The PolgD257A mutant allele has a D257A mutation in the N-terminal "proofreading" exonuclease domain of the DNA polymerase γ gene (Polg), rendering the expressed mutant protein devoid of polymerase proofreading function in mitochondria. These mitochondrial mutator mice may be useful for studying the accumulation of mtDNA mutations in apoptosis, impaired energy metabolism, aging, and age-related conditions/diseases.
Tomas A Prolla, University of Wisconsin-Madison
The PolgD257A mutant allele has a mitochondrial DNA polymerase editing amino acid substitution, D257Am in the N-terminal "proofreading" exonuclease domain II of the DNA polymerase γ gene (Polg). The expressed mutant protein, Polg-D257A, lacks the polymerase proofreading function/mismatch repair mechanism in mitochondria (although mtDNA replication is not significantly altered). As such, homozygous PolgD257A mice exhibit ~2500-fold increased rate of mtDNA mutation compared to wild-type mice. The accumulation of mtDNA mutations leads to increased apoptosis (particularly in cells/tissues that are metabolically active or have rapid cellular turnover). Homozygous (PolgD257A/D257A) mice grossly demonstrate a premature aging phenotype beginning at ~nine months of age with alopecia, graying hair, impaired mobility and kyphosis, and die prematurely by ~13-15 months of age with severe anemia. Homozygous mice exhibit several age-related defects, including thymic involution, weight loss, testicular atrophy, and cardiac hypertrophy/dysfunction, as well as progressive loss of skeletal muscle (sarcopenia), bone mass, circulating red blood cells, hearing, and intestinal crypts.The testicular atrophy manifests around five months of age concurrent with depletion of spermatogonia.The cardiovascular phenotype includes macrocytic anemia with abnormal erythroid maturation and megaloblastic changes, as well as profound defects in lymphopoiesis (characteristics of human myelodysplastic syndromes). The hearing loss is attributed to cochlear hair cell degeneration (presbycusis), with onset around nine months of age.Histologically, increased apoptosis in PolgD257A/D257A tissues is observable by three months of age in the duodenum, liver, testes, and thymus.Heterozygous mice (PolgD257A/+) also have an increased mutation burden, but no gross observable age-related phenotype, fertility problems, or other abnormalities are reported.
When PolgD257A mice are bred with Ins2Akita mice (Stock No. 003548), the double mutant offspring may be useful in studying appetite and diabetes.
To create the targeting vector, Dr. Tomas A. Prolla (University of Wisconsin, Madison) isolated a mouse DNA polymerase γ (Polg) DNA sequence and used site-directed mutagenesis to introduce an AC-->CT two-base substitution corresponding to positions 1054-1055 in exon 3. The targeting vector also had a loxP-flanked PGK-neo cassette inserted into intron 3. The point mutations create a D257A amino acid substitution in the conserved N-terminal "proofreading" exonuclease domain II of Polg. This targeting construct was electroporated into 129S7/SvEvBrd-Hprtb-m2-derived AB2.2 embryonic stem (ES) cells, and correctly targeted ES cells were injected into blastocysts. Chimeric males were bred to C57BL/6J females. The PolgAD257Aneo mice were then bred with CMV-Cre transgenic mice (on a mixed 129/ICR genetic background) to remove the floxed PGK-neo cassette. The resulting mice harboring the PolgAD257A allele (also called PolgD257A, Polgmut, or PolgA) were subsequently backcrossed 16 generations to C57BL/6J wildtype mice (and the CMV-Cre transgene was removed) prior to sending to The Jackson Laboratory Repository. Upon arrival, mice were bred to C57BL/6J inbred mice (Stock No. 000664) for at least one generation.
|Allele Name||targeted mutation 1, Tomas A Prolla|
|Allele Synonym(s)||PolgA; Polgm; Polgmut; Polgtm1Tprol; PolgAD257A; mtDNA-mutator|
|Gene Symbol and Name||Polg, polymerase (DNA directed), gamma|
|Gene Synonym(s)||AA409516; MDP1; MIRAS; MTDPS4A; MTDPS4B; PEO; POLG1; POLGA; Pol gamma; Polga; SANDO; SCAE; expressed sequence AA409516; mitochondrial DNA polymerase gamma; mitochondrial DNA polymerase-gamma; polymerase gamma|
|Promoter||Polg, polymerase (DNA directed), gamma, mouse, laboratory|
|Strain of Origin||129S7/SvEvBrd-Hprt |
|Molecular Note||A targeting construct was designed to introduce an AC to CT two-base substitution in positions 1054 and 1055, resulting in a critical residue substitution in the conserved exonuclease domain.|
|Mutations Made By|| |
Tomas Prolla, University of Wisconsin-Madison
Both heterozygous and homozygous mice have progressive accumulation of mtDNA point mutations. Considering that paternal mitochondrial DNA is actively eliminated following fertilization in mice, one can minimize the (undesirable) accumulation of these mutations in the germline by breeding wildtype females or C57BL/6J inbred females (Stock No. 000664) with heterozygous males.
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