Research Article: Protease-sensitive prions with 144-bp insertion mutations

Date Published: March 28, 2013

Publisher: Impact Journals LLC

Author(s): Xiangzhu Xiao, Ignazio Cali, Zhiqian Dong, Gianfranco Puoti, Jue Yuan, Liuting Qing, Heming Wang, Qingzhong Kong, Pierluigi Gambetti, Wen-Quan Zou.

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Abstract

Insertion of 144-base pair (bp) containing six extra octapeptide repeats between residues 51 and 91 of prion protein (PrP) gene is associated with inherited prion diseases. Most cases linked to this insertion examined by Western blotting showed detectable proteinase K-resistant PrPSc (rPrPSc) resembling PrPSc type 1 and type 2 in sporadic Creutzfeldt-Jakob disease (sCJD), or PrP7-8 in Gerstmann-Sträussler-Scheinker disease. However, cases lacking detectable rPrPSc also have been reported. Which PrP conformer is associated with neuropathological changes in the cases without detectable rPrPSc remains to be determined. Here we report that while all six but one subjects with the 144-bp insertion mutations examined display the pathognomonic PrP patches in the cerebellum, one of them exhibits no detectable typical rPrPSc even in PrPSc-enriched preparations. Instead, a large amount of abnormal PrP is captured from this case by gene 5 protein and sodium phosphotungstate, reagents that have been proved to specifically capture abnormal PrP. All captured abnormal PrP from the cerebellum and other brain regions is virtually sensitive to PK-digestion (termed sPrPSc). The presence of the predominant sPrPSc but absence of rPrPSc in this 144-bp insertion-linked inherited CJD case suggests that mutant sPrPSc is the main component of the PrP deposit patches and sPrPSc is sufficient to cause neurotoxicity and prion disease.

Partial Text

Mutations of prion protein gene (PRNP) are associated with a group of inherited prion diseases that are characterized clinically by dementia, ataxia, and myoclonus and pathologically by spongiform de-generation, astrocytic gliosis, and neuronal loss [1]. Like sporadic and acquired forms of prion diseases, the molecular hallmark of inherited prion diseases is the deposition in the central nervous system (CNS) of an abnormal isoform of prion protein (PrPSc) that is derived from a host-encoded cellular prion protein (PrPC) via a structural transition from α-helices into β-sheet structures [2]. However, unlike the other two forms of the diseases, the conversion of PrPC into the pathological PrPSc in inherited prion diseases is believed to be spontaneously triggered by the mutated PrP allele (PrPM). The wild-type PrP allele (PrPWt) may or may not be recruited into PrPSc by the PrPM allele [3-10]. Regardless of distinct etiologies, the PrPSc molecules present in all human prion diseases share some common structural, physicochemical, and biological properties, including a β-sheet-rich structure, resistance to proteinase K (PK) digestion, insolubility in non-denaturing detergents, and infectivity [2]. It has been well-documented that the co-existence of PrPC and PK-resistant PrPSc (rPrPSc) is a prerequisite for the pathogenesis of various prion diseases; however, what type of PrPSc conformers are directly responsible for the PrP deposition in the brain and the neuropathological changes in the prion diseases remains poorly understood [11].

Neurodegenerative disorders are all associated with misfolding of various cellular proteins [42-44]. Human prion diseases including sporadic, inherited and infectious forms are highly heterogeneous in terms of their broad range of clinical and pathological phenotypes [12]. In addition to the transmissible spongiform encephalopathy (TSE), non-transmissible prion diseases have also been reported [45-47], which has brought about a proposal that the spectrum of prion diseases should be beyond the classic definition of TSE [48]. The high heterogeneity of prion diseases may be associated with the chameleon-like conformations of the PrPSc molecule [11], the only component identified in the infectious prion pathogen to date [2]. Presence of a variety of rPrPSc including PrPSc type 1 and type 2 in sCJD and PrP7-8 in GSS detected by Western blotting and distinct brain PrP deposits detected by immunohistochemistry might be attributable to the variable PrPSc conformation [11, 12]. Recently, novel PK-resistant PrP species with a distinctive ladder-like gel profile have also been identified in a new prion disease termed VPSPr [34, 39]. These newly-identified PK-resistant PrP fragments are preferentially detected by 1E4 but much less immunoreactivity with the widely used 3F4, which is similar to those PK-resistant PrP species detected in the normal brain and uninfected cultured cells [33, 37, 49, 50]. Studies on the correlation between the phenotypic heterogeneity of the diseases and the chameleon-like conformation of PrPSc molecule are often complicated by the diversity in the etiologies of the diseases. Therefore, an investigation on cases with a single etiology will be critical for understanding the molecular mechanism responsible for this high heterogeneity in disease phenotypes.

 

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