In order to find association of RPSA with sporadic CJD, we screened sequence of the RPSA in all available samples of sporadic CJD patients from Korean populations and investigated SNPs in the LRP/LR gene. In this study, we identified four polymorphisms including 519G>A (at codon 173) in the coding region and three SNPs located in 5'-UTR and intron on the RPSA. By two-hybrid system in the yeast and cell-binding assay, two PrP-LRP-LR interaction domains on PRNP were identified: a direct binding domain amino acids 144-179, termed PrPLRPbd1 and an indirect heparin sulfate proteoglycan (HSPG)-dependent LRP binding domain amino acids 53-93, termed PrPLRPbd2 . On RPSA, the direct PrP binding site is amino acids 161-180 and the heparin sulfate-dependent PrP-binding site is amino acids 205-229 . The RPSA polymorphism at codon 173 is a synonymous SNP, which encodes leucine, and does not lead to substitution of amino acid. Nevertheless, it remains of interest, because the polymorphism at codon 173 is located in the direct PrP binding domain. Similar to our study, ovine RPSA polymorphisms in the regions related to PrP-LRP-LR interaction did not lead to a change in amino acid sequence . Although the effects of this SNP on LRP/LR biology or prion biology are unknown, it is possible that the differences in combinations of amino acids may modify its specificity with other proteins affecting PrP-LRP-LR interaction, strengthening or weakening the species barrier . The RPSA amino acid sequences associated with resistance barrier to scrapie infection have been identified at positions 241, 272 and 291 in goat , and at positions 241, 271 and 290 in sheep . SNPs in UTR are known to have an effect on the expression or stability of genes. It is possible that the 5'-UTR -8T>C polymorphism may be responsible for genetic susceptibility to sporadic CJD.
In addition to differences in LRP/LR amino acid sequence, which affects the interaction with PrPSc, differences in the production/expression of LRP/LR bioavailability at the cell surface and particular conformations might be regional leading to cellular differences in binding/accumulation of prion proteins; the result of these differences could lead to variable response to scrapie infection [30, 32]. LRP is relatively abundant in developing brain tissue, and its expression is limited to a few neurons in cortex known to be especially sensitive to abnormal prion accumulation, and these cells rapidly degenerate during the early stages of CJD [32, 33]. LR is expressed in most adult neurons, and a subset of glial cells and astrocytes, which can accumulate PrPSc in scrapie-infected hamsters . PrPC-LRP-LR interaction, related to prion protein binding/accumulation, may take place at the cell soma and apical dendrites . LRP is localized in the cytoplasm and in the nucleus, and LR can be present as a laminin-binding protein on the cell surface or in a free form in the extracellular matrix [11, 30].
As shown in Table 4, although we found no significant difference in the genotype and allele frequencies of the RPSA polymorphisms between controls and sporadic CJD groups, RPSA 793+58C>T showed a tendency to lower frequency of heterozygotes in sporadic CJD patients than those of controls (P = 0.058). Since the number of RPSA 793+58C>T was much smaller (n = 50 for controls, n = 50 for sporadic CJD patients), further study on whether this result relates to our small sample sizes is required. In our study, the genotype and allele frequencies of RPSA polymorphisms were not influenced by gender. Lloyd et al.  and Stephenson et al.  reported that prion disease incubation time is not affected by sex. In contrast, the studies of Moreno et al.  showed that the survival time of female mice is shorter than those of male mice. The source of this sex effect could be hormonal factors, body size, fat composition, appetite and so on, acting at the end of the survival time .
One major haplotype (TCGT) is present in both controls and sporadic CJD patients with frequencies of 67-69% (Table 5). The additional four haplotypes were not significantly different in the two populations.
Some SNPs may act through LD with other mutations, even if it may not have direct influence on sporadic CJD susceptibility. From this point of view, we tested for LD between RPSA polymorphisms and PRNP codon 129. We found that RPSA polymorphisms had no effect on the susceptibility of 129MM individuals to sporadic CJD. To fully elucidate the association of the RPSA polymorphisms with sporadic CJD susceptibility, further studies are required using various populations because different linkage patterns can occur in different populations.
The PRNP associated resistance/susceptibility for prion disease is likely to coexist with other genes modulating its effect . In previous studies, we found that sporadic CJD shows a susceptibility effect at codon 129 and 219 on PRNP in Koreans . The PRNP polymorphism at codon 219 is unique to Asian populations, and PRNP 219EK heterozygous genotype has a protective effect on sporadic CJD development . In analysis stratified by PRNP codon 129 or 219 status, we found no significant difference in the genotype and allele frequencies of RPSA 5'-UTR -8T>C polymorphism according to the PRNP codon 129 or 219 status (Table 6). These results are a function of the fact that all sporadic CJD patients had only PRNP 129MM or 219EE genotype.