Under normal circumstances, a child is born to two parents. A male (father) donates sperm and the female (mother) donates egg for fertilization either in-vivo or in-vitro to generate a zygote which develops further in the form of a complete baby. In this process, both the sperm and egg donors are called as the biological father and mother of the child respectively. The concept of parenthood is changed with scientific advancements when a baby Abrahim Hassanis born to a previously infertile couple through in-vitro fertilization (IVF) technique in 2016, the first of its kind in the world in Mexico. The uniqueness of this baby is that, it carried its extranuclear mitochondrial DNA from a third individual in addition to the parent’s nuclear DNA. Due to the presence of DNA from three individuals, the child is being called as triparental child and this is going to be the technique of the future to deal with many currently untreatable diseases associated with mitochondrial DNA.

Nuclear and extranuclear DNA

 A human cell contains two types of DNA i.e. nuclear DNA and extranuclear DNA. Nuclear DNA is present within the nucleus as chromosomesin its condensed form. Half of the nuclear DNA of a child is inherited from its mother and another half is inherited from its father. Extranuclear DNA is present in the cellular organelles mostly as mitochondrial DNA in animalsand chloroplast DNA in plants. The major difference of between nuclear DNA and extranuclear DNA is that, the amount of nuclear DNA in a cell is constant over a period of time, whereas the amount of extranuclear DNA is dependent on the metabolic stage of the cells.

 During the process of sexual reproduction, sperm cells from male and egg cells from female individuals are required which are haploid (n) in nature in comparison to the other somatic cells of a body which are diploid (2n) in nature for nuclear DNA. Considering the structure of a human sperm cell,it has two parts i.e. head and tail. Head portion of the sperm cells contain the haploid nuclear DNA, whereas mitochondria as well as its DNA are present in the tail part which provides energy to the sperm cell till fertilization. Prior to fertilization the tail of spermatozoa is shed and only the sperm head is fertilized to egg leading to the zygote formation. Hence, the zygote contains nuclear DNA of both sperm and egg, whereas its mitochondrial DNA is inherited exclusively from the mother’s egg.

Human Mitochondrial DNA and associated disorders

Mitochondria is responsible for the generation of energy required for smooth maintenance of the cell. Hence, it is called as the powerhouse of the cell. The process of energy production in mitochondria is called as oxidative phosphorylation. For this process, a set of enzyme complexes are required. Additionally, mitochondria also helps in the process of regulation of self-destruction of cell called as apoptosis. Cholesterol and heme are also synthesized in the mitochondria. In order to carry out the aforementioned functions mitochondriarequires many enzymes, most of which are synthesized by mitochondrial DNA. In this context, mitochondria contains double stranded circular DNA of 16,569 bp size, containing 37 genes coding for two rRNAs, 22 tRNAs and 13 polypeptides.

Mitochondrial DNA (mtDNA) encounters a higher rate of mutation in comparison to the nuclear DNA. Lacking of histone proteins as well as possessing low intensity repair mechanismscontribute to accumulation of DNA damagesleading to alterations in mtDNA sequences.Subsequently, diseases appear due to mtDNAmalfunctioning. However, the prevalence of mitochondrial diseases are relatively rare in nature occurring as many as 1 in 10,000 cases.A famous mtDNA associated maternally inherited disorder include maternally inherited diabetes and deafness (MIDD). Other diseases associated with mtDNA malfunction includeLeigh syndrome, Depletion syndromes, Kearns-Sayre syndrome (KSS), Pearson syndrome, mitochondrial encephalopathy, lactic acidosis, and stroke-likeepisodes (MELAS),chronic progressive external ophthalmoplegia (CPEO), Neuropathy, ataxia, and retinitis pigmentosa (NARP), Leber's hereditary optic neuropathy (LHON), Myoclonic epilepsy and ragged red fibres (MERRF).

Triparental child and mitochondrialreplacement therapy

To eradicate pre-diagnosed mitochondrial diseases in the future baby, an in-vitro fertilization technique is being carried out called as mitochondrial replacement therapy (MRT). This therapy is used mostly in cases when the mother carries mitochondrial diseases related genes. In this technique, the genomic DNA comes from the original father and mother whereas the mitochondria as well as the mitochondrial DNA comes from a third individual with healthy mitochondria. Due to the contribution of three individual’s DNA for the development of a healthy child, he/she is called as triparental child.

 In the first of its kind case, the couple married since ten years were losing their child each time due to miscarriages. Though they had a baby girl in 2005, it survived for only six months, whereas another child also survived for eight months. Extensive tests on the mother showed that though she was healthy, about one-quarter of her mitochondria harbored the genes for Leigh syndrome, which had affected her nervous system. As the fetus gets its mitochondrial DNA from its mother, there was every possible chance of transmission of the fatal disease to the child through her mitochondrial DNA. Hence, the consulting doctor John Zhang and his team from New Hope Fertility Center in New York decided to go for the controversial procedure of mitochondria transfer from a third individual for delivery of a healthy child.

 During this in-vitroprocess, the fertility expert used the spindle nuclear transfer technique. In this process, the nucleus from the mother’s egg is recovered and inserted it to a healthy donor’s anucleated egg. Finally, the egg was fertilized with the husband’s sperm. Though Dr. Jhang and his team created five embryos, only one of them survived. The survived embryo was implanted to the mother’s womb and in this process the first triparental child in history, Abrahim Hassan was born nine months later on 6^th April, 2016. Currently Abrahim Hassan is healthy and doing well.

In addition to the spindle nuclear transfer technique, another technique has also been developed by Scientists at the Wellcome Trust Centre for Mitochondrial Research, Newcastle for removal of faulty mitochondria called as pronuclear transfer. In this technique, two zygotes are created in-vitro, first by fertilizing the mother’s egg and father’s sperm and the second one by the father’s sperm and the donor’s egg. During first 24 h pronuclei are removed from both the zygotes to generate the enucleated cells. Finally, the parent’s pronuclei is transferred to the enucleated cell produced from the donor’s egg and the reconstructed zygote is implanted in the mother’s womb.

Ethical issues

Though development of triparental child attracted a huge round of controversy since its inception, there is a widespread support for this from the patients and their families with mitochondrial disease, noted Nobel winning scientists and other intellectuals of as many as 14 different countries. This technique has been proved to be safe in mice and monkeys experimentally. Though extensive study is pending with human subject, the technique has been tagged with ‘not unsafe’. Additionally, the result of mitochondrial donation leads to the avoidance of the transmission of a serious mitochondrial disease, it will be passed down to the future generations. Still the department of Health, UK is not considering it as genetic modification. In this regard, with known risks, the individuals sensing uncertainty and complexity of mitochondrial diseases, assessment of the risk of having a child with mitochondrial disease, negotiation of correct IVF technique are of paramount importance to prevent mitochondrial associated genetic disease and its transmission.