the French population, the distributions of PRND genotype and allele frequencies in sporadic CJD patients did not significantly differ from those of controls. The genotype frequency of the T174 M PRND polymorphism in the normal Korean population differs significantly from that previously reported for the British (χ2 = 17.3604, P =0.0002), French (χ2=20.1895, P<0.0001), German (χ2=6.072, P =0.048), and Dutch populations (χ2=9.947, P =0.007) (Mead et al. 2000; Peoc’h et al. 2000; Schroder et al. 2001; Croes et al. 2004). These results suggest that there is a difference in the genotype frequency of this polymorphism between Europeans and Asians. In Koreans, neither the genotype nor allele frequency of the T174 M PRND polymorphism is associated with sporadic CJD. This result supports the British, French, and Dutch findings but not the German data. We have not found evidence that polymorphisms in ORF of the PRND are associated with susceptibility to sporadic CJD. Also, there were no significant differences in genotype frequencies between definite sporadic CJD or probable sporadic CJD and controls, even after stratifying by onset of age, duration of disease, and PrPSc type (Table 2). The positive results in the German population may have arisen by chance due to the very small sample size of sporadic CJD patients studied (21 cases) (Schroder et al. 2001).