Ted and Julie McMasters were a regular couple, with one son Jason and had just found out Julie was pregnant with their second child.
On the way home from the babysitter’s, Julie and Jason had gotten into a car accident. At a red light an 18 wheeler slammed into the passenger side of the station wagon and both Julie and Jason were rushed to the hospital and into surgery. The doctor came to Ted and updated him that Julie hemorrhaged and lost the baby, but was otherwise okay and would be able to have more children. Unfortunately, Jason could not be revived and died on the operating table.
The couple had always wanted a large family and after being diagnosed with testicular cancer, leaving Ted sterile, and Julie had several miscarriages before having Jason, the couple was unable to have another child naturally. The doctor talking to Ted suggested the couple speak with a doctor from a human cloning company regarding the possibility of cloning Jason. This came to Ted as a shock, leaving him unsure as to what he and Julie should do.Cloning is a method of asexual reproduction in which one parent produces genetically identical cells because there is no fusion of gametes or mixing of genetic material (“Cloning and Stem Cells”). This process of asexual reproduction can occur naturally in some plants and some bacteria but is not the process by which organisms reproduce sexually. However, through genetic manipulation, sexually reproducing organisms can reproduce asexually. In the laboratory setting, a method of cloning is somatic cell nuclear transfer, where the nucleus of a somatic cell is inserted into an enucleated donor egg and the nucleus is reprogrammed by the host cell (Brownlee).
Then to stimulate division, the cell is shocked and the embryo is grown to produce the clone (Brownlee). This would be the process used to attempt to clone Baby Jason, in which a nucleus from one of Baby Jason’s somatic cells would be inserted into a donor egg. The idea of cloning came about in the late 1800s when Hans Adolf Eduard Driesch demonstrated artificial embryo twinning in sea urchins, and advances have been made in over the past one hundred years.
The first successful nuclear transfer was completed in 1952 by Robert Briggs and Thomas King when they transferred the nucleus of a tadpole embryo into an enucleated frog egg, resulting in a developed tadpole (“The History of Cloning”). The scientists, through their experiment, showed that nuclear transfer was a viable cloning technique. The first mammalian embryo created by nuclear transfer was a rabbit by J. Derek Bromhall in 1975, when an advanced embryo developed a couple days after he transferred the nucleus form a rabbit embryo cell into an enucleated rabbit egg cell (“The History of Cloning”).
With his experiment, Bromhall proved that mammalian embryos could be created by the process of nuclear transfer. In the 1980s and 1990s, scientists attempted to experiment further with developing mammals by nuclear transfer; but the first to be truly successful was Dolly the sheep. The first mammal to be created by somatic cell nuclear transfer was Dolly the sheep in 1996 by Ian Wilmut and Keith Campbell (“Cloning Dolly the Sheep”). The scientists used an udder cell from an adult sheep and injected the cells into unfertilized egg cells that had their nuclei removed. Of the cell fusions, the embryos that developed were implanted into a surrogate mother (“Cloning Dolly the Sheep”). This process had a low success rate, as it took many trials to produce a single viable clone. It took 277 attempts to produce Dolly, only 29 embryos developing, and Dolly was born from the only pregnancy that went to full term (“Cloning Dolly the Sheep”). Dolly, unfortunately, was euthanized at age six, because she was suffering from arthritis in a hind leg and a virus-induced lung tumor common in sheep raised indoors called sheep pulmonary adenomatosis (“Cloning Dolly the Sheep”).
Cloning in mammals is complex, and often fails even in modern studies because of the complications faced when removing a nucleus from an adult cell or remodeling of the chromatin in the nucleus of a somatic cell when inserting it into an enucleated egg cell. When removing a nucleus from an adult human cell, there are difficulties faced that interfere with cloning. For example, the two spindle proteins that are necessary for cell division are located near the nucleus in humans (“Cloning”). When removing the egg’s nucleus, spindle fibers are also removed, which interferes with chromosome segregation (“Cloning”).
This could result in cells having twice the required amount of chromosomes, odd combinations, or even none at all (“Cloning”). Other animals do not have this problem as their spindles are spread throughout the egg. This makes cloning humans more difficult in comparison to other mammals and is a reason why clones produced are often not viable. In addition, difficulties are faced when inserting a somatic nucleus into an enucleated egg cell, impacting the viability of clone. When placing a somatic nucleus into a gamete donor egg, the chromatin is remodeled and reprogrammed to appear similar to the nucleus of a cell that had just been fertilized (Xu, Rong, et al. 5).
Modifications in chromosome often determine the genes that are expressed or not within an organism by adding a group to the chromatin to alter its properties. Chromatin is comprised of DNA wrapped around an octet of histones called a nucleosome. Phosphorylation, a chromatin modification, is when negatively charged phosphate groups are added to the protein tails of histones (Phillips and Shaw 209). This results in the tails having a more negative net charge and losing affinity of the DNA, creating open or loose chromatin, where it is not as tightly wound around the nucleosome, allowing more transcription to take place. Similarly, the modification of lysines in the histone tails by adding acetyl groups, called acetylation changes the net charge of the histone tail to be more negative, resulting in open chromatin and allowing transcription to take place (Phillips and Shaw 209). Methylation or the addition of methyl groups to residues in the histone tail dependent upon where the modification is located can induce or repress transcription by loosening or tightening the DNA, respectively (Phillips and Shaw 209). Phosphorylation, acetylation, and methylation of histones all directly impact the genes expressed in an organism. Chromatin modifications are epigenetic modifications.
This means “tags” such as amino acids are added to the DNA structure, altering the accessibility of the DNA and chromatin structure, but not the genetic sequence itself. The reprogramming of these modifications to the chromatin directly impacts the viability of the clone. It has been suggested that developmental and morphological defects can be seen in clones, as early zygotic genes are expressed incorrectly because of chromatin reprogramming (Byrne, J. A., et al.
6063). In cattle, a recent study has been done and had results indicating that less than ten percent of the cloned animals live to birth, mainly due to embryonic death, a failure during the implantation process, or the development of a defective placenta (Biase, Fernando H., et al. 14492).
Using RNA sequencing, the researchers found that SCNT led to abnormal expression of more than 5,000 genes, and 121 of them led to lethal phenotypes in mice embryos (Biase, Fernando H., et al. 14492). In addition, studies have been performed in mice that have suggested that cloning leads to morphological defects such as obesity, pneumonia, liver failure, and premature death (Holden). These inaccuracies in gene expression lead to morphological and developmental disparities in clones, making Jason II phenotypically different from Jason I.
The incorrect chromatin modifications of imprinted genes also impacts the viability of the clone when placing a somatic nucleus into an enucleated egg cell. Imprinting is where one working copy of a gene is inherited, while the other copy is epigenetically silenced by placing markers on the DNA. Imprinting, which is unique to mammals and flowering plants, can have a tremendous impact on a cell. In humans, depending on the gene, one from the father or mother may be silenced in imprinted genes. Specifically, 80 to 90 genes are silenced in humans through the process of imprinting (Peters 517).
Silencing occurs when methyl groups are added during egg and sperm formation to prevent transcription of the gene (“Genomic Imprinting”). The epigenetic tags on the imprinted genes stay for the lifetime of the organism but are reset in sperm and egg formation. When cloning, a cell’s nucleus is transferred from the host cell to the surrogate cell, the surrogate cell will attempt to erase the epigenetic tags it contains (Xu, Rong, et al. 5). However, the surrogate cell will oftentimes fail in this process, resulting in two active copies of a gene where only one is meant to be active or two inactive copies of a gene when one is meant to be active, which impacts the viability of the clone.It has been suggested that the reprogramming of chromatin when placing a somatic nucleus into an enucleated egg cell impacts the viability of the clone because it leads to issues in development and physiology.
For example, incorrect imprinting can result in the organism having two active or two inactive copies of a gene which can lead to severe developmental abnormalities, cancer and other disorders (“Genomic Imprinting”). This is because there is improper gene expression because of the incorrect silencing. Diseases such as Prader-Willi Syndrome and Angelman Syndrome can be caused by the inheritance of a defect on chromosome fifteen as a result of incorrect imprinting (“Genomic Imprinting”). Specifically, Prader-Willi Syndrome occurs as a result of two active maternal copies of a gene or an absence in the paternal gene on chromosome fifteen. Symptoms of the syndrome include learning difficulties, short stature, and compulsive eating (“Genomic Imprinting”).
In Angelman Syndrome, the maternal gene is faulty or missing, or there are two paternal copies of the gene on chromosome fifteen. Symptoms of the syndrome include learning difficulties, speech problems, seizures, and jerky movements (“Genomic Imprinting”). Therefore if Jason were to be cloned, the resulting clone would likely be phenotypically different to Jason I. Chromatin reprogramming epigenetically alters what genes are expressed by changing the histone modifications and imprinting. The modifications of the DNA change, resulting in a phenotypic change, but the DNA sequence or genotype is the same, so while the clone and original organism would have the same DNA, they would phenotypically look different. Because of the overexpression or faulty expression of genes, based on the reprogramming of the nucleus when inserting a somatic nucleus into an enucleated egg cell, the clone looks different in appearance to the original organism. Therefore, Jason II would not be perfectly identical to Jason I if the clone were viable. If Julie and Ted decided to clone Jason it would be an exhausting process physically on Julie if she were to donate her own eggs.
In order to donate eggs, they would be extracted the same way as if the eggs were required for In Vitro Fertilization. For this process, women take hormone shots and several medications and on average 2 to 8 eggs are retrieved from the process (“In Vitro Fertilization (IVF)”). This can be taxing on the donor as for a clone to work it would take many trials for a clone to be viable. Therefore, Julie would need to provide many eggs if the couple chose to proceed with cloning Jason. Cloning Jason would not be the best idea for Ted and Julie McMasters as cloning would not truly provide them a perfect replica of Jason. Cloning Jason would be very taxing physically on Julie as she would need to provide many eggs for a viable clone to be produced. Additionally, cloning Jason would be emotionally taxing on the couple as many trials are needed for a viable clone to be produced and often the clone does not live for long or has morphological defects.
This would get the couple’s hopes up, though the process would likely not produce a viable clone let alone a clone that is identical to Jason. Based on chromatin remodeling, there would be epigenetic changes which would alter gene expression and the phenotype. If Ted and Julie were to clone Jason, they would never get an exact replica of the child they once had.