Loss of glycosylation associated with the T183A mutation in human prion disease

Loss of post-translational modification sites in disease

Understanding and predicting molecular cause of disease is one of the major challenges for biology and medicine. One particular area of interest continues to be computational analyses of disease-associated amino acid substitutions. To this end, various studies have been performed to identify molecular functions disrupted by disease-causing mutations. Here, we investigate the influence of disease-associated mutations on post-translational modifications. In particular, we study the loss of modification target sites as a consequence of disease mutation. We find that about 5% of disease-associated mutations may affect known modification sites, either partially (4%) of fully (1%), compared to about 2% of putatively neutral polymorphisms. Most of the fifteen post-translational modification types analyzed were found to be disrupted at levels higher than expected by chance. Molecular functions and physiochemical properties at sites of disease mutation were also compared to those of neutral polymorphisms involved in the process of post-translational modification site disruption. Disease-associated mutations in the neighborhood of post-translationally modified sites were found to be enriched in mutations that change polarity, charge, and hydrophobicity of the wild-type amino acids. Overall, these results further suggest that disruption of modification sites is an important but not the major cause of human genetic disease.

Phosphorylation of prion protein at serine 43 induces prion protein conformational change

The cause of the conformational change of normal cellular prion protein (PrP) into its disease-associated form is unknown. Posttranslational modifications, such as glycosylation, acetylation, S-nitrosylation, and phosphorylation, are known to induce protein conformational changes. Here, we investigated whether phosphorylation could induce the conformational change of PrP because PrP contains several kinase motifs and has been found recently in the cytosol, in which kinases generally reside. Neuronal cyclin-dependent kinase 5 (Cdk5) phosphorylated recombinant PrP(23-231) at serine 43 (S43) in an in vitro kinase assay. Cdk5-phosphorylated PrP became proteinase K resistant, formed Congo Red-positive fibrils, and formed aggregates that were immunostained with anti-PrP and anti-phospho-PrP(S43) (anti-pPrP(S43)). pPrP(S43) was detected in PrP/Cdk5/p25 cotransfected N2a cells. Roscovitine inhibition of Cdk5 activity or transfection of N2a cells with mutant PrP S43A eliminated the anti-pPrP(S43)-immunopositive protein. Alkaline phosphatase-sensitive and proteinase K-resistant pPrP(S43) immunoreactivity was observed in scrapie-infected but not control-injected mice brains. These results raise the possibility that phosphorylation could represent a physiological mechanism of PrP conversion in vivo.

The consequences of pathogenic mutations to the human prion protein

Prion diseases, in which the conformational transition of the native prion protein (PrP) to a misfolded form causes aggregation and subsequent neurodegeneration, have fascinated the scientific community as this transmissible disease appears to be purely protein-based. Disease can arise due to genetic factors only. At least 30 single point mutations have been indicated to cause disease in humans. Somehow, these mutations must influence the stability, processing and/or cellular interactions of PrP, such that aggregation can occur and disease develops. In this review, the current evidence for such effects of single point mutations is discussed, indicating that PrP can be affected in many different ways, although questions remain about the mechanism by which mutations cause disease.

A novel PRNP-P105S mutation associated with atypical prion disease and a rare PrPSc conformation

OBJECTIVE

To define the clinicopathologic, genetic, and pathogenic prion protein (PrP(Sc)) characteristics associated with a novel mutation of the prion protein gene (PRNP).

METHODS

The coding segment of PRNP from the proband and family members was sequenced and the brain of the proband was histologically studied. The Western blot profile of the proteinase K (PK) resistant fraction of PrP(Sc), an approximation of its conformation, or "PrP(Sc)-type," was determined.

RESULTS

We detected a novel mutation at codon 105 of PRNP that results in a serine (S) substitution of proline (P) (P105S), in a young woman who developed progressive aphasia, behavioral changes, dementia, and parkinsonism, lasting 10 years to her death. Histopathologic findings included an intense focus of multicentric PrP-plaques within the hippocampus, punctate plaques scattered throughout the cerebellum, and intense spongiform degeneration focally within the putamen, suggesting a variant of Gerstmann-Sträussler-Scheinker syndrome (GSS). However, PrP(Sc)-typing revealed two PK-resistant PrP(Sc) fragments (approximately 21 and 26 kDa), a pattern not previously detected in GSS.

CONCLUSIONS

This mutation is the third sequence variation at codon 105 of PRNP. The unusual phenotype and PrP(Sc)-type distinguishes this genetic prion disease from typical Gerstmann-Sträussler-Scheinker syndrome and other codon 105 substitutions, suggesting that, in addition to the loss of proline at this position, the PrP(Sc) conformation and phenotype is dependent on the specific amino acid substitution.

Regulating factors of PrP glycosylation in Creutzfeldt-Jakob disease--implications for the dissemination and the diagnosis of human prion strains

OBJECTIVE

The glycoprofile of pathological prion protein (PrP(res)) is widely used as a diagnosis marker in Creutzfeldt-Jakob disease (CJD) and is thought to vary in a strain-specific manner. However, that the same glycoprofile of PrP(res) always accumulates in the whole brain of one individual has been questioned. We aimed to determine whether and how PrP(res) glycosylation is regulated in the brain of patients with sporadic and variant Creutzfeldt-Jakob disease.

METHODS

PrP(res) glycoprofiles in four brain regions from 134 patients with sporadic or variant CJD were analyzed as a function of the genotype at codon 129 of PRNP and the Western blot type of PrP(res).

RESULTS

The regional distribution of PrP(res) glycoforms within one individual was heterogeneous in sporadic but not in variant CJD. PrP(res) glycoforms ratio significantly correlated with the genotype at codon 129 of the prion protein gene and the Western blot type of PrP(res) in a region-specific manner. In some cases of sCJD, the glycoprofile of thalamic PrP(res) was undistinguishable from that observed in variant CJD.

INTERPRETATION

Regulations leading to variations of PrP(res) pattern between brain regions in sCJD patients, involving host genotype and Western blot type of PrP(res) may contribute to the specific brain targeting of prion strains and have direct implications for the diagnosis of the different forms of CJD.

Viral infection and human disease--insights from minimotifs

Short functional peptide motifs cooperate in many molecular functions including protein interactions, protein trafficking, and posttranslational modifications. Viruses exploit these motifs as a principal mechanism for hijacking cells and many motifs are necessary for the viral life-cycle. A virus can accommodate many short motifs in its small genome size providing a plethora of ways for the virus to acquire host molecular machinery. Host enzymes that act on motifs such as kinases, proteases, and lipidation enzymes, as well as protein interaction domains, are commonly mutated in human disease, suggesting that the short peptide motif targets of these enzymes may also be mutated in disease; however, this is not observed. How can we explain why viruses have evolved to be so dependent on motifs, yet these motifs, in general do not seem to be as necessary for human viability? We propose that short motifs are used at the system level. This system architecture allows viruses to exploit a motif, whereas the viability of the host is not affected by mutation of a single motif.

Gain-of-glycosylation mutations

Disease-causing missense (and other in-frame) mutations can exert their deleterious effects at the cellular level through multiple mechanisms. A pathogenic mechanism involves the addition of a novel N-linked glycan. Up to 1.4% of known disease-causing missense mutations are predicted to give rise to gains-of-glycosylation. For some of these mutations, the novel glycans have been shown to be both necessary and sufficient to account for the deleterious impact of the mutation. The chemical complementation of cells from patients in vitro with various modifiers of glycosylation has been demonstrated and raises the possibility of specific chemical treatments for patients bearing gain-of-glycosylation mutations.

V180I mutation of the prion protein gene associated with atypical PrPSc glycosylation

A valine to isoleucine mutation at residue 180 was identified in a French patient with Creutzfeldt-Jakob disease (CJD). The mutation is located in the close vicinity of one of the two N-glycosylation sites of the cellular prion protein (PrP(C)). Western blot analysis revealed accumulation in the brain of the pathogenic proteinase K-resistant PrP (PrP(Sc)) isoform with the notable absence of the diglycosylated band. The mutant protein expressed in CHO cells was correctly glycosylated, suggesting that the atypical glycosylation pattern of PrP(Sc) was not due to the mutation at position 180. These results suggest that the diglycosylated form of the mutant PrP(180I) prevents its conversion into the pathogenic mutant form PrP(Sc180I), supporting a central role of N-linked glycan chains in the PrP conversion process.

Intracellular accumulation of a 46 kDa species of mouse prion protein as a result of loss of glycosylation in cultured mammalian cells

Prion diseases are fatal neurodegenerative disorders characterized by the accumulation of an abnormal isoform (PrPSc) of the normal cellular prion protein (PrPC) in the brain. Reportedly, abnormal N-linked glycosylation patterns in PrPC are associated with disease susceptibility; thus, we compared the glycosylation status of normal and several mutant forms of the murine prion protein (MuPrP) in cultured mammalian cells. Substitution of the N-terminal signal sequence of normal MuPrP with a heterologous signal peptide did not alter glycosylation. When expressed without the C-terminal glycophosphatidylinositol anchor signal, the majority of MuPrP remained intracellular and unglycosylated, and a 46 kDa species (p46) of the unglycosylated PrPC was detected on reducing gels. p46 was also observed when wild-type MuPrP was expressed in the presence of tunicamycin or enzymatically deglycosylated in vitro. A rabbit polyclonal anti-serum raised against dimeric MuPrP cross-reacted with p46 and localized the signal within the Golgi apparatus. We propose that the 46 kDa signal is a dimeric form of MuPrP and in the light of recent studies, it can be argued that a relatively stable, non-glycosylated, cytoplasmic PrPC dimer, produced as a result of compromised glycosylation is an intermediate in initiating conversion of PrPC to PrPSc in sporadic transmissible spongiform encephalopathies.

Altered Glycosylated PrP Proteins Can Have Different Neuronal Trafficking in Brain but Do Not Acquire Scrapie-like Properties

N-Linked glycans have been shown to have an important role in the cell biology of a variety of cell surface glycoproteins, including PrP protein. It has been suggested that glycosylation of PrP can influence the susceptibility to transmissible spongiform encephalopathy and determine the characteristics of the many different strains observed in this particular type of disease. To understand the role of carbohydrates in influencing the PrP maturation, stability, and cell biology, we have produced and analyzed gene-targeted murine models expressing differentially glycosylated PrP. Transgenic mice carrying the PrP substitution threonine for asparagine 180 (G1) or threonine for asparagine 196 (G2) or both mutations combined (G3), which eliminate the first, second, and both glycosylation sites, respectively, have been generated by double replacement gene targeting. An in vivo analysis of altered PrP has been carried out in transgenic mouse brains, and our data show that the lack of glycans does not influence PrP maturation and stability. The presence of one chain of sugar is sufficient for the trafficking to the cell membrane, whereas the unglycosylated PrP localization is mainly intracellular. However, this altered cellular localization of PrP does not lead to any overt phenotype in the G3 transgenic mice. Most importantly, we found that, in vivo, unglycosylated PrP does not acquire the characteristics of the aberrant pathogenic form (PrPSc), as was previously reported using in vitro models.