Single-point mutations of the M protein of a measles virus variant obtained from a patient with subacute sclerosing panencephalitis critically affect solubility and subcellular localization of the M protein and cell-free virus production

Nairovirus polymerase mutations associated with the establishment of persistent infection in human cells

Crimean-Congo hemorrhagic fever virus (CCHFV), a tick-borne virus of the Orthonairovirus genus, persistently infects tick cells. It has been reported to establish persistent infection in non-human primates, but virological analysis has not yet been performed in human cells. Here, we investigated whether and how nairoviruses persistently infect human cells using Hazara orthonairovirus (HAZV), a surrogate model for CCHFV. We established a human cell line that was persistently infected with HAZV. Surprisingly, virions of persistently infected HAZV (HAZVpi) were not observed in the culture supernatants. There were five mutations (mut1, mut2, mut3, mut4, and mut5) in L protein of HAZVpi. Mutations in L protein of HAZVpi contribute to non-detection of virion in the supernatants. Lmut4 was found to cause low viral growth rate, despite its high polymerase activity. The low growth rate was restored by Lmut2, Lmut3, and Lmut5. The polymerase activity of Lmut1 was extremely low, and recombinant HAZV carrying Lmut1 (rHAZV/Lmut1) was not released into the supernatants. However, genomes of rHAZV/Lmut1 were retained in the infected cells. All mutations (Lmut1-5) found in L protein of HAZVpi were required for experimental reproduction of HAZVpi, and only Lmut1 and Lmut4 were insufficient. We demonstrated that point mutations in viral polymerase contribute to the establishment of persistent HAZV infection. Furthermore, innate immunity was found to be suppressed in HAZVpi-infected cells, which also potentially contributes to viral persistence. This is the first presentation of a possible mechanism behind how nairoviruses establish persistent infection in human cells.

IMPORTANCE

We investigated whether and how nairoviruses persistently infect human cells, using Hazara orthonairovirus (HAZV), a surrogate model for Crimean-Congo hemorrhagic fever virus. We established a human cell line that was persistently infected with HAZV. Five mutations were found in L protein of persistently infected HAZV (HAZVpi): mut1, mut2, mut3, mut4, and mut5. Among them, Lmut1 and Lmut4 restricted viral growth by low polymerase activity and low growth rate, respectively, leading to inhibition of viral overgrowth. The restriction of viral growth caused by Lmut1 and Lmut4 was compensated by other mutations, including Lmut2, Lmut3, and Lmut5. Each of the mutations found in L protein of HAZVpi was concluded to cooperatively modulate viral growth, which facilitates the establishment of persistent infection. Suppression of innate immunity also potentially contributes to virus persistence. This is the first presentation of a possible mechanism behind how nairoviruses establish persistent infection in human cells.

Suppression of viral RNA polymerase activity is necessary for persistent infection during the transformation of measles virus into SSPE virus

Subacute sclerosing panencephalitis (SSPE) is a fatal neurodegenerative disease caused by measles virus (MV), which typically develops 7 to 10 years after acute measles. During the incubation period, MV establishes a persistent infection in the brain and accumulates mutations that generate neuropathogenic SSPE virus. The neuropathogenicity is closely associated with enhanced propagation mediated by cell-to-cell fusion in the brain, which is principally regulated by hyperfusogenic mutations of the viral F protein. The molecular mechanisms underlying establishment and maintenance of persistent infection are unclear because it is impractical to isolate viruses before the appearance of clinical signs. In this study, we found that the L and P proteins, components of viral RNA-dependent RNA polymerase (RdRp), of an SSPE virus Kobe-1 strain did not promote but rather attenuated viral neuropathogenicity. Viral RdRp activity corresponded to F protein expression; the suppression of RdRp activity in the Kobe-1 strain because of mutations in the L and P proteins led to restriction of the F protein level, thereby reducing cell-to-cell fusion mediated propagation in neuronal cells and decreasing neuropathogenicity. Therefore, the L and P proteins of Kobe-1 did not contribute to progression of SSPE. Three mutations in the L protein strongly suppressed RdRp activity. Recombinant MV harboring the three mutations limited viral spread in neuronal cells while preventing the release of infectious progeny particles; these changes could support persistent infection by enabling host immune escape and preventing host cell lysis. Therefore, the suppression of RdRp activity is necessary for the persistent infection of the parental MV on the way to transform into Kobe-1 SSPE virus. Because mutations in the genome of an SSPE virus reflect the process of SSPE development, mutation analysis will provide insight into the mechanisms underlying persistent infection.

M protein of subacute sclerosing panencephalitis virus, synergistically with the F protein, plays a crucial role in viral neuropathogenicity

Subacute sclerosing panencephalitis (SSPE) is a rare fatal neurodegenerative disease caused by a measles virus (MV) variant, SSPE virus, that accumulates mutations during long-term persistent infection of the central nervous system (CNS). Clusters of mutations identified around the matrix (M) protein in many SSPE viruses suppress productive infectious particle release and accelerate cell–cell fusion, which are features of SSPE viruses. It was reported, however, that these defects of M protein function might not be correlated directly with promotion of neurovirulence, although they might enable establishment of persistent infection. Neuropathogenicity is closely related to the character of the viral fusion (F) protein, and amino acid substitution(s) in the F protein of some SSPE viruses confers F protein hyperfusogenicity, facilitating viral propagation in the CNS through cell–cell fusion and leading to neurovirulence. The F protein of an SSPE virus Kobe-1 strain, however, displayed only moderately enhanced fusion activity and required additional mutations in the M protein for neuropathogenicity in mice. We demonstrated here the mechanism for the M protein of the Kobe-1 strain supporting the fusion activity of the F protein and cooperatively inducing neurovirulence, even though each protein, independently, has no effect on virulence. The occurrence of SSPE has been estimated recently as one in several thousand in children who acquired measles under the age of 5 years, markedly higher than reported previously. The probability of a specific mutation (or mutations) occurring in the F protein conferring hyperfusogenicity and neuropathogenicity might not be sufficient to explain the high frequency of SSPE. The induction of neurovirulence by M protein synergistically with moderately fusogenic F protein could account for the high frequency of SSPE.

Aprepitant in the Treatment of Subacute Sclerosing Panencephalitis: A Randomized, Double-Blind, Placebo-Controlled Study

BACKGROUND

Aprepitant is a neurokinin-1 receptor antagonist approved for the treatment of chemotherapy-induced nausea. We aimed to investigate the safety and efficacy of aprepitant in patients with subacute sclerosing panencephalitis.

METHODS

A randomized, double-blind, placebo-controlled study was conducted in patients with subacute sclerosing panencephalitis assigned to receive two courses of aprepitant 250 mg/day orally or placebo for 15 days with an interval of two months between courses. Primary end points were safety and tolerability, and secondary end point was clinical improvement or stabilization assessed by subacute sclerosing panencephalitis scoring system. Electroencephalography (EEG), brain magnetic resonance imaging, and cerebrospinal fluid measles-specific immunoglobulin G index were evaluated before and after treatment.

RESULTS

Sixty-two patients with subacute sclerosing panencephalitis were allocated to aprepitant (n = 31, median age 18 years) or placebo (n = 31, median age 22 years) group. Fifteen patients left the study within the first six months and 12 patients left between six and 12 months. Aprepitant was well tolerated and treatment-associated adverse events were similar to those described in the treatment of nausea. Clinical status at six and 12 months' follow-up did not differ between aprepitant and placebo groups. Post-treatment EEG scores at 12 months were better in the aprepitant group (P = 0.015). Cerebral atrophy on magnetic resonance imaging increased in both groups, whereas measles-specific immunoglobulin G index decreased in the placebo group.

CONCLUSIONS

In this first clinical trial of aprepitant treatment in patients with subacute sclerosing panencephalitis, the drug was safe and well tolerated. No clinical effect was observed. A modest improvement in EEG findings might justify trials for longer periods because EEG changes can precede clinical findings in subacute sclerosing panencephalitis.

Genetic characterization of measles virus genotype D6 subacute sclerosing panencephalitis case, Alberta, Canada

Subacute sclerosing panencephalitis (SSPE) is a progressive and eventually fatal neurological disease arising from a persistent infection with measles virus (MV) acquired at a young age. SSPE measles virus strains are defective and unable to produce progeny virions, due to multiple and extensive mutations in a number of key genes. We sequenced the full MV genome from our recently reported SSPE case, which typed as genotype D6, and compared it with other genotype D6 wild type and SSPE sequences. The Alberta D6 strain was significantly different from other reported SSPE D6 sequences. Mutations were observed in all the genes of the Alberta strain, with the greatest sequence divergence noted in the M gene with 17.6% nucleotide and 31% amino acid variation. The L gene showed the least variation with 1.3% nucleotide and 0.7% amino acid differences respectively. The nucleotide variability for 15,672 bases of the complete genome compared to the wild type and other SSPE D6 strains was around 3%.

Measles Virus Matrix Protein Inhibits Host Cell Transcription

Measles virus (MeV) is a highly contagious virus that still causes annual epidemics in developing countries despite the availability of a safe and effective vaccine. Additionally, importation from endemic countries causes frequent outbreaks in countries where it has been eliminated. The M protein of MeV plays a key role in virus assembly and cytopathogenesis; interestingly, M is localised in nucleus, cytoplasm and membranes of infected cells. We have used transient expression of M in transfected cells and in-cell transcription assays to show that only some MeV M localizes to the nucleus, in addition to cell membranes and the cytoplasm as previously described, and can inhibit cellular transcription via binding to nuclear factors. Additionally, MeV M was able to inhibit in vitro transcription in a dose-dependent manner. Importantly, a proportion of M is also localized to nucleus of MeV infected cells at early times in infection, correlating with inhibition of cellular transcription. Our data show, for the first time, that MeV M may play a role early in infection by inhibiting host cell transcription.

Constraints on the Genetic and Antigenic Variability of Measles Virus

Antigenic drift and genetic variation are significantly constrained in measles virus (MeV). Genetic stability of MeV is exceptionally high, both in the lab and in the field, and few regions of the genome allow for rapid genetic change. The regions of the genome that are more tolerant of mutations (i.e., the untranslated regions and certain domains within the N, C, V, P, and M proteins) indicate genetic plasticity or structural flexibility in the encoded proteins. Our analysis reveals that strong constraints in the envelope proteins (F and H) allow for a single serotype despite known antigenic differences among its 24 genotypes. This review describes some of the many variables that limit the evolutionary rate of MeV. The high genomic stability of MeV appears to be a shared property of the Paramyxovirinae, suggesting a common mechanism that biologically restricts the rate of mutation.

Subacute sclerosing panencephalitis in pregnancy

We present a case of subacute sclerosing panencephalitis that developed in a previously healthy 29-year-old pregnant woman who had returned from a trip to rural India shortly before the onset of symptoms. She was admitted to hospital at 27 weeks' gestation with a history of cognitive decline and difficulty completing simple tasks. She had no clinical signs of infection. The working diagnosis was autoimmune encephalitis, although extensive investigations did not lead to a final classifying diagnosis. The patient became comatose and developed hypertension, and an emergency caesarean section was done at 31 weeks to deliver the child, who seemed healthy. The patient died about 6 weeks after the onset of symptoms. The patient was found to have had subacute sclerosing panencephalitis at autopsy. In this Grand Round, we review the clinical features and treatment of subacute sclerosing panencephalitis, and the epidemiological and public health aspects of the case.

The first genetic characterization of a D4 measles virus strain derived from a patient with subacute sclerosing panencephalitis

Measles virus (MV) strains derived from patients with subacute sclerosing panencephalitis (SSPE), SSPE strains, possess numerous mutations when compared to viruses belonging to the same genotype and circulating in similar time period. Although many SSPE strains have been extensively characterized, none of them belongs to D4 genotype which currently predominates in Europe where it has caused a number of recent outbreaks/epidemics. We sequenced an MV derived from a patient with long-term SSPE; the virus was named MVs/Zagreb.CRO/30.06[D4] (SSPE). Initial genetic analysis showed that it belongs to D4 genotype. The sequences of genes encoding matrix and fusion proteins indicate premature protein terminations. Putative hemagglutin (H) protein is lengthened for 20 amino acids, which is the longest H protein elongation so far found in SSPE viruses. Nucleotides 1421 A, 1422 G, 1507 C and 1542 C in nucleoprotein gene open reading frame seem to be specific for this D4 strain, differentiating it from other D4 non-SSPE strains. Besides, a unique mutation at position 543 of H protein was found, histidine instead of tyrosine. As persistent MV infections are initially established by "normal" wild-type MV strains, the presented comparative analyses describe alterations that could be involved in the maintenance of persistent infection, disease development and progression.

F-actin modulates measles virus cell-cell fusion and assembly by altering the interaction between the matrix protein and the cytoplasmic tail of hemagglutinin

Actin filament (F-actin) is believed to be involved in measles virus (MV) assembly as a cellular factor, but the precise roles remain unknown. Here we show that Phe at position 50 of the MV matrix (M) protein is important for its association with F-actin, through which the function of the M protein is regulated. In plasmid-expressed or MV-infected cells, a coimmunoprecipitation study revealed that the wild-type M (M-WT) protein associated strongly with F-actin but only weakly with the cytoplasmic tail of the hemagglutinin (H) protein. Since the F50P mutation allowed the M protein the enhanced interaction with the H protein in return for the sharply declined association with F-actin, the mutant M (M-F50P) protein strongly inhibited MV cell-cell fusion and promoted the uptake of the H protein into virus particles. The abundantly incorporated H protein resulted in the increase in infectivity of the F50P virus, although the virus contained a level of genome RNA equal to that of the WT virus. When the structure of F-actin was disrupted with cytochalasin D, the M-WT protein liberated from F-actin interacted with the H protein as tightly as the M-F50P protein, suppressing cell-cell fusion and promoting virus assembly comparably efficiently as the M-F50P protein. The cell-cell fusion activity of the WT virus appeared to be upheld by F-actin, which prevents the M protein interaction with the H protein. Our results indicate that F-actin in association with the M protein alters the interaction between the M and H proteins, thereby modulating MV cell-cell fusion and assembly.