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Genetic strategies used to create a mouse model of SMA. A, Mutant mice carrying a heterozygous deletion of Smn exon 7 (left) have been crossed with a mouse line carrying, in addition to the heterozygous deletion of Smn exon 7, a 115-kb human genomic DNA transgene containing SMNc (Hu-SMNc), part of NAIP (NAIPp), and H4F5 (right). Boxes indicate the exons and black solid line indicates the transgene. Mice carrying both the homozygous deletion of SMN exon 7 and the human transgene display abnormal phenotypes. B, Mutant mice carrying a heterozygous deletion of Smn from exon 2 (left) have been crossed with a mouse line carrying, in addition to the same Smn mutation, a human genomic DNA transgene containing SMNc only (Hu-SMNc, right). Mice carrying both the homozygous deletion of Smn and the human SMNc gene transgene display abnormal phenotypes. C, Mutant mice carrying Source: Spinal Muscular Atrophy, The Online Metabolic and Molecular Bases of Inherited Disease two loxP sites flanking Smn exon 7 (left, SMNF7) have been crossed with a mouse line carrying, in addition to the heterozygous deletion of Smn exon 7 Valle D, Beaudet B, Kinzler KW, (indicated Antonarakis Ballabio A, The Gibson K, Mitchell G. The Online Metabolic andgenotype Molecular (SMNΔ7), a Citation: transgene expressing the AL, Cre Vogelstein recombinase in neurons bySE, N-Cre, right). mutant mice carrying the SMNF7/SMNΔ7 Bases of Inherited Disease; 2014 Available at: http://mhmedical.com/ Accessed: May 15, 2017 and the N-Cre transgene display an abnormal phenotype. In neuronal tissues, Cre-mediated deletion of the SMNF7 allele leads to a homozygous deletion McGraw-Hill Education. rights reserved of Smn exonCopyright 7 (Cre+). © In 2017 nonneuronal tissues, SMNF7All remains intact because the Cre recombinase is not expressed (Cre−). LoxP sites are indicated by arrows. Human SMNc transgene or conditional targeting of SMN restricted to neurons are able to avoid early embryonic lethality.