STOCK Smn1^tm2Cdid Tg(SMN2)1Cdid/J

Stock number: 034286
Product name: DD CTN

Research areas: Neurobiology Research

Description

DDCTN/CTN mice carry a BAC transgene expressing the human SMN2 gene and the targeted Smn^C-T-Neo allele with a C to T substitution in exon 7 of the Smn1 (survival motor neuron) gene. 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). The hypomorphic Smn^C-T-Neo allele is designed to substitute a C for a T at position 6 in exon 7 of the ESE region. This C-T-Neo (CTN) mutation changes the exon splice enhancer to an exon splice silencer and reduces inclusion of exon 7 in transcripts to a degree similar to human SMN2. Functionally, the transcriptionally silent C-T substitution alters SMN1(C) to mimic SMN2(T). Removal of the neomycin cassette from the CTN allele increases the amount of the full length SMN1 transcript. In the homozygous state the CTN allele causes embryonic lethality by E12.5. However, when combined with a BAC transgene expressing human SMN2 (Tg(SMN2)1Cdid or D), the lethality phenotype is rescued. Mice homozygous for both alleles (D/D CTN/CTN) are viable, fertile and have normal lifespan.

In addition, when DDCTN/CTN mice are combined with a heterozygous Smn1 KO (K) allele (Smn1^tm1Msd: Stock No. 034287), mice represent a mild model of SMA that presents with an adult phenotype without ear, tail or foot necrosis. If a cre/ERT fusion allele such as Gt(ROSA)26Sor^{tm1(cre/ERT)Nat} (Stock No. 004847) is bred into the DDCTNK model an inducible model of mild SMA is generated. Removing the neo from the CTN allele increases the amount of full length SMN1 transcript.

Development

The C-T-Neo targeting vector was designed to insert a single nucleotide mutation (C to T) at position 6 in 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 transcriptionally silent C-T substitution functionally alters SMN1(C) to mimic SMN2(T). The construct was electroporated into 129 derived embryonic stem (ES) cells. Correctly targeted ES cells were injected into recipient blastocysts. The resulting chimeric animals were crossed to C57BL/6J females and then crossed to C57BL/6J for approximately 3 generations.

The SMN2-CJD or D transgene was designed using an approximately 61 kb fragment of the human PAC clone 215P15 microinjected into C57BL/6 X C3H fertilized eggs. PAC 215P15 contains the promoter and exons 1-7 of the SMN2 gene and the promoter and exons 1-3 of SERF1Bgene. The transgene insertion site is located on Chr 18, approximately 635 kb downstream of Gata6 and approximately 84 kb upstream of Rbbp8.  The transgenic mice were backcrossed to FVB/NJ for at least 1 generation before being crossed to mice carrying the Smn1^tm2Cdid allele (on the C57BL/6 background).

Allele

Symbol: Smn1^tm2Cdid
Name: targeted mutation 2, Christine DiDonato
MGI accession ID: MGI:5290070
Generation method: Targeted
Attributes: Hypomorph
Synonyms: Smn^C-T-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
Molecular note: Exon 7 was replaced with one in which a C to T transition at position 6 of this exon. A floxed pgk-neo selection cassette was inserted about 180 bp distal to exon 7 in the antisense orientation. Analysis of protein expression indicates very low levels of Smn1 and that this is a hypomorphic allele.

Allele

Symbol: Tg(SMN2)1Cdid
Name: transgene insertion 1, Christine DiDonato
MGI accession ID: MGI:6379745
Generation method: Transgenic
Attributes: Inserted expressed sequence; Humanized sequence
Synonyms: D; SMN2-CJD
Marker symbol: Tg(SMN2)1Cdid
Marker name: transgene insertion 1, Christine DiDonato
Chromosome: 18
Strain of origin: C57BL/6 x C3H
Site of expression: widespread expression, encodes a protein required by all cells

Molecular note: The transgene is designed using an approxmately 61 kb fragment of the human PAC clone 215P15. PAC 215P15 contains the promoter and exons 1-7 of the SMN2 gene and the promoter and exons 1-3 of SERF1B gene. The transgene insertion site is located on Chr 18, approximately 635 kb downstream of Gata6 and approximately 84 kb upstream of Rbbp8.