Synthetic Prions Provide Clues for Understanding Prion Diseases

Clinical profiles and ethnic heterogeneity of sporadic fatal insomnia

BACKGROUND AND PURPOSE

This study was undertaken to elucidate the clinical profile of sporadic fatal insomnia (sFI), assess the similarities and differences between sFI and fatal familial insomnia (FFI), and evaluate the influence of ethnicity on the phenotype of sFI patients.

METHODS

The data of sFI and FFI patients were retrieved from our case series and through literature review. The clinical and diagnostic features of sFI and FFI were compared, as were the phenotypes of Asian and Caucasian sFI patients.

RESULTS

We identified 44 sFI and 157 FFI cases. The prevalence of sleep-related, neuropsychiatric, and autonomic symptoms among the sFI patients were 65.9%, 100.0%, and 43.2%, respectively. Compared to FFI, sFI exhibited longer disease duration and a higher proportion of neuropsychiatric symptoms, whereas FFI was characterized by a higher incidence of sleep-related and autonomic symptoms in the early stages of the disease or throughout its course. In addition, a higher proportion of the sFI patients showed hyperintensity on magnetic resonance imaging (MRI) and periodic sharp wave complexes on electroencephalography compared to the FFI patients, especially those presenting with pathological changes associated with MM2-cortical type sporadic Creutzfeldt-Jakob disease. The Asian sFI patients had a higher proportion of males and positivity for cerebrospinal fluid 14-3-3 protein, and fewer sleep-related symptoms compared to Caucasian sFI patients. The age at onset and duration of sFI differed between ethnic groups, but the difference failed to reach statistical significance.

CONCLUSIONS

Despite its similarities to FFI, sFI is characterized by longer disease duration, higher proportion of neuropsychiatric symptoms, and hyperintensity on MRI, along with differences in the clinical characteristics based on ethnicity.

Fatal familial insomnia and Agrypnia Excitata: Autonomic dysfunctions and pathophysiological implications

Fatal Familial Insomnia (FFI) is a hereditary prion disease caused by a mutation at codon 178 of the prion-protein gene leading to a D178N substitution in the protein determining severe and selective atrophy of mediodorsal and anteroventral thalamic nuclei. FFI is characterized by physiological sleep loss, which polygraphically appears to be a slow wave sleep loss, autonomic and motor hyperactivation with peculiar episodes of oneiric stupor. Alteration of autonomic functions is a great burden for FFI patients consisting in sympathetic overactivation, dysregulation of its physiological responses and disruption of circadian rhythms. The cardiovascular system is the most frequently and severely affected confirming the increased sympathetic drive with preserved parasympathetic responses. Sleep loss, autonomic and motor hyperactivation define Agrypnia Excitata (AE), which is not exclusive to FFI, but it has been canonically described also in Morvan Syndrome and Delirium Tremens. These three conditions present different pathophysiological mechanisms but share the same thalamo-limbic impairment of which AE is one of the possible clinical presentations. FFI, and consequently also AE, is a model for the investigation of the essential role of the thalamus in the organization of body homeostasis, integrating both sleep and autonomic function control.

Fatal familial insomnia and sporadic fatal insomnia

Fatal familial insomnia (FFI) and sporadic fatal insomnia (sFI), or thalamic form of sporadic Creutzfeldt-Jakob disease MM2 (sCJDMM2T), are prion diseases originally named and characterized in 1992 and 1999, respectively. FFI is genetically determined and linked to a D178N mutation coupled with the M129 genotype in the prion protein gene (PRNP) at chromosome 20. sFI is a phenocopy of FFI and likely its sporadic form. Both diseases are primarily characterized by progressive sleep impairment, disturbances of autonomic nervous system, and motor signs associated with severe loss of nerve cells in medial thalamic nuclei. Both diseases harbor an abnormal disease-associated prion protein isoform, resistant to proteases with relative mass of 19 kDa identified as resPrP^TSE type 2. To date at least 70 kindreds affected by FFI with 198 members and 18 unrelated carriers along with 25 typical cases of sFI have been published. The D178N-129M mutation is thought to cause FFI by destabilizing the mutated prion protein and facilitating its conversion to PrP^TSE. The thalamus is the brain region first affected. A similar mechanism triggered spontaneously may underlie sFI.

Strains of Pathological Protein Aggregates in Neurodegenerative Diseases

The presence of protein aggregates in the brain is a hallmark of neurodegenerative disorders such as Alzheimer’s disease (AD) and Parkinson’s disease (PD). Considerable evidence has revealed that the pathological protein aggregates in many neurodegenerative diseases are able to self-propagate, which may enable pathology to spread from cell-to-cell within the brain. This property is reminiscent of what occurs in prion diseases such as Creutzfeldt-Jakob disease. A widely recognized feature of prion disorders is the existence of distinct strains of prions, which are thought to represent unique protein aggregate structures. A number of recent studies have pointed to the existence of strains of protein aggregates in other, more common neurodegenerative illnesses such as AD, PD, and related disorders. In this review, we outline the pathobiology of prion strains and discuss how the concept of protein aggregate strains may help to explain the heterogeneity inherent to many human neurodegenerative disorders.

Novel strain properties distinguishing sporadic prion diseases sharing prion protein genotype and prion type

In most human sporadic prion diseases the phenotype is consistently associated with specific pairings of the genotype at codon 129 of the prion protein gene and conformational properties of the scrapie PrP (PrP^Sc) grossly identified types 1 and 2. This association suggests that the 129 genotype favours the selection of a distinct strain that in turn determines the phenotype. However, this mechanism cannot play a role in the phenotype determination of sporadic fatal insomnia (sFI) and a subtype of sporadic Creutzfeldt-Jakob disease (sCJD) identified as sCJDMM2, which share 129 MM genotype and PrP^Sc type 2 but are associated with quite distinct phenotypes. Our detailed comparative study of the PrP^Sc conformers has revealed major differences between the two diseases, which preferentially involve the PrP^Sc component that is sensitive to digestion with proteases (senPrP^Sc) and to a lesser extent the resistant component (resPrP^Sc). We conclude that these variations are consistent with two distinct strains in sFI and sCJDMM2, and that the rarer sFI is the result of a variant strain selection pathway that might be favoured by a different brain site of initial PrP^Sc formation in the two diseases.