Sickle Cell Disease or Anemia and CRISPR-Cas9 Genome Editing
Genome Editing for Treating Disease
Sickle cell anemia is a type of sickle cell disease, or SCD. It's a very unpleasant and often painful condition in which red blood cells are misshapen, stiff, and sticky. The abnormal cells may block blood vessels. The blockages can lead to tissue and organ damage. The disorder is caused by a gene mutation in a specific type of stem cell. A process known as CRISPR-Cas9 has been used to correct the mutation in stem cells placed in lab equipment. The edited cells may one day be placed in the bodies of people with sickle cell anemia. The process will hopefully cure the disorder.
Many people working in molecular biology and biomedicine are excited by the CRISPR-Cas9 process. It offers the potential for huge benefits in our lives. There are some concerns about the process, however. Our genes give us our fundamental characteristics. While it's hard to imagine that anyone would object to replacing genes in order to help people with a life threatening, painful, or debilitating disease, there are worries that the new technology will be used for less benign purposes.
What Is Sickle Cell Disease or SCD?
SCD exists in several forms. Sickle cell anemia is the most common form of the disease. For this reason, the term "sickle cell disease" is often synonymous with sickle cell anemia. This article refers specifically to the sickle cell anemia version of SCD, though some of the information may apply to the other forms as well.
Patients with SCD make an abnormal form of hemoglobin due to a gene mutation. Hemoglobin is a protein in red blood cells that transports oxygen from the lungs to the body's tissues.
Normal red blood cells are round and flexible. In someone with the sickle cell anemia form of SCD, the red blood cells are sickle-shaped, stiff, and inflexible due to the presence of the abnormal hemoglobin inside them. Normal cells can squeeze through narrow passages in the circulatory system. Sickled cells may get stuck. They sometimes collect and stick together, forming a bottleneck. The clump of cells reduces or prevents oxygen from getting to the tissue beyond the bottleneck and may cause damage to the tissue.
Sickle cell disease requires a doctor's diagnosis and treatment recommendations. Treatments vary and depend on a person's symptoms, age, and other health problems as well as the type of SCD.
Types of SCD
Sickle cell disease is caused by a mutation in a gene that codes for part of the hemoglobin molecule. Each of our chromosomes has a partner chromosome that contains genes for the same characteristics, so we have two copies of the hemoglobin gene. The effects of the mutated gene depend on the way in which it's altered and whether an alteration occurs in both copies of the gene or in only one.
Normal hemoglobin is also known as hemoglobin A. In certain situations, an abnormal form of the protein known as hemoglobin S causes red blood cells to become sickled. Some examples of sickle cell disease and their relationship to hemoglobin S are listed below. Other types of SCD exist in addition to the ones that are listed, but they are rarer.
- If one hemoglobin gene codes for hemoglobin S and the other gene codes for hemoglobin A, the individual won't have sickle cell disease. The normal gene is dominant and the mutated one is recessive. The dominant one "overrules" the recessive one. The person is said to be a carrier for sickle cell trait and may pass it to their children, however.
- If both genes code for hemoglobin S, the person has sickle cell anemia. The condition is symbolized by hemoglobin SS or HbSS. This is generally the most serious form of the disease as well as the most common.
- If one gene codes for hemoglobin S and the other codes for an abnormal form of hemoglobin called hemoglobin C, the condition is symbolized as hemoglobin SC or HbSC.
- If one gene codes for hemoglobin S and the another codes for a disease called beta thalassemia, the condition is symbolized as HbS beta thalassemia or HbSβ thalassemia. Beta thalassemia is caused by a mutation in the HBB gene. This codes for a protein called beta globin, which is part of the hemoglobin molecule.
People with any of last three conditions in the list above have a problems in carrying a sufficient amount of oxygen in their blood due to the alterations in their hemoglobin molecules.
Possible Symptoms of SCD (Sickle Cell Anemia Form)
Symptoms of SCD vary considerably. They depend on a person's age and the type of sickle cell disease that they have. Some symptoms are more common than others, however. A patient often experiences pain when sickled red blood cells block a vessel and prevent oxygen from reaching tissues. The painful episode is known as a crisis. The frequency and severity of crises is different in different people. As a person grows older, they may experience chronic pain.
Patients with SCD frequently suffer from anemia. This is a condition in which the body contains an insufficient number of red blood cells and is therefore unable to transport enough oxygen to the tissues. Sickled red blood cells live for a much shorter time than normal ones. The body may not be able to keep up with the demand for new cells. The main symptom of anemia is fatigue.
Other symptoms of SCD may include:
- an increased risk of infection due to spleen damage
- jaundice due to the presence of yellow bilirubin released by excessive red blood cell breakdown
- stroke due to the blockage of blood travelling to the brain
- acute chest syndrome (sudden breathing problems due to the presence of sickled cells in the blood vessels of the lungs)
Medications and other treatments are available to treat sickle cell disease. A person may need to seek medical aid during a crisis. As the doctor in the video above says, SCD must be managed carefully because there are several symptoms associated with the disorder that are potentially life threatening. As long as this management takes place, however, the outlook for patients today is much better than it was in the past.
According to the NIH (National Institutes of Health), in the United States the predicted lifespan for SCD patients is currently forty to sixty years. In 1973 it was only fourteen years, which shows how much treatment has improved. Nevertheless, we need to find ways to increase lifespan to a normal length and to reduce or preferably eliminate crises. It would be wonderful to eliminate the disease altogether. Correcting the mutation that causes the disorder might enable us to do this.
Mutations in Hematopoietic Stem Cells
Our blood cells are made in the bone marrow, which is located inside some of our bones. The starting point for blood cell production is the hematopoietic stem cell, as shown in the illustration above. Stem cells are unspecialized, but they have the wonderful ability to produce the specialized cells that our body needs and well as new stem cells. The mutation that produces SCD is present in the hematopoietic stem cells and passed to the red blood cells, or erythrocytes. If we could give SCD patients normal stem cells, we could cure the disease.
At the moment, the only cure for sickle cell disease is a bone marrow or hematopoietic stem cell transplant using cells from someone that lacks the mutation. Unfortunately, this isn't a suitable treatment for everyone due to their age or the incompatibility of donor cells with the recipient's body. CRISPR may be able to correct the mutation in the patient's own stem cells, eliminating the problem of incompatibility.
In order to get a basic understanding of the gene editing process, some knowledge of cell biology is needed.
DNA and Chromosomes
DNA stands for deoxyribonucleic acid. There are forty-six DNA molecules in the nucleus of each of our body cells (but only twenty-three in our eggs and sperm). Each molecule is associated with a small amount of protein. The union of a DNA molecule and protein is known as a chromosome.
Genome and Genes
Our genome is the complete set of all the DNA in our cells. Most of our DNA is in the nucleus of our cells, but some is located in the mitochondria. Genes are located in DNA molecules and contain the code for making proteins. Part of each DNA molecule is non-coding, however.
Structure of DNA and the Nature of the Genetic Code
A DNA molecule consists of two strands consisting of smaller molecules. The strands are bonded together to form a ladder-like structure. The ladder is twisted to form a double helix. A flattened section of the "ladder" is shown in the illustration below.
The most significant molecules in a strand of DNA as far as the genetic code is concerned are known as nitrogenous bases. There are four of these bases—adenine, thymine, cytosine, and guanine. Each base appears multiple times in the strand. The sequence of bases on one strand of the DNA forms a code that provides instructions for making proteins. The code resembles a sequence of letters from the alphabet arranged in a specific order to form a meaningful sentence. The length of DNA that codes for a particular protein is called a gene.
The proteins that are made by cells are used in many ways. Enzymes are one type of protein and are vitally important in our body. They control the myriad of chemical reactions that keep us alive.
The "building blocks" of a DNA molecule are known as nucleotides. A nucleotide consists of a phosphate-deoxyribose group and a nitrogenous base.
Messenger RNA, Complementary Base Pairing, and Mutations
Although the code for making proteins is located in the nuclear DNA, the proteins are made outside the nucleus. DNA is unable to leave the nucleus. RNA, or ribonucleic acid, is able to leave it, however. It copies the code and transports it to the site of protein synthesis in the cell.
There are several versions of RNA. They have a similar structure to DNA but are single-stranded. The version that copies and transports information out of the nucleus during protein synthesis is known as messenger RNA. The copying process is based on the idea of complementary bases.
Complementary Base Pairing
There are two pairs of complementary bases in nucleic acids. Adenine on one strand of nucleic acid always bonds to thymine on another strand, and vice versa. The bases are said to be complementary. Similarly, cytosine on one strand always binds to guanine on another strand, and vice versa. This feature can be seen in the DNA illustration above.
The messenger RNA that leaves the nucleus contains a base sequence that is complementary to the one in DNA. The two strands of the DNA molecule temporarily separate in the region where messenger RNA is being made. Once the RNA is complete, it separates from the DNA molecule and the strands of DNA reattach.
In a mutation, the order of bases in a region of a DNA molecule is changed. As a result, the RNA that is made from the DNA will also have the wrong sequence of bases. This will in turn cause an altered protein to be made.
RNA transcribed from DNA is also used for purposes other than protein synthesis.
CRISPR and Spacers in Bacteria
In the 1980s, researchers noticed that several species of bacteria contained a strange pattern in part of their DNA. The pattern consisted of repeating sequences of bases alternating with spacers, or sections with a unique sequence of bases. The researchers called the repeating sequences CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats).
The researchers eventually discovered that the unique sections or spacers in the CRISPR region of the bacterial DNA came from viruses that had entered the bacteria. The bacteria were maintaining a record of their invaders. This enabled them to recognize the viral DNA if it appeared again and then mount an attack against it. The system is reminiscent of the action of our immune system. The process is important in bacteria because intact viral DNA takes over a bacterial cell and forces it to make and release new viruses. The bacterium is often killed as a result.
A Method of Destroying Viruses in Bacterial Cells
Once the viral DNA is incorporated into the DNA of a bacterium, the bacterium is able to attack that type of virus if it enters the cell again. The "weapon" in the bacterial attack against viruses is a set of Cas (CRISPR-associated) enzymes that cut the viral DNA into pieces, thereby preventing it from overtaking the cell. The steps in the attack are as follows.
- The viral genes in the bacterial DNA are copied into RNA (via complementary bases).
- Cas enzymes surround the RNA. The resulting structure resembles a cradle.
- The cradle travels through the bacterium.
- When the cradle encounters a virus with complementary DNA, the RNA attaches to the viral material and the Cas enzymes break it up. This process prevents the viral DNA from harming the bacterium.
Today the term CRISPR refers to the gene-editing technique in which specific base sequences in a cell are found and modified as well as to the repeated sequences of bases in bacteria. The gene-editing process is also known as CRISPR-Cas9 because an enzyme known as Cas9 is involved in the process.
How Does CRISPR-Cas9 Edit Human Cells?
CRISPR technology in human cells follows a similar pattern to the process in bacteria. In human cells, however, the RNA and enzymes attack the cell's own DNA instead of the DNA of an invading virus.
The most common form of CRISPR at the moment involves the use of an enzyme called Cas9 and a molecule known as guide RNA. Understanding all the details of CRISPR requires an in-depth knowledge of molecular biology. The overall process as it applies to correcting mutations is as follows.
- The guide RNA contains bases that are complementary to those in the mutated (altered) region of the DNA and therefore binds to this region.
- By binding to the DNA, the RNA "guides" the molecules of the Cas9 enzyme to the correct place on the altered molecule.
- The enzyme molecules break the DNA, removing the target section.
- A harmless virus is used to add the correct strand of nucleotides to the broken area. The strand is incorporated into the DNA as it repairs itself.
CRISPR-Cas9 and Sickle Cell Disease Treatment
In 2016, the results of some interesting research into treating SCD with CRISPR were reported. The research was performed by scientists from UC Berkeley, UC San Francisco Benioff Children's Hospital Oakland Research Institute, and the University of Utah School of Medicine.
The scientists extracted hematopoietic stem cells from the blood of people with sickle cell disease. The researchers were able to correct the mutations in the stem cells by using the CRISPR process. The plan is to eventually put the edited cells into the bodies of people with SCD. The transplantation of edited cells has not yet been done, however. The researchers have to perform large scale tests with mice and lots of safety tests before they will be allowed to perform clinical trials in humans.
Adding normal stem cells to the body will be useful only if the cells stay alive. To discover whether this is possible, the researchers placed edited hematopoietic stem cells in the bodies of mice. After four months, two to four percent of the mouse stem cells that were examined were the edited version. The researchers say this percentage is likely the minimum level needed to be beneficial for humans. They want to increase the survival rate of the cells before they begin clinical trials, however.
Unlike other gene-editing methods, it (CRISPR) is cheap, quick and easy to use, and it has swept through labs around the world as a result. Researchers hope to use it to adjust human genes to eliminate diseases, create hardier plants, wipe out pathogens and much more besides.— Heidi Ledford at nature.com
Heading Towards a Clinical Trial
In 2018, Stanford University said that they were hoping to soon perform a clinical trial of CRISPR-Cas9 technology for sickle cell disease treatment. They plan to edit one of the two problematic hemoglobin genes in a patient's stem cells by replacing it with a normal gene. This would lead to a genetic situation similar to the one found in a carrier of the sickle cell gene. It would also be a less extreme process than editing both genes.
A scientist involved in the research says that the CRISPR-Cas9 process doesn't have to replace all of the damaged stem cells. Normal red blood cells live longer than the damaged ones and soon outnumber them, as long as there aren't too many damaged cells to replace in proportion to the normal ones.
Symptoms of the disease occur only if the proportion of sickled cells in the bloodstream is above 30 percent. If at least 70 percent of the red blood cells are healthy, the patient is symptom-free.— Dr. Mark Porteus, Stanford University
Hope for the Future
Some people with SCD might be eager to receive a transplant of genetically corrected stem cells. Scientists need to be cautious, however. Changing the DNA of a living person is a very significant event. Researchers must make sure that the altered stem cells are safe.
The Stanford researchers have said that they would like to perform a clinical trail sometime in 2019. It will be interesting to see if this happens. If it does, I hope the trial is successful. The wait could be very worthwhile if it enables scientists to provide both effectiveness and safety with their genome editing technique.
Sickle cell disease information from the National Heart, Lung, and Blood Institute
Facts about sickle cell anemia from the Mayo Clinic
CRISPR overview from Harvard University
CRISPR and SCD from the Nature journal
Gene editing for sickle cell disease from the National Institutes of Health
A 2018 report about a potential treatment for SCD from Stanford Medicine
This content is accurate and true to the best of the author’s knowledge and does not substitute for diagnosis, prognosis, treatment, prescription, and/or dietary advice from a licensed health professional. Drugs, supplements, and natural remedies may have dangerous side effects. If pregnant or nursing, consult with a qualified provider on an individual basis. Seek immediate help if you are experiencing a medical emergency.
© 2016 Linda Crampton