Monday, September 17, 2012

Genetics, Stem Cells and Life Expectancy

by Dr. Deepti Dileep Deobagkar


In recent years, there have been path breaking advances in biology and medicine, which have resulted in significant increase in life expectancy and health and well-being. It is said that in this century, the advances in biology in general and Genetics in particular, will drive the developments in science and this will have significant impact on life style.

Several traits are passed on from parents to children and so on. We notice similarities amongst relatives and can identify features and characters. When a child is born there is a discussion about whom the baby resembles. We are well aware that we inherit a combination of characters from our grandparents and parents and have some typical features of our own. We often notice that if a little girl comes running and an old lady walks behind from their appearances we can easily make out that the little girl is a granddaughter and the older women is the grandmother. Genetics, the study of inheritance of traits (phenotypes), provides us with concepts and paradigms which revolve around how we look, behave, etc. Both parents pass on a complement of genes to us and each individual has a unique genetic composition. We are often concerned about effects of aging and possibilities of suffering from lifestyle and other diseases. This becomes very important in view of increased life expectancy and demographic changes. The study of science of genetics provides us with the possibility of prediction of susceptibility to diseases, accurate diagnosis of disease and provides directions about future treatment options. Whether a person will have black eyes, will he be tall, whether he will be obese or thin, beautiful or handsome are all decided by a combination of genetics and environmental factors. The rapid advances in the field are helping us to unravel these secrets. Although Mendel was the first person to provide us laws of inheritance and the structure of DNA was solved 61 years ago, the last 25 years saw an explosion of information. We inherit the possibilities of a range of external appearances (phenotypes) but it is the interaction of genetic and epigenetic components which give rise to form and function.

The genetic material is the DNA or the Deoxyribose nucleic acid. The language of life and inheritance is made up of 4 letters namely A, T G, C (adenine, thymine, guanine and cytosine). These letters pair in a specific manner where A always partners with T and G with C. They are arranged as two long chains which run antiparallel to each other. Due to this pairing of A on one strand with T and G with C, if we know the sequence of one chain we can automatically generate the second chain. In fact it is this property of DNA that is responsible for continuity of genetic material for generations. The sequence of the entire human genome was deciphered 12 years ago i.e., determined the sequence of A T G and C in the trillion nucleotides. This genetic information carries a blueprint for design of form and function for all animals and plants, in fact in all living beings. DNA sequence carries the code for traits or characters. Although we have many similar genes each one has some differences in the sequence and organisation and these are important in giving different features, shapes and sizes. There is uniformity in this code and in the principles governing its working in all life forms.

Scientists have unravelled the working of these codes and genes. There are some disorders, which occur due to a fault in one’s gene. It is therefore possible to predict in advance (even when the baby is in the womb) what diseases the baby may come down with. More than 3000 diseases can be mapped in this fashion. In some cases, accurate diagnosis is possible while in some others it has important predictive value. This will help in predicting the genetic load and help in determining the proportion of individuals who are likely to suffer. It is possible to diagnose and predict the development of inherited disease or gauge the susceptibility of individuals. This has unraveled the cause of the disease and provided valuable insights for designing treatment. Such analysis thus provides a way of better treatment along with accurate prediction of diseases and also of possibility of susceptibility. This raises an important and interesting question that is who should have access to this information. Should the information be made available to the person or should the employer, insurance company, family, husband or wife also know about it. This has many implications. In our country due to the stigma attached to such diagnosis there will be a tendency to keep this as a secret. In fact one technique of prenatal diagnosis (ultrasound) is routinely being misused for sex determination leading to missing girl child.

Along with understanding the principles of inheritance, scientists have acquired a new kind of therapy based on tissue and cell engineering. What a wonderful thing it will be if one day we can get parts of our body rejuvenated, replaced. Just like when the car is damaged we contact a mechanic, could we have the damaged liver, kidney or pancreas repaired? If a person has damaged some body part or skin during an accident, can it be replaced? The concept of renewal and rejuvenation particularly to recover from effects of ageing and find remedies for many ailments and diseases has always been a fascinating concept. In Mythology, we have description of “Amrut” and also revival of a person. We are aware that this is very easy in plants and we experience it in our day to day life. Grafting from different parts of the plant can generate new plants. Every onset of monsoon, we cut down the plants and trees and they grow back. Although our body is endowed with limited repair capacity it was thought, based on our experience, that such regeneration is not feasible in higher animals. If a finger is lost or leg has to be amputated plastic surgery and artificial limbs need to be utilized. If kidneys are damaged then kidney transplant from matched donor is carried out. This technology though surgically perfect, faces problems because our body has a system of recognizing self from non self. It accepts our own cells and organs but is not happy with organs from another donor. It mounts a fight with non-self (foreign) cells and tissues. An embryo develops into an animal and generates all body parts but adult animal body cells were thought to have limited regeneration capacity.

All this changed one day! In 1996, the birth of Dolly the sheep challenged this belief. Dolly was a clone who acquired her genetic material from an adult cell. In case of cloning genetic material of an adult cell was transplanted in an egg nucleus and this generated a live animal. This showed that it was possible to carry out cloning where genetic material from adult cell can be reprogramed to give rise to another animal. Since then many animals such as bullock, cow, etc. have been cloned. Our body has trillions of cells and therefore many genetically identical individuals can be generated in this manner. Cloning generates a new individual from body cells and not from fertilised egg as is the normal case. Cloning of non-mammals was first accomplished in 1952. However, cloning of mammals proved much more difficult, with the first successful clone being the sheep, Dolly. Dolly died a premature death, probably due to the use of aged chromosomes (from 6 year old nucleus) in her nuclear transfer. Other mammalian species followed rapidly, with mice and cows being cloned in 1998, and pigs in 2000.This has opened up many new avenues of fundamental research.

This technology opened up a can of worms and has raised many social, ethical, legal issues and led to paradigm change in our basic concepts. There are many questions and also strong opposition to reproductive cloning where cloning generates another individual. It is banned in humans. However therapeutic cloning, ie, use of cloning for generation of cells, tissues and organs, is a possibility. The genetic material of a clone is identical to the animal whose genetic material is used. There is also no involvement of a father. A female (surrogate mother) who will carry the baby in her womb is required.

Both males and females have been generated in this manner. There are proposals for creating a stem-cell line from a cloned human embryo that could be used either for research or to develop tissue for therapy. Stem cells have an ability of self-renewal and ability to differentiate. Some of them can form only one type of cells while others can form many different types of cells. Medical researchers believe that stem cell therapy has the potential to dramatically change the treatment of human disease.

Stem cells are cells that have ability to self-replicate for indefinite periods and in response to specific signals can give rise to different cell types that make up the organism –i.e., make mature cells that carry out different functions. Stem cells can be generated from embryonic cells, undifferentiated progenitor cells in tissue and by using induced pluripotent (which can form multiple cell types) cells generated from adult cells. They are immortal and continue to proliferate. Adult cells are thus reprogrammed by making them “forget” their original cell type and inducing pluripotency, i.e. ability to generate different types of body cells and tissues. This has led to generation of tooth, functional liver, skin, lymphocytes and can be successfully used for treatment of macular degeneration, spinal cord injury, liver damage, generating pancreatic islet cells, etc. It has cosmetic applications for improvement of skin, treatment of scars, burn injuries, etc. When a scooter or car is damaged it is taken to the mechanic and the faulty parts are repaired and replaced. Stem cell therapy will create such possibilities for human beings. It is believed that stem cell therapy will help in treatment of cancer, brain damage, and organ and tissue damage during accidents and fatal injuries, wound healing as well as many genetic and metabolic syndromes. Although doctors can transplant organs, non self (from donor) grafts often have problems with rejection. Generating organs, cells, tissues from cells from our own body cells by reprogramming them would help in overcoming this problem. For example, liver cirrhosis can be treated in this fashion. Embryonic stem cells are thought by most scientists and researchers to hold potential cures for spinal cord injuries and hundreds of rare immune system and genetic disorders and much more. Stem cells represent a significant opportunity for medical research. Stem cells are primal cells found in multi-cellular organisms which have the ability to not only differentiate into a wide range of specialized cell types but also to renew themselves through mitotic cell division. Human mesenchymal stem cells can generate liver while neural stem cells generate different neural tissues. Stem cells can be classified into three broad categories, based on their ability to differentiate. Totipotent stem cells are found only in early embryos. Each such cell can form a complete organism (e.g., identical twins). Pluripotent stem cells exist in the undifferentiated inner cell mass of the blastocyst and can form any of the over 200 different cell types found in the body. Multipotent stem cells are derived from fetal tissue, cord blood and adult stem cells. Although their ability to differentiate is more limited than pluripotent stem cells, they already have a track record of success in cell-based therapies. Here is a current list of the sources of stem cells:
  • Embryonic stem cells - are harvested from the inner cell mass of the blastocyst seven to ten days after fertilization.
  • Fetal stem cells - are taken from the germline tissues that will make up the gonads of aborted fetuses.
  • Umbilical cord stem cells - Umbilical cord blood contains stem cells similar to those found in bone marrow.
  • Placenta derived stem cells - up to ten times as many stem cells can be harvested from a placenta as from cord blood.
  • Adult stem cells - Many adult tissues contain stem cells that can be isolated.
  • Does life begin at fertilization, in the womb, or at birth?
  • Is a human embryo equivalent to a human child?
  • Does a human embryo have any rights?
  • Could the destruction of a single embryo be justified if it provides a cure for a countless number of patients?
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Although cloning and stem cell therapy has opened up new avenues, caution has to be exercised since all side effects and possible medico-legal implications have not yet been fully realised. In order to ensure that this technology is not misused, increased participation and awareness needs to be generated. The pace of development in this field is so rapid that new legislations need to be formulated to cater to the social and ethical aspects. Embryonic stem cells are thought by most scientists and researchers to hold potential cures for spinal cord injuries, multiple sclerosis, diabetes, Parkinson's disease, cancer, Alzheimer's disease, heart disease, hundreds of rare immune system and genetic disorders and much more. There are a lot of questions about the possible side effects. Although substantial progress has been made, there are many features, nuances which have not been completely unravelled. Given the controversy around embryonic stem cell research it would be better if adult stem cells offered the same potential as embryonic stem cells. Embryonic stem cells are thought to have much greater developmental potential than adult stem cells. It is the ability of the embryonic cell, in theory, to become all types of tissue that offers such hope of cure for many diseases. There have been a number of breakthroughs. It is believed that one of them came as long ago as 1968 when Dr Robert Good, at the University of Minnesota, performed the world's first bone marrow transplant on a four month old boy. It is believed that the stem cells in the bone marrow grew and created specialized immune cells needed to fight the inherited disease the boy had. Bone marrow treatments have been the only type of stem cell treatment, which is commonly used to treat human diseases. More advanced techniques of "harvesting" human stem cells are now used to treat leukemia, lymphoma and several inherited blood disorders. Scientists see almost infinite value in the use of embryonic stem cell research to understand human development and the growth and treatment of diseases. Treatments for diabetes and advanced kidney cancer have also been shown to have great clinical potential. With a projected market for regenerative medicine of $500 billion by 2020 in the U.S. alone, stem cell and regenerative medicine can become an engine for economic growth. Chris Mason, director of the London Regenerative Medicine Network in the U.K., said that while regenerative medicine is relatively new to the marketplace — with more than 323,000 patients worldwide have receiving cell-based therapies and there are a handful of products on the market, including a synthetic skin — he predicted that one day the market for stem cell and other regenerative therapies will rival that of medical devices and drugs from large pharmaceutical companies. To put this in an economic perspective, according to an interesting report that attempts to create a model that values human life at the economic level, it suggests decreasing the mortality rate of all cancers by 1% would be worth about 6% of the GDP.

Human embryonic stem cell research has been promoted as being the best way to pursue cell-based therapies for a number of diseases. Although embryonic stem cells are the most versatile type of stem cells, they are sometimes unacceptable for therapy because they may spontaneously form tumors when transplanted into a compatible host. Embryonic stem cells also suffer from the usual tissue compatibility problems associated with donor transplants. The proposed solution to tissue compatibility problems, therapeutic cloning, is technically challenging (i.e., it hasn't been accomplished yet) and fiscally prohibitive (costs on the order of hundreds of thousands of dollars per patient). In contrast to embryonic stem cell technologies, adult stem cells have been used to treat dozens of diseases, with the list growing every year. Pursuing this technology would eliminate the tissue rejection problems associated with embryonic stem cells, and the high cost associated with therapeutic cloning. A new technique involving reprogramming of adult skin cells (iPS) has proved feasible, producing pluripotent ESC-like stem cells, potentially from individual patients. Adult stem cells have been isolated from amniotic fluid, peripheral blood, umbilical cord blood, umbilical cord, brain tissue, muscle, liver, pancreas, cornea, salivary gland, skin, tendon, heart, cartilage, thymus, dental pulp, and adipose tissue. However, because individualized adult stem cell therapies cannot be patented, this research does not appeal to biotech companies and scientists and research centers seeking royalty payments for patents. With the announcement that embryonic stem cell-like lines can be produced by reprogramming adult human skin cells, the potential usefulness of embryonic stem cell research has been lost for many stem cell researchers, as they are now pursuing the new technology, which will be cheaper and provide fewer problems for use in patient-directed therapies.

In theory, stem cell technology could be used to produce replaceable tissues or organs. Defective tissues/organs could be repaired using healthy cells. It would also be possible to genetically engineer stem cells to accomplish activities that they would not ordinarily be programmed to do. Part of this engineering could involve the delivery of chemotherapeutic agents for treatment of cancers and tumors. Human embryonic stem cells have been studied only recently, so their capabilities are, as of now, unknown. In theory, the embryonic stem cells are able to form every cell type (which is what they do in the embryo). However, the conditions in culture might not be able to recreate the conditions that give rise to many tissues in the intact embryo. In addition to these unknowns, it is uncertain whether the cultured stem cells will possess the same properties as cells that have been developed within the embryo. What happens if they undergo changes and this alters their function. For example, in one recent study, insulin-producing cells derived from murine embryonic stem cells failed to produce the insulin when transplanted into mice, but only formed tumors. In addition, it is uncertain that these lines will continue to proliferate indefinitely without undergoing genetic mutations that render them useless.

Like all immortal cell lines, embryonic stem cell lines must be protected and checked for contamination with viruses, bacteria, fungi, and Mycoplasma. The use of infected lines in patient treatment could have devastating effects. Many embryonic stem cell lines are grown using mouse feeder cells. The mouse cells help the embryonic lines to grow, but pose risks for transplantation due to compatibility problems in human bodies. Initially many countries and states all over the world had banned cloning.

The first human cloned embryos were not produced until 2001, when a private company, Advanced Cell Technology, produced 6-cell embryos. The first cloned human blastocyst was reported in 2004 by a group in Korea. However, subsequently, it was found that much of the information in the publication in science was fabricated and the paper was officially withdrawn in January, 2006. Also, contrary to the claims in the paper, not hundreds of oocytes, but over 2000 were used, although no cell lines could be established.

A subsequent publication by Hwang et al. in 2005 was also found to have been fabricated by intentionally submitting duplicate patient samples in place of patient samples and cloned cells. Both papers were withdrawn by the journal Science. This data was shown to be fabricated, using embryos from a fertilization clinic. The publication making this claim has been officially withdrawn which was a severe blow to the idea human embryonic stem cells can be cloned for use in treating patients.

There are still many hurdles to clear before embryonic stem cells can be extensively used therapeutically. For example, because undifferentiated embryonic stem cells can form tumors after transplantation in histo- compatible animals, it is important to determine an appropriate state of differentiation before transplantation. Differentiation protocols for many cell types have yet to be established. Targeting the differentiated cells to the appropriate organ and the appropriate part of the organ is a challenge. Delivery of stem cells and regulating their ability to differentiate in a particular lineage is also a challenge.

This raises many questions about when does life begin, what is life, who has the right to generate embryos, who would be responsible if something in the treatment goes wrong, etc. Proponents of human embryonic research claim that frozen embryos from in vitro fertilization treatments are going to be destroyed, so they might as well be used for stem cell research. However, using that logic, prisoners on death row should also be used for medical research, since they are also going to be killed anyway. Until recently, the only way to get pluripotent stem cells for research was to remove the inner cell mass of an embryo and put it in a dish. The thought of destroying a human embryo can be unsettling, even if it is only five days old.

Stem cell research thus raised difficult questions:
  • Does life begin at fertilization, in the womb, or at birth?
  • Is a human embryo equivalent to a human child?
  • Does a human embryo have any rights?
  • Could the destruction of a single embryo be justified if it provides a cure for a countless number of patients? 
With iPS (induced pluripotent adult) cells now available as an alternative to hES (human embryonic) cells, the debate over stem cell research is becoming increasingly irrelevant. But ethical questions regarding hES cells may not entirely go away. Inevitably, some human embryos will still be needed for research. iPS cells are not exactly the same as hES cells, and hES cells still provide important controls: they are a gold standard against which the "stemness" of iPS cells is measured. Some experts believe it's wise to continue the study of all stem cell types, since we're not sure yet which one will be the most useful for cell replacement therapies.

An additional ethical consideration is that iPS cells have the potential to develop into a human embryo, in effect producing a clone of the donor. Many nations are already prepared for this, having legislation in place that bans human cloning. Since ES cells can grow indefinitely in a dish and can, in theory, still grow into a human being, is the embryo really destroyed?

The implications of genetic analysis and possibilities of many new treatments due to increased understanding of the mechanisms will certainly lead to increased life expectancy. The use of embryonic cells though promising has to address many legal, social, moral ethical issues. The new science of generating stem cells from adult cells and tissue has opened up new avenues and remarkable breakthroughs in regenerative and recuperative medicine. This new paradigms and technological inventions promise to open up new ways of treatment which may become very popular and acceptable and beneficial. It will lead to improving the quality of life and increasing longevity. Such treatments must be taken only after they are scientifically well established and increased awareness in very important. This would prevent it from becoming a marketing gimmick and taking people on a ride with tall claims. This promising science is still in its infancy and is developing in leaps and bounds. This will affect all predictive processes related to human life in terms of health and other policy issues. It also brings to the fore the necessity to formulate new laws and governance systems to cater to the misuse and side effects. Issues such as accessibility, affordability and availability have to be evaluated for this promising therapy.

The average life expectancy has increased from 47 years in 2005 to 67.6 in 2008 and is expected to rise further. The process of organismal ageing is characterized by functional decline due to histologic and biochemical changes in tissues and organ systems with the passage of time. Declining functionality is paralleled by diminishing capacity to respond to injury or stress. One barely remembered when one fell off the bike at a young age, but as one grows old we remember it for weeks. There are reports that stem cell treatment increased life span of some experimental mice. The scientists started with a genetically modified variety of mice which ages rapidly. The mice typically die after only 28 days while normally they can live till 21 months. In the trial these mice were injected with stem cells taken from young mice. The cells were actually muscle specific progenitor cells taken from non-mutated mice. The injections caused the experimental mice to live longer, up to 66 days, and appeared healthier. Regenerative medicine appeared to help natural healing processes to work faster and better. Research undertaken since 2004 suggests that the stem cells in the adult body - which become less effective at their job of repair with age - could be rejuvenated, restored to action with the right biochemical cues. Furthermore, researchers already regularly manipulate the genes and biochemistry of stem cells taken from patients for use in trials of new therapies: there is every reason to expect that future medicine will involve the repair and restoration of aged stem cells prior to their use in regenerative medicine. Amazing things like bone regrowth, formation of teeth, retinal repair, cure for deafness, skin replacement are some things that have been demonstrated in laboratory. In addition to life saving and regenerative abilities cosmetic uses of stem cells could also be many. Today, hundreds of millions of people live in pain and suffering - and will eventually die - as a result of degenerative conditions of aging. If in future strategies could be worked out to relieve the pain and suffering even partially, it would be a boon. If stem cell therapy would end up delivering some of what it promises it would almost be like a dream come true!

References


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About the author

Dr. Deepti Dileep Deobagkar is a Professor, Centre of Advanced Studies, Department of Zoology, and Director, Bioinformatics Centre, University of Pune, 411007. This is an abstract of the 10th Prof. G. S. Diwan memorial lecture she delivered in Mumbai on Saturday September 15th, 2012.

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