B6.129(Cg)-Smn1^tm1.1Cdid/J

Stock number: 034285
Product name: Smn^2B
Research areas: Neurobiology Research

Description

Smn^2B mice have a GGA to TTT knock-in to exon 7 of the Smn1 (survival motor neuron) gene. The knock-in alters an exon splice enhancer and reduces SMN1 protein levels by 32%. This strain may be useful for studying spinal muscular atrophy (SMA).

Detailed description

The Smn1 (survival motor neuron) gene is one of at least 4 genes found in a 500 kb inverted duplication on human chromosome 5q13. The telomeric and centromeric copies of Smn1 both encode the same protein, with the exception of a single nucleotide in exon 7 identified as an exon splice enhancer (ESE). Mutations in Smn1 (telomeric copy) are associated with spinal muscular atrophy (SMA). This hypomorphic Smn^2B allele is designed to substitute a GGA to TTT in exon 7 of the ESE region. This 2B mutation alters the central portion of the ESE and alters SMN exon 7 processing. SMN1 protein levels are reduced to 32% of wild-type in 2B/2B mice. Homozygotes are viable, fertile and have normal lifespan.

When combined with an Smn1 KO (Smn1^tm1Msd: Stock No. 006214), mice develop motor deficits and denervation by postnatal day 12. On the FVB/N background, median survival for 2B/- mice is 21 days, on the C57BL/6 background median survival is 28 days. These mice may be useful for studying spinal muscular atrophy.

Of note, the 2B allele is also available on the FVB/N background (Stock No. 034496).

Development

The targeting vector was designed to insert 3 nucleotides (GGA to TTT) into exon 7 of the Smn1 gene followed by loxP flanked neomycin cassette 180 bp distal to exon 7 and placed in the antisense orientation. The mutation alters the central portion of the exon splice enhancer (ESE) and effects exon 7 processing. The construct was electroporated into embryonic stem (ES) cells derived from 129/SvJ mice.  Correctly targeted ES cells were injected into recipient blastocysts. The resulting chimeric animals were crossed to C57BL/6J females and the crossed to mice carrying an unspecified cre recombinase to remove the neomycin cassette. The 2B strain was further backcrossed to C57BL/6J for 7 generations. Upon arrival at The Jackson Laboratory, sperm was cryopreserved. To establish our live colony, an aliquot of frozen sperm was used to fertilize C57BL/6J oocytes.

Allele

Symbol: Smn1^tm1.1Cdid
Name: targeted mutation 1.1, Christine DiDonato
MGI accession ID: MGI:4836034
Generation method: Targeted
Attributes: Hypomorph; Modified regulatory region
Synonyms: Smn^2B; Smn2B- no Neo
Marker symbol: Smn1
Marker name: survival motor neuron 1
Marker synonyms: BCD541; C-BCD541; GEMIN1; SMA; SMA@; SMA1; SMA2; SMA3; SMA4; Smn; SMNC; SMNT; T-BCD541; TDRD16A; TDRD16B
Chromosome: 13
Strain of origin: 129/SvJ
Molecular note: A targeting vector was used to insert a 3 nucleotide mutation (gga-TTT) in exon 7 of Smn1, within an important splice enhancer element, and a floxed Neo cassette (in the opposite orientation to transcription) in intron 7. The mutation changes the exon splice enhancer into an exon splice silencer, thus greatly reducing the inclusion of exon 7 in transcripts, which results in producing Smn2-like transcripts. Cre mediated recombination deleted the neo cassette, which disrupted efficient transcription of Smn1.