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Influenza virus-susceptible mice carry Mx genes with a large deletion or a nonsense mutation
1Department of Molecular Biology, Research Institute of Scripps Clinic, La Jolla, California 92037.
Abstract:
The interferon-regulated mouse Mx gene encodes the 72-kilodalton nuclear Mx protein that selectively inhibits influenza virus replication. Mice carrying Mx+ alleles synthesize Mx protein and resist influenza virus infection, whereas mice homozygous for Mx- alleles fail to synthesize Mx protein and, as a consequence, are influenza virus susceptible. Southern blot analysis allowed us to define the following three distinct Mx restriction fragment length polymorphism (RFLP) types among classical inbred strains: RFLP type 1 in the Mx+ strains A2G and SL/NiA, RFLP type 2 in BALB/c and 33 other Mx- strains, and RFLP type 3 in CBA/J and 2 other Mx- strains. cDNA clones of Mx mRNAs from BALB/c and CBA/J cells were isolated, and their sequences were compared with that of the wild-type Mx mRNA of strain A2G. Mx mRNA of BALB/c mice has 424 nucleotides absent from the coding region, resulting in a frame shift and premature termination of Mx protein. The missing sequences correspond exactly to Mx exons 9 through 11. These three exons, together with some flanking intron sequences, are deleted from the genomes of all Mx RFLP type 2 strains. The Mx- phenotype of the Mx RFLP type 3 strain CBA/J is due to a point mutation that converts the lysine codon in position 389 to a termination codon. Mx RFLP type 3 strains have an extra HindIII site which maps to an intron and thus probably does not affect the coding capacity of Mx mRNA. We further show that the Mx mRNA levels in interferon-treated BALB/c and CBA/J cells are about 15-fold lower than in similarly treated Mx+ cells. This is probably due to decreased metabolic stabilities of the mutant mRNAs.
Insights
Mice with Mx+ alleles resist influenza, while Mx- mice are susceptible due to mutations in the Mx gene. Genetic analysis revealed specific deletions and point mutations causing this resistance difference.
Area of Science:
- Immunology
- Genetics
- Virology
Background:
- The mouse Mx gene produces a 72-kDa nuclear protein that inhibits influenza virus replication.
- Mx+ alleles confer resistance to influenza, while Mx- alleles lead to susceptibility due to lack of Mx protein synthesis.
Purpose of the Study:
- To investigate the genetic basis of Mx protein deficiency and influenza susceptibility in mice.
- To characterize the molecular defects in Mx- strains.
Main Methods:
- Southern blot analysis to identify Mx restriction fragment length polymorphism (RFLP) types.
- cDNA cloning and sequencing of Mx mRNAs from different mouse strains.
- Comparison of wild-type and mutant Mx mRNA sequences.
Main Results:
- Three distinct Mx RFLP types were identified among inbred strains.
- Mx- BALB/c mice have a deletion of exons 9-11 in their Mx mRNA, causing a frame shift and premature protein termination.
- Mx- CBA/J mice possess a point mutation converting a lysine codon to a termination codon at position 389.
- Mutant Mx mRNAs in BALB/c and CBA/J cells exhibit reduced metabolic stability, leading to lower Mx mRNA levels.
Conclusions:
- Genetic variations, including exon deletions and point mutations, in the mouse Mx gene underlie differential influenza virus resistance.
- These molecular defects result in non-functional Mx protein or reduced mRNA levels, conferring susceptibility to influenza.