Thursday, 8 October 2015

Consequences of zygote injection and germline transfer of mutant human mitochondrial DNA in mice

http://www.pnas.org/content/early/2015/09/30/1506129112
 
Hong Yu, Rajeshwari D. Koilkonda, Tsung-Han Chou, Vittorio Porciatt, Arpit Mehta, Ian D. Hentall, Vince A. Chiodo, Sanford L. Boye, William W. Hauswirth, Alfred S. Lewin and John Guy

In trying to understand mutations of mitochondrial DNA, biologists tend to use cells from either: 1) biopsies directly from patients; 2) early cell culture (where a biopsy from a patient has been purified in some way, then grown in the lab for a small number of generations); or 3) immortal cell lines (these are often cancer cells, which have been manipulated to contain the mtDNA mutation of interest).

Whilst all of these model system have their pros and cons (e.g. cell purity vs similarity to a natural environment), what has been lacking in this field is the ability to study human mitochondrial mutations in an animal model. This has the advantage of being able to introduce further genetic/drug perturbations to a living biological system, in the background of a mtDNA mutation. Whilst introducing nuclear mutations into mice is commonplace, it has never been attempted for mitochondrial genomes, which are held in multiple copy number.

In this study, the authors use an adeno-associated virus, which usually targets the nucleus, and add a particular protein (MT-COX8) which causes the virus to localise to mitochondria. The virus then delivers mutated human mtDNA to a mitochondrion. By exposing a mouse zygote to the virus, the authors were able to generate a mouse with a mutation in complex I, associated with LHON. The inserted viral mtDNA existed separately from the endogenous mtDNA, and was able to replicate and be transferred between generations after cross-breeding with wild-type animals. 

In humans, LHON is typically associated with retinal degeneration. The mutated mtDNA was able to express the mutant form of complex I in the mice, and cause a visual deficiency. Using mice which showed a visual defect, the authors used ocular injection with a virus containing wild-type mtDNA, and show that this was partially able to restore the visual defect, 1 month after treatment. 



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