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Connie Karambelas/MEDILL

New procedure can leave mutant cell structures behind, protecting babies from disease

by Connie Karambelas
April 21, 2010

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The effects of mitochondrial disease on two families

Two types of mitochondrial disease

Leigh Disease/Syndrome:  This disease generally starts in infancy or childhood, but can also occur in teens and adults. Symptoms include the loss of basic skills such as head control, walking and talking. These can also be accompanied by loss of appetite, vomiting and seizures.  Eventually the child may have heart, kidney, vision and breathing complications. There is no cure. An estimate of the prevalence of Leigh disease is one in 77,000 births.

Complex I Deficiency: Three major forms of Complex 1 deficiency result from mitochondrial disease. The first is fatal infantile multisystem disorder, which is characterized by poor muscle tone, developmental delay, heart disease, and respiratory failure. Myopathy, which starts in childhood or adulthood, is characterized by weakness and limited ability to move.The third major form is mitochondrial encephalamyopathy, a muscle and brain disease. It can begin in childhood or adulthood and involves different symptom including hearing loss, nerve damage involving the sense organs, seizures, dementia, involuntary movements, eye muscle paralysis and involuntary movements. This form of Complex I deficiency may cause Leigh Syndrome. There is no cure for this deficiency as with all mitochondrial diseases.

There are 44 types of known mitochondrial disease.  

Source: The United Mitochondrial Disease Foundation

Scientists in England have successfully transferred the DNA from a human fertilized egg into another without carrying over the mitochondria, the cells energy production plant.

The new procedure could be used to prevent babies from inheriting their mother’s defective mitochondrial DNA, which can cause an array of incurable disorders called mitochondrial diseases.

The mitochondria are considered the powerhouses of the cell, combining food and oxygen to produce all the energy needed for cell function. But some residual DNA resides in the mitochondria and it can mutate. According to the scientists from Newcastle University in England, where the transfer procedure was developed, many people carry a normal mix of defective mitochondrial DNA. But 50 percent or more of mutant mitochondrial DNA cause diseases such as Type 2 diabetes, hearing loss, and multiple sclerosis-type diseases. 

The problem of mitochondrial disease is prevalent. “Every 30 minutes, a child is born who will develop a mitochondrial disease by age 10,” according to the United Mitochondria Disease Foundation,

The British researchers reported last week in the journal Nature that their procedure may be the answer for women who have mutant mitochondria but still want to have children.

The procedure isn't available in the U.S. as yet, however, and it could be 10 years before it becomes available here, U.S. scientists estimate. It may be in use in England within about three years but, so far, the research was done with abnormal fertilized eggs that would have been discarded. 

“What needs to be understood is that this is very preliminary work,” said Dr. Edmond Confino, a professor at the Feinberg School of Medicine, Northwestern University. The researchers are now planning further studies that will test the safety of the procedure.

Ethical issues regarding embryos and who should be considered the parent are concerns regarding this development. “Once that nucleus is removed, much like what happens with stem cell research, that embryo, from which the nucleic material is removed, no longer exists and has been essentially destroyed,” said Nanette Elster, a lawyer at Chicago-based Spence & Elster, P.C. and a bioethicist who teaches at DePaul University.

“This is one of the arguments that might be raised: By treating one embryo you’re essentially destroying another,” she said.

The fertilized eggs that were used for the research at Newcastle were donated by women who had undergone in vitro fertilization.  In a normal IVF, process 20-30 percent of the embryos that are made are genetically abnormal and cannot be used and these are normally discarded, according to Allison Murdoch, a professor at Newcastle Fertility Centre at Life North East England Stem Cell Institute.

“If those embryos were set to be discarded anyway, is there a reason that they shouldn’t be put to some use [in research]?" Elster said.  “This way they aren’t really created and then destroyed for naught.”

An unfertilized egg wouldn't have worked for testing the procedure because the egg would not work for the procedure because the egg would not be ready for implantation, according to Suresh Pillai, a Chicago-based lawyer with a doctorate in physiology.

Another ethical query concerning the new procedure pertains to the offspring essentially having three parents. There will be DNA contributed to the child from three different people: the two parents who provide the genetic material and the mitochondrial DNA that is provided by the egg donor. This however, may be a stretch. “It all depends on what your definition of a parent is,” Pillai said.

According to an article in the journal Nature, a mother’s egg contains 23 chromosomes and also the DNA for her mitochondria, which is exclusively used to generate the energy for the cell.  The amount of genetic material that is in the mitochondria is very small; the equivalent of 13 protein-coding genes, compared with the 23,000 genes that are inherited from the parents.

The compromise of having a small portion of the donor’s DNA is “easier to make because it is preventing a disease,” Confino said. “Nothing in medicine is 100 percent, there is always risk of contamination.”

“A parent is a legal status and it imbues certain rights and it imparts certain responsibilities on people based on being legally identified as parents,” Elster said. “And if we want to be technical we would have to balance the amount of genetic contribution to determine the two people who provide the highest percentage of genetic material as the equal parents. It really depends what we are trying to define by calling someone a parent,” she said.

If the person who is donating the fertilized egg does not want to be a mother, or has any intent of being the mother, and the intended parents are assuming all the responsibilities that pertain to caring for the child, “than we don’t need to have this debate at all,” Elster said. “There’s a presumption that the women who gives birth to the child is the legal mother and I don’t see why that presumption would have to change here.”