Examination of the human prion protein-like gene doppel for genetic susceptibility to sporadic and variant Creutzfeldt-Jakob disease

Bovine spongiform encephalopathy (BSE) associated polymorphisms of the prion-like protein gene (PRND) in Korean dairy cattle and Hanwoo

Bovine spongiform encephalopathy (BSE) involves insertion/deletion (in/del) polymorphisms in the prion protein gene (PRNP) promoter region that are associated with vulnerability to disease progression. Recently, a second member of the prion gene family, prion-like protein gene (PRND), has been reported to show the PRND R132Q polymorphism, which is associated with the susceptibility to BSE in German Fleckvieh breeds. The objective of this study was to examine the genotype, allele, and haplotype frequencies of PRND gene in Korean cattle and evaluate their susceptibility to BSE. We did this in 277 Korean native cattle (Hanwoo) and 124 Korean dairy cattle (Holstein) by direct sequencing and compared the R132Q genotype frequency between BSE-affected German cattle and Korean cattle. The results indicated a total of 5 single nucleotide polymorphisms (SNPs) including PRND c.149G > A (p.50Arg > His; R50H), PRND c.285C > T (C4819T), PRND c.395G > A (p.132Arg > Gln; R132Q) and PRND c.528T > A (T5063A) in the open reading frame (ORF) and c.602C > G in the 3′ untranslated region (UTR) of exon 2 in Korean Holstein and Hanwoo cattle. Except for c.149G > A, the remaining 4 SNPs showed significantly different genotype and allele frequencies between the Korean Holstein and Hanwoo (P < 0·01). There were no significant differences in genotype distribution of c.395G > A SNP between BSE-affected German and Korean Holstein cattle (P = 0·6778), but a significant difference was detected between BSE-affected German cattle and Hanwoo cattle (P = 0·0028). The results suggest that Hanwoo cattle may possess a relatively more BSE-resistant genotype than Korean Holstein cattle.

Prion disease: experimental models and reality

The understanding of the pathogenesis and mechanisms of diseases requires a multidisciplinary approach, involving clinical observation, correlation to pathological processes, and modelling of disease mechanisms. It is an inherent challenge, and arguably impossible to generate model systems that can faithfully recapitulate all aspects of human disease. It is, therefore, important to be aware of the potentials and also the limitations of specific model systems. Model systems are usually designed to recapitulate only specific aspects of the disease, such as a pathological phenotype, a pathomechanism, or to test a hypothesis. Here, we evaluate and discuss model systems that were generated to understand clinical, pathological, genetic, biochemical, and epidemiological aspects of prion diseases. Whilst clinical research and studies on human tissue are an essential component of prion research, much of the understanding of the mechanisms governing transmission, replication, and toxicity comes from in vitro and in vivo studies. As with other neurodegenerative diseases caused by protein misfolding, the pathogenesis of prion disease is complex, full of conundra and contradictions. We will give here a historical overview of the use of models of prion disease, how they have evolved alongside the scientific questions, and how advancements in technologies have pushed the boundaries of our understanding of prion biology.

Genetic studies in human prion diseases

Human prion diseases are fatal neurodegenerative disorders that are characterized by spongiform changes, astrogliosis, and the accumulation of an abnormal prion protein (PrP(Sc)). Approximately 10%-15% of human prion diseases are familial variants that are caused by pathogenic mutations in the prion protein gene (PRNP). Point mutations or the insertions of one or more copies of a 24 bp repeat are associated with familial human prion diseases including familial Creutzfeldt-Jakob disease (CJD), Gerstmann-Sträussler-Scheinker syndrome, and fatal familial insomnia. These mutations vary significantly in frequency between countries. Here, we compare the frequency of PRNP mutations between European countries and East Asians. Associations between single nucleotide polymorphisms (SNPs) of several candidate genes including PRNP and CJD have been reported. The SNP of PRNP at codon 129 has been shown to be associated with sporadic, iatrogenic, and variant CJD. The SNPs of several genes other than PRNP have been showed contradictory results. Case-control studies and genome-wide association studies have also been performed to identify candidate genes correlated with variant and/or sporadic CJD. This review provides a general overview of the genetic mutations and polymorphisms that have been analyzed in association with human prion diseases to date.

PRND 3'UTR polymorphism may be associated with behavioral disturbances in Alzheimer disease

The etiology of behavioral and psychological symptoms of dementia (BPSD) is complex, including putative biological, psychological, social and environmental factors. Recent years have witnessed accumulation of data on the association between genetic factors and behavioral abnormalities in Alzheimer disease (AD). In this research paper, our aim is to evaluate the association between the APOE, CYP46, PRNP and PRND genes and the profile of neuropsychiatric symptoms in Polish subjects with AD and mild cognitive impairment (MCI). We studied 99 patients with AD and 48 subjects with MCI. The presence and profile of BPSD were evaluated at baseline and prospectively with the Neuropsychiatric Inventory (NPI). Patients were dichotomized into those having ever experienced a particular symptom and those who did not over the whole disease period. Genotyping was performed using previously described standard protocols. The prevalence of comorbid behavioral symptoms and the overall level of behavioral burden were significantly greater in AD compared with the MCI group. In AD patients, carrier status of the T allele of the 3'UTR (untranslated region) PRND polymorphism was associated with an increased cumulative behavioral load and an elevated risk for delusions, anxiety, agitation/aggression, apathy and irritability/emotional ability. Among MCI subjects, APOE ε4 carriers demonstrated a reduced risk for nighttime behavior change. No other statistically significant genotype-phenotype correlations were observed, including the APOE, CYP46 and PRNP genes. A precise estimation of the exact significance of particular polymorphisms in BPSD etiology requires future studies on large populations.

A polymorphism in the YWHAH gene encoding 14-3-3 eta that is not associated with sporadic Creutzfeldt-Jakob disease (CJD)

14-3-3 proteins are abundantly expressed in the brain, particularly neuronal tissue and are thought to serve multiple biological functions involved in neuronal development and cell growth and death. Recent studies have shown associations of 14-3-3 genes with neurodegenerative disorders based on their chromosomal linkage to these diseases and to regulatory functions for the nervous system. Although the role of 14-3-3 proteins in the pathogenesis of prion diseases remains unknown, the detection of altered levels of isoforms of the 14-3-3 protein in the cerebrospinal fluid is considered a biomarker for diagnosis of sporadic Creutzfeldt-Jakob disease (sCJD). To identify other susceptibility genes for prion disease, we examined nucleotide variations in YWHAH, a gene encoding 14-3-3 eta. This case-control study included 182 sCJD patients and 206 healthy Koreans. Polymerase chain reaction was used to amplify open reading frame and some 3'-untranslated region (UTR) in exon 2, and direct sequencing was carried out. One polymorphism, 753 G/A, was detected in the 3'-UTR of exon 2 on the YWHAH. The genotype distribution and allele frequencies of the YWHAH 753 G/A polymorphism were not significantly different between controls and sCJD patients. This finding indicates that YWHAH 753 G/A polymorphism is unlikely to be linked to genetic susceptibility or have a modifying effect in sCJD. On analysis stratified by the prion protein gene 129 or 219 genotype, no significant relation was found in genotype and allele frequencies of the YWHAH 753G/A. This is the first genetic association study of YWHAH with sCJD populations.

Mapping the interaction site of prion protein and Sho

The cellular prion protein (PrP(C)) is a highly conserved protein among mammals and is considered to have important cellular functions. Despite decades of intensive research, however, the physiological function of PrP(C) remains unclear. Sho (Shadoo, shadow of prion protein) and PrP(C) have similar N-terminals, which suggests that the two proteins share biological functions. Using truncation mutants of both proteins and yeast two-hybrid analysis, with validation by co-immunoprecipitation and surface plasmon resonance (SPR), we have identified an interaction between Sho 61-77 and PrP(C) 108-126 domains. This indicates that Sho may play a role in the physiological function of PrP(C) and prion pathogenesis.

Rationale for diagnosing human prion disease

Human prion diseases (PrD) like Creutzfeldt-Jakob disease (CJD) include sporadic, acquired and familial neurodegenerative disorders. The central events in the neuropathological process of PrDs are severe neuronal loss, spongiform change and accumulation of abnormal prion protein (PrPSc). The latter is a conformational variant of the host-encoded cellular PrP (PrPC), a copper-binding protein. The physiological role of PrPC is debated. Definitive diagnosis of PrD is based on post mortem demonstration of PrPSc by immunohistochemistry or Western blot. Mutations in the PrP gene (PRNP), the polymorphic site at codon 129, and the molecular characteristic of protease resistant PrP influence the phenotype. Clinical symptoms, cranial MRI scan, EEG and investigation of 14-3-3 protein in cerebrospinal fluid (CSF) suggest a diagnosis of probable CJD. Variant CJD, related to bovine spongiform encephalopathy, shows a different clinical course, symmetrical high intensity MRI signal in the pulvinar, presence of PrPSc in tonsil biopsy tissue, and a lower sensitivity of CSF 14-3-3 protein compared to sporadic CJD. Future possibilities in diagnosis of PrDs include either the demonstration of PrPSc in body fluids or disease associated changes in laboratory variables or gene expression.

Molecular pathology of human prion diseases

Prion diseases are fatal neurodegenerative conditions in humans and animals. In this review, we summarize the molecular background of phenotypic variability, relation of prion protein (PrP) to other proteins associated with neurodegenerative diseases, and pathogenesis of neuronal vulnerability. PrP exists in different forms that may be present in both diseased and non-diseased brain, however, abundant disease-associated PrP together with tissue pathology characterizes prion diseases and associates with transmissibility. Prion diseases have different etiological background with distinct pathogenesis and phenotype. Mutations of the prion protein gene are associated with genetic forms. The codon 129 polymorphism in combination with the Western blot pattern of PrP after proteinase K digestion serves as a basis for molecular subtyping of sporadic Creutzfeldt-Jakob disease. Tissue damage may result from several parallel, interacting or subsequent pathways that involve cellular systems associated with synapses, protein processing, oxidative stress, autophagy, and apoptosis.

Association of a null allele of SPRN with variant Creutzfeldt-Jakob disease

Background:

No susceptibility genes have been identified in human prion disase, apart from the prion protein gene (PRNP). The gene SPRN, encodes Shadoo (Sho, shadow of prion protein) which has protein homology and possible functional links with the prion protein.

Methods:

A genetic screen was carried out of the open reading frame of SPRN by direct sequencing in 522 patients with prion disease, including 107 with variant Creutzfeldt–Jakob disease (vCJD), and 861 healthy controls.

Results:

A common coding variant of SPRN, two further single nucleotide polymorphisms (SNPs) and three rare insertion or deletion variants were found. A single base-pair insertion at codon 46, predicted to cause a frameshift and potentially a novel protein, was found in two patients with vCJD but not in controls (p = 0.01). Two linked SNPs, one in intron 1 and the other a missense variant at codon 7, were associated with risk of sporadic CJD (p = 0.009).

Conclusion:

These data justify the functional genetic characterisation of SPRN and support the involvement of Shadoo in prion pathobiology.

The prion protein family: Diversity, rivalry, and dysfunction

The prion gene family currently consists of three members: Prnp which encodes PrP(C), the precursor to prion disease associated isoforms such as PrP(Sc); Prnd which encodes Doppel, a testis-specific protein involved in the male reproductive system; and Sprn which encodes the newest PrP-like protein, Shadoo, which is expressed in the CNS. Although the identification of numerous candidate binding partners for PrP(C) has hinted at possible cellular roles, molecular interpretations of PrP(C) activity remain obscure and no widely-accepted view as to PrP(C) function has emerged. Nonetheless, studies into the functional interrelationships of prion proteins have revealed an interesting phenomenon: Doppel is neurotoxic to cerebellar cells in a manner which can be blocked by either PrP(C) or Shadoo. Further examination of this paradigm may help to shed light on two prominent unanswered questions in prion biology: the functional role of PrP(C) and the neurotoxic pathways initiated by PrP(Sc) in prion disease.

The doppel gene biology: a scientific journey from brain to testis, and return

Doppel is a newly recognized prion-like molecule encoded by a novel gene locus, PRND, located on the same chromosomal region of the prion (PRNP) coding gene. Doppel was considered a paralogue and the first member of the prion-gene family, possibly originated through an ancestral gene duplication event. Prion and doppel have different expression patterns, suggesting that the gene products exhibit different biological functions. Actually, doppel is not involved in the aetiology of the Transmissible Spongiform Encephalopathies (TSEs) or “prion diseases” and is highly expressed only within the testicular tissue, suggesting an important physiological role in the process of spermatogenesis. The restricted spatial and temporal expression profile of doppel has suggested its investigation within particular pathological contexts, such as cancers, showing that it might represent a novel and attractive diagnostic molecular marker and that might provide insights into the regulatory pathways of tumor-cell transformation.

Polymorphic microsatellite sites in the PRNP region point to excess of homozygotes in Creutzfeldt–Jakob disease patients

Polymorphic microsatellite sites within 148 kb of the human prion gene complex, including the genes PRNP, PRND and PRNT, were analysed together with the Codon129 variants regarding 50 CJD (Creutzfeldt-Jakob Disease) patients and 46 non-diseased control persons. Three of the sites (MM03, MM04, Codon129) differed significantly (P<0.05) for their allele frequencies between the two groups--the predominant allele being always more frequent in the CJD group. Deviations from Hardy-Weinberg Equilibrium were mainly obtained in the CJD group--in all cases with a reduction of the observed heterozygosity. The sites MM03, MM04 and Codon129 were also analysed for their haplotypes. The predominant homozygous haplotype combination was more frequently observed in the CJD group (0.875) than in the non-diseased group (0.38). Thus the different polymorphic sites indicate that high CJD disposition is associated with homozygosity in the PRNP gene.

Kuru in the 21st century--an acquired human prion disease with very long incubation periods

BACKGROUND

Kuru provides the principal experience of epidemic human prion disease. Its incidence has steadily fallen after the abrupt cessation of its route of transmission (endocannibalism) in Papua New Guinea in the 1950s. The onset of variant Creutzfeldt-Jakob disease (vCJD), and the unknown prevalence of infection after the extensive dietary exposure to bovine spongiform encephalopathy (BSE) prions in the UK, has led to renewed interest in kuru. We investigated possible incubation periods, pathogenesis, and genetic susceptibility factors in kuru patients in Papua New Guinea.

METHODS

We strengthened active kuru surveillance in 1996 with an expanded field team to investigate all suspected patients. Detailed histories of residence and exposure to mortuary feasts were obtained together with serial neurological examination, if possible.

FINDINGS

We identified 11 patients with kuru from July, 1996, to June, 2004, all living in the South Fore. All patients were born before the cessation of cannibalism in the late 1950s. The minimum estimated incubation periods ranged from 34 to 41 years. However, likely incubation periods in men ranged from 39 to 56 years and could have been up to 7 years longer. PRNP analysis showed that most patients with kuru were heterozygous at polymorphic codon 129, a genotype associated with extended incubation periods and resistance to prion disease.

INTERPRETATION

Incubation periods of infection with human prions can exceed 50 years. In human infection with BSE prions, species-barrier effects, which are characteristic of cross-species transmission, would be expected to further increase the mean and range of incubation periods, compared with recycling of prions within species. These data should inform attempts to model variant CJD epidemiology.

Prion disease genetics

Prion diseases have stimulated intense scientific scrutiny since it was proposed that the infectious agent was devoid of nucleic acid. Despite this finding, genetics has played a key role in understanding the pathobiology and clinical aspects of prion disease through the effects of a series of polymorphisms and mutations in the prion protein gene (PRNP). The advent of variant Creutzfeldt-Jakob disease has confirmed one of the most powerful human genetic susceptibility factors, as all tested patients have an identical genotype at polymorphic codon 129 of PRNP. This review will also consider the accrued reports of inherited prion disease and attempt a genotype-phenotype correlation. The prospects for detection of novel genetic susceptibility factors using mouse models and human genetic association studies will be explored.

Prion-like Doppel gene (PRND) in the goat: genomic structure, cDNA, and polymorphisms

The genomic structure of the caprine Doppel gene (PRND) was determined using the ovine sequence as a scaffold to generate PCR fragments that were aligned with a cDNA sequence obtained from testicular mRNA. The caprine gene contains two exons, 89 and >2291 bp long, separated by a 1689-bp intron. Two mRNA isoforms of 3.2 and 4.8 kb were identified in the testis, as well as the exact transcription start site by fluorescently labeled oligonucleotide extension (FLOE). Like in sheep and cattle, the open reading frame (ORF) (537 bp) lies within exon 2 and is very much conserved in sheep (99.3%) and cattle (97%). The intronic sequence is also highly conserved (95.3%) compared with sheep, with the only exception of a 47-bp insertion. The PRND ORF was sequenced in 47 healthy and 17 TSE-affected goats of the Italian Ionica breed. Seven nucleotide positions showed variation: T28C, C65T, A151G, G286A, C385G, T451C, and T528C. Five were commonly represented polymorphisms: T28C, T451C, and T528C are silent mutations at codons L10, L151, and I176, respectively, while A151G and C385G determine a T51A and L129V amino acid change, respectively. The two remaining variants, C65T and G286A, were rare, leading to the amino acid substitutions S22F and E96K, respectively. None of the polymorphisms was significantly relatable to the TSE status, and the same result was obtained by the analysis of the combined haplotypes at the five major polymorphic sites, namely, T28C, C65T, A151G, G286A, and C385G.

DNA polymorphisms of the prion doppel gene region in four different German cattle breeds and cows tested positive for bovine spongiform encephalopathy

Polymorphisms of the prion protein gene PRNP have been shown to influence the susceptibility/resistance to prion infections in human and sheep. In addition, the T174M polymorphism within the flanking prion doppel gene (PRND) was thought to be involved in susceptibility to sporadic Creutzfeldt-Jacob disease. To study a possible influence of DNA polymorphisms of the bovine PRND gene in bovine spongiform encephalopathy (BSE), previously identified and newly isolated DNA polymorphisms were genotyped in all available German cattle that tested positive for BSE. Genotypes and calculated haplotypes were compared with breeding bulls serving as controls. Analysis of the four major breeds Schwarzbunt (Holstein Friesian), Rotbunt (Holstein Red), Fleckvieh (Simmental), and Braunvieh (Swiss Brown) resulted in the isolation of the previously known polymorphisms R50H and R132Q and two novel synonymous single nucleotide polymorphisms (SNPs) C4820T and A5063T. Comparative genotype and haplotype analysis of BSE and control animals revealed a significantly different distribution of polymorphisms C4815T and R132Q in Fleckvieh animals but not in the other breeds tested. No association to BSE susceptibility was detectable for polymorphisms R50H and A5063T.

Polymorphism at 3' UTR +28 of the prion-like protein gene is associated with sporadic Creutzfeldt-Jakob disease

The downstream prion-like protein (doppel or Dpl) shares significant biochemical and structural homology with the cellular prion protein, PrP(C), which is considered as a responsible protein for the transmissible spongiform encephalopathies (TSEs) or prion diseases. Recently, polymorphisms in open reading frame (ORF) of the prion-like protein gene (PRND) have been analysed in relation to the occurrence of prion diseases and other neurodegenerative disorders. We examined the role of a single-nucleotide polymorphism (SNP) at 3' untranslated region (UTR) +28 of PRND. We analysed this polymorphism in 110 Korean patients with sporadic Creutzfeldt-Jakob disease (CJD) and 102 healthy control subjects. Significant differences in genotype (P=0.005) and allele (P=0.032) frequencies at 3' UTR +28 were observed between sporadic CJD and normal controls. This result suggests that the PRND polymorphism at 3' UTR +28 might be associated with the occurrence of sporadic CJD.

Polymorphisms at codons 56 and 174 of the prion-like protein gene (PRND) are not associated with sporadic Creutzfeldt-Jakob disease

Association between sporadic Creutzfeldt-Jakob disease (CJD) and the prion-like protein gene (PRND) has been reported in the German population. To investigate whether the PRND polymorphisms are associated with an increased risk for developing sporadic CJD in the Korean population, we compared the genotype and allele frequencies of PRND polymorphisms in 110 sporadic CJD patients with those in 102 healthy Koreans. Two polymorphisms (P56L, T174 M) in Koreans were found in the open reading frame (ORF) of PRND. One heterozygote of P56L was observed in normal controls but not in sporadic CJD patients. A strong significant difference of PRND genotype frequency at codon 174 was found between the normal Korean population and various European populations. In contrast to results in the German population, our study did not show a significant difference in PRND genotype or allele frequency at codon 174 between sporadic CJD and normal controls. This was the first genetic association study of the ORF of PRND in an Asian CJD population.

Polymorphisms in the prion protein gene and in the doppel gene increase susceptibility for Creutzfeldt-Jakob disease

The prion protein gene (PRNP) plays a central role in the origin of Creutzfeldt-Jakob disease (CJD), but there is growing interest in other polymorphisms that may be involved in CJD. Polymorphisms upstream of PRNP that may modulate the prion protein production as well as polymorphisms in the prion-like doppel gene (PRND) have been studied, with inconsistent findings. We investigated the role of a single-nucleotide polymorphism (SNP 1368) located upstream of PRNP and three polymorphisms in PRND (T26M, P56L and T174M) in CJD. The study included a population-based sample of 52 patients with sporadic CJD and 250 controls. We analysed our data as single markers and haplotypes. Further, we conducted a meta-analysis on PRND T174M comparing the data of the four studies conducted to date. For SNP 1368 and PRNP M129V, we found significant evidence for linkage disequilibrium. No evidence was found for a relation of SNP 1368 to CJD independent of PRNP M129V. We further found a significant increased prevalence of M homozygotes at PRND T174M among sporadic CJD patients, when adjusting the analyses for the other genotypes. In the haplotype analyses, the association was strongest for persons homozygous for PRNP 129M and PRND 174M (odds ratio 4.35, 95% confidence interval 1.05-8.09; P=0.04). The meta-analysis on the PRND T174M polymorphism did not show a consistent effect across studies, raising the question as to whether PRND 174M is causally related to CJD, or whether the PRND allele is in linkage disequilibrium with another polymorphism related to CJD.

Male infertility and DNA damage in Doppel knockout and prion protein/Doppel double-knockout mice

The prion protein (PrP) and Doppel (Dpl) have many structural and biochemical properties in common, leading to the suggestion that the lack of an obvious phenotype in PrP-deficient mice maybe because of compensation by Dpl. To test this hypothesis and also investigate the function of Dpl we have generated Prnd(-/-) and Prnp(-/-)/Prnd(-/-) mouse lines. Both develop normally and display an identical male sterility phenotype that differs from that reported for another Prnd(-/-) mouse line. Sperm from both our mutant lines were present at normal concentrations, had normal motility, and no morphological abnormalities. Despite only rarely fertilizing oocytes in vivo, because of an inability to perform the acrosome reaction, mutant sperm were capable of fertilization in vitro, albeit at reduced rates compared to wild type. Elevated levels of oxidative DNA damage were found in both types of mutant sperm and resulting embryos failed at an early stage. Therefore we found no evidence that Dpl compensates for the loss of PrP function in mutant mouse lines, but it does have an important anti-oxidant function necessary for sperm integrity and male fertility.

Prion proteins and infertility: insight from mouse models

A wealth of evidence points to an abnormal form of the prion protein called PrP(Sc) as the transmissible agent responsible for prion diseases. However, the physiological function of its normal conformer, the cellular prion protein (PrP(C)), is still unknown. Recently, a homologue of PrP(C) was discovered and denoted Doppel (Dpl). In contrast to PrP, mice deficient for Dpl suffer from an important pathological phenotype: male sterility. This phenotype shifts the attention from the brain, where most of the investigations on Dpl have been performed, to testis, raising hope to resolve the long lasting search of PrP(C) function.

Selective PrP-like protein, doppel immunoreactivity in dystrophic neurites of senile plaques in Alzheimer's disease

Doppel (Dpl) is a prion-like protein encoded by the gene PRND, which has been found downstream of the prion gene PRNP in several species. The present study examines by immunohistochemistry Dpl expression in brain samples from 10 patients with Alzheimer's disease (AD), three patients with Pick's disease, four patients with Parkinson's disease, eight patients with diffuse Lewy body disease (DLBD), six patients with sporadic Creutzfeldt-Jakob disease (CJD) methionine/methionine at the codon 129, two patients with sporadic CJD methionine/valine at the codon 129 and numerous kuru plaques in the cerebellum, one patient with fatal familial insomnia (FFI), and 10 age-matched controls. In the adult human brain, Dpl immunoreactivity was restricted to scattered granule cells of the cerebellum and scattered small granules in the cerebral cortex. Dpl immunoreactivity was seen around betaA4 amyloid deposits in neuritic plaques, but not in diffuse plaques, AD and the common form of DLBD. Neurofibrillary tangles, Pick bodies and Lewy bodies were not stained with anti-Dpl antibodies. No modifications in Dpl immunoreactivity were observed in CJD excepting those associated with accompanying senile plaques. No Dpl-positive deposits were seen in FFI. Whether Dpl in neuritic plaques may attenuate amyloid-induced oxidative stress and participate in the glial response around amyloid cores is discussed in light of the few available data on Dpl functions.

Analysis of doppel protein toxicity

The recently described doppel protein (Dpl) is a homologue of the prion protein (PrP(c)). This protein, expressed in the brains of mice that lack the expression of PrP(c), causes neuronal death as the mice age. Previous studies have suggested this neuronal damage is caused by oxidative assault and changes in the activity of NOS proteins. We investigated the toxicity of Dpl in cell culture models and showed that Dpl was toxic to neurons. This toxicity was inhibited by the expression of PrP(c) and possibly involved direct interaction between the two proteins. The mechanism of toxicity involved stimulation of nitric oxide production via activation of the nitric oxide synthases, nNOS and iNOS. This mechanism of toxicity is quite different from that of PrP(Sc) and does not require the protein to change conformation. These results provide the first evidence for the mechanism of Dpl toxicity.

Prions and the immune system: a journey through gut, spleen, and nerves

For more than two decades it has been contended that prion infection does not elicit immune responses: transmissible spongiform encephalopathies do not go along with conspicuous inflammatory infiltrates, and antibodies to the prion protein are typically undetectable. Why is it, then, that prions accumulate in lymphoid organs, and that various states of immune deficiency prevent peripheral prion infection? This review revisits the current evidence of the involvement of the immune system in prion diseases, while attempting to trace the elaborate mechanisms by which peripherally administered prions invade the brain and ultimately cause damage. The investigation of these questions leads to unexpected detours, including the neurophysiology of lymphoid organs, and even the function of a prion protein homolog in male fertility.

Physiological and pathological functions of the prion protein homologue Dpl

A misfolded version of the prion protein PrP(C), known as PrP(Sc), is the major component of scrapie infectivity, the pathological agent in transmissible spongiform encephalopathies. The Prnp gene that encodes the cellular PrP(C) protein was cloned almost 20 years ago, but remained without sequence or structural relatives for over a decade. Only recently a novel protein, named Doppel (Dpl), was identified, which shares significant biochemical and structural homology with PrP(C). When overexpressed, Dpl is neurotoxic and causes a neurological disease. Strikingly, Dpl neurotoxicity is counteracted and prevented by PrP(C). In contrast to its homologue PrP(C), Dpl is dispensable for prion disease progression and for the generation of PrP(Sc), but Dpl appears to have an essential function in male spermatogenesis. Although Dpl research is still in its infancy, the discovery of Dpl has already solved some enigmas of prion biology and an understanding of its physiological function is emerging.

The human "prion-like" protein Doppel is expressed in both Sertoli cells and spermatozoa

The prion-like Doppel protein (Dpl) has many biochemical and structural properties in common with the cellular prion protein (PrP(c)), and the physiological role of neither protein is known. Experimental data suggest either direct or indirect interaction between the two proteins. In this study, we investigated the expression pattern and biochemical characteristics of Dpl in human tissues and in Chinese hamster ovary cells transfected with wild-type or variant human Dpl gene constructs. Human Dpl appears to be a glycosylphosphatidylinositol-anchored glycoprotein with N- and O-linked sugars. It was found on Sertoli cells in the testis, on the flagella of epididymal and mature spermatozoa, and in seminal plasma. Dpl coexists only with N-terminally truncated isoforms of PrP(c) on mature spermatozoa. The localization of human Dpl on both Sertoli cells (somatic cells) and spermatozoa (germinal cells) strongly suggests that this protein may play a major role in human male fertility. Finally, our data indicate that spermatozoa are thus an interesting model for studies of the potential interaction between Dpl and PrP(c).

Polymorphism at codon 174 of the prion-like protein gene is not associated with sporadic Alzheimer's disease

Alzheimer's disease (AD) and prion diseases are associated with the occurrence of protein aggregates called amyloid fibrils, containing the amyloid-beta peptide in AD, and a modified form (PrP(Sc)) of the normal cellular prion protein (PrP(c)) in prion diseases. PrP(c) is encoded by the prion protein gene, and a common polymorphism at codon 129 of this gene is a determinant of susceptibility to acquired and sporadic prion diseases but not for sporadic AD. A recently identified novel protein, named Doppel, shares biochemical and structural homology with PrP(c). Preliminary evidence in a German population indicates that a polymorphism at codon 174 of the prion-like protein (PRND) gene encoding for Doppel protein is a predisposing factor for both prion diseases and sporadic AD. A case-control study utilizing a clinically well-defined group of 283 sporadic AD patients and 288 control subjects was performed to test this association. The current study does not demonstrate any significant difference in T174M PRND genotype or allele frequencies between AD patients and controls. Our study in the Spanish population argues against the hypothesis that the PRND gene is causally related to AD.

Absence of the prion protein homologue Doppel causes male sterility

The agent that causes prion diseases is thought to be identical with PrP(Sc), a conformer of the normal prion protein PrP(C). PrP(C)-deficient mice do not exhibit major pathologies, perhaps because they express a protein termed Dpl, which shares significant biochemical and structural homology with PrP(C). To investigate the physiological function of Dpl, we generated mice harbouring a homozygous disruption of the Prnd gene that encodes Dpl. Dpl deficiency did not interfere with embryonic and postnatal development, but resulted in male sterility. Dpl protein was expressed at late stages of spermiogenesis, and spermatids of Dpl mutants were reduced in numbers, immobile, malformed and unable to fertilize oocytes in vitro. Mechanical dissection of the zona pellucida partially restored in vitro fertilization. We conclude that Dpl regulates male fertility by controlling several aspects of male gametogenesis and sperm-egg interaction.

Molecular diagnostics of transmissible spongiform encephalopathies

Clinical criteria for the diagnosis of sporadic, iatrogenic and variant Creutzfeldt-Jakob diseases are now available and show an excellent sensitivity and specificity ( approximately 98%). Post-mortem diagnosis, based upon the identification in the brain of the pathological conformer of the prion protein (PrP(Sc)), is also very accurate, and several diagnostic kits are now available that facilitate the immunochemical measurement of PrP(Sc). Several new molecular diagnostic techniques aimed at increasing the sensitivity and specificity of PrP(Sc) detection, and at identifying markers of disease that are other than PrP(Sc), are the subject of ongoing studies. The aim of these studies is to develop preclinical screening tests for the identification of infected, but still healthy, individuals. These tests are also badly needed to check the safety of blood or blood-derived products, and to ensure meat safety in European countries.

Small is not beautiful: antagonizing functions for the prion protein PrP(C) and its homologue Dpl

A conformational variant of the normal prion protein PrP(C) is believed to be identical to PrP(Sc), the agent that causes prion diseases. Recently, a novel protein, named Doppel (Dpl), was identified that shares significant biochemical and structural homology with PrP(C). In specific strains of PrP(C)-deficient mouse lines, Dpl is overexpressed and causes a neurological disease. Dpl neurotoxicity is counteracted and prevented by PrP(C), but the mechanism of antagonistic PrP(C)-Dpl interaction remains elusive. In contrast to its homologue PrP(C), initial studies suggest that Dpl is dispensable for prion disease progression and for the generation of PrP(Sc). Although we are only beginning to understand its function, the discovery of Dpl has already provided some answers to long-standing questions and is transforming our understanding of prion biology.

Expression of doppel in the CNS of mice does not modulate transmissible spongiform encephalopathy disease

Late onset ataxia reported in three independently derived PrP null lines of mice has been attributed to the overexpression of the doppel protein in the CNS of these mice rather than to the loss of PrP. The central role of PrP in the transmissible spongiform encephalopathies (TSEs), the proximity of the gene which encodes doppel (Prnd) to the PrP gene (Prnp) and the structural similarity shared by PrP and doppel have led to the proposition that ataxia which develops during TSE disease could, in part, be due to doppel. In order to address this hypothesis, we have crossed our two inbred lines of PrP null mice, which either express (RCM) or do not express (NPU) the Prnd gene in the CNS, with mice expressing two Prnp(a[108F189V]) alleles of the PrP gene. We have found that the TSE infection does not influence the level of expression of Prnd in the CNS at the terminal stages of disease. Moreover, we have demonstrated that the level of expression of Prnd in the CNS has no influence on the incubation period, vacuolar pathology nor amount or distribution of PrP(Sc) deposition in the brains of the TSE-infected mice. Doppel has therefore no apparent influence on the outcome of TSE disease in transgenic mice, suggesting it is unlikely to be involved in the naturally occurring TSE diseases in other species.

Influence of the prion protein gene, Prnp, on scrapie susceptibility in sheep

Natural scrapie in sheep occurs through a complex interplay between host genetic elements and various strains of the infectious scrapie agent. Scrapie-related polymorphisms in the coding region of the prion protein (PrP) gene, Prnp, have been studied in a number of breeds. The disease-promoting V136 allele, and the susceptibility-reducing R171 allele, have proved to be most important. However, variation in the coding region of Prnp cannot alone explain the diverse patterns of scrapie susceptibility in various breeds. For instance, in many breeds plagued with scrapie, the V136 allele appears to be a rarity. The R171 allele greatly reduces scrapie susceptibility This lays the molecular foundation for marker-assisted breeding for reduced scrapie susceptibility now underway in many countries. Although potentially important, and still under investigation, variable expression level and pattern of the ovine Prnp appears to be of little importance for the occurrence of natural scrapie. Studies of scrapie in mice also indicate that genetic elements other than Prnp may have a strong influence on scrapie incubation time, and hence susceptibility. Narrowing down the search to focus on these elements and identification of candidate genes are important tasks for future research in sheep scrapie.

Sporadic--but not variant--Creutzfeldt-Jakob disease is associated with polymorphisms upstream of PRNP exon 1

Human prion diseases have inherited, sporadic, and acquired etiologies. The appearance of the novel acquired prion disease, variant Creutzfeldt-Jakob disease (vCJD), and the demonstration that it is caused by the same prion strain as that causing bovine spongiform encephalopathy, has led to fears of a major human epidemic. The etiology of classical (sporadic) CJD, which has a worldwide incidence, remains obscure. A common human prion-protein-gene (PRNP) polymorphism (encoding either methionine or valine at codon 129) is a strong susceptibility factor for sporadic and acquired prion disease. However, a quantitative-trait-locus study of prion incubation periods in mice has demonstrated an important factor that is close to Prnp but is independent of its coding sequence or that of the nearby prion-like doppel gene (Prnd). We have analyzed the PRNP locus for such tightly linked susceptibility factors. Fifty-six polymorphic sites have been identified within 25 kb of the PRNP open reading frame, including sites within the PRNP promoter and the PRNP 3' untranslated region. These have been characterized in 61 Centre d'Etude du Polymorphisme Humain (CEPH) families, demonstrating extensive linkage disequilibrium around PRNP and the existence of 11 major European PRNP haplotypes. Haplotype frequencies estimated in healthy U.K. control individuals were very similar to those deduced in the CEPH families. A common haplotype was overrepresented in patients with sporadic CJD (sCJD). Through use of a log-linear modeling approach to simultaneously model Hardy-Weinberg and linkage disequilibria, a significant independent association was found between sCJD and a polymorphism upstream of PRNP exon 1 (P=.005), in addition to the strong susceptibility conferred by codon 129 (P=2x10(-8)). However, although our sample size was necessarily small, no association was found between these polymorphisms and vCJD or iatrogenic CJD, in keeping with their having distinct disease mechanisms. In addition, there was no evidence of a PRNP founder effect in the first reported geographical cluster of vCJD.

The prion gene complex encoding PrP(C) and Doppel: insights from mutational analysis

The prion protein gene, Prnp, encodes PrP(Sc), the major structural component of prions, infectious pathogens causing a number of disorders including scrapie and bovine spongiform encephalopathy (or BSE). Missense mutations in the human Prnp gene cause inherited prion diseases such as familial Creutzfeldt-Jakob disease. In uninfected animals Prnp encodes a glycophosphatidylinositol (GPI)-anchored protein denoted PrP(C) and in prion infections PrP(C) is converted to PrP(Sc) by templated refolding. Though Prnp is conserved in mammalian species, attempts to verify interactions of putative PrP binding proteins by genetic means have proven frustrating and the ZrchI and Npu lines of Prnp gene-ablated mice (Prnp(0/0) mice) lacking PrP(C) remain healthy throughout development. This indicates that PrP(C) serves a function that is not apparent in a laboratory setting or that other molecules have overlapping functions. Current possibilities involve shuttling or sequestration of synaptic Cu(II) via binding to N-terminal octapeptide residues and/or signal transduction involving the fyn kinase. A new point of entry into the issue of prion protein function has emerged from identification of a paralogue, Prnd, with 24% coding sequence identity to Prnp. Prnd lies downstream of Prnp and encodes the doppel (Dpl) protein. Like PrP(C), Dpl is presented on the cell surface via a GPI anchor and has three alpha-helices: however, it lacks the conformationally plastic and octapeptide repeat domains present in its well-known relative. Interestingly, Dpl is overexpressed in the Ngsk and Rcm0 lines of Prnp(0/0) mice via intergenic splicing events. These lines of Prnp(0/0) mice exhibit ataxia and apoptosis of cerebellar cells, indicating that ectopic synthesis of Dpl protein is toxic to central nervous system neurons: this inference has now been confirmed by the construction of transgenic mice expressing Dpl under the direct control of the PrP promoter. Remarkably, Dpl-programmed ataxia is rescued by wild-type Prnp transgenes. The interaction between the Prnp and Prnd genes in mouse cerebellar neurons may have a physical correlate in competition between Dpl and PrP(C) within a common biochemical pathway that when mis-regulated leads to apoptosis.

Normal neurogenesis and scrapie pathogenesis in neural grafts lacking the prion protein homologue Doppel

The agent that causes prion diseases is thought to be identical to PrPSc, a conformer of the normal prion protein PrPC. Recently a novel protein, termed Doppel (Dpl), was identified that shares significant biochemical and structural homology with PrPC. To investigate the function of Dpl in neurogenesis and in prion pathology, we generated embryonic stem (ES) cells harbouring a homozygous disruption of the Prnd gene that encodes Dpl. After in vitro differentiation and grafting into adult brains of PrPC-deficient Prnp0/0 mice, Dpl-deficient ES cell-derived grafts contained all neural lineages analyzed, including neurons and astrocytes. When Prnd-deficient neural tissue was inoculated with scrapie prions, typical features of prion pathology including spongiosis, gliosis and PrPSc accumulation, were observed. Therefore, Dpl is unlikely to exert a cell-autonomous function during neural differentiation and, in contrast to its homologue PrPC, is dispensable for prion disease progression and for generation of PrPSc.