Normal and Sickled Red Blood Cells

While scientists have long known that carriers for Sickle Cell Trait are more resistant to Malaria infection, the mechanism by which protection is conferred has not been well understood—until now.  Scientists at Heidelberg University used an electron microscope to observe what happens when the parasite that causes Malaria in humans, Plasmodium falciparum, infects red blood cells containing both healthy and mutant hemoglobin.

Scientists noticed that in red blood cells with healthy hemoglobin, the parasite hijacks the actin cytoskeleton to transport its own “adhesin” protein to the cell membrane.  The adhesin, also called Plasmodium falciparum Erythrocyte Membrane Protein 1 (PfEMP1), causes the red blood cells to adhere to each other and to the walls of small blood vessels so that the infected cells don’t circulate through the spleen, where they would otherwise be destroyed.   The parasite continues to reproduce within the infected cells and causes cell lysis, destroying the red blood cells and releasing more Plasmodium into the blood stream.  As a result, Malaria patients become severely anemic and suffer from symptoms related to insufficient oxygen delivery.

While mutant hemoglobin polymerizes into long rods and causes red blood cells to sickle dangerously in low oxygen conditions, it appears that in sufficient oxygen conditions the mutant protein prevents Plasmodium from borrowing the actin cytoskeleton to ferry adhesin to the cell surface.  Without knobs of adhesin protein, infected cells travel to and are destroyed in the spleen so that patients do not suffer Malaria symptoms.

Each individual carries two copies of the information needed to make a protein.  People with two mutated hemoglobin genes may be highly resistant to Malaria infection, but they also suffer from Sickle Cell Disease, in which abnormally shaped red blood cells clog blood vessels and cut off the blood supply to organs.  People with one copy of the mutant hemoglobin gene, however, make both enough healthy protein to maintain red blood cell shape and enough mutant protein to interfere with Malaria infection.

HIV infects white blood cells by latching onto two protein receptors, CD4 and CCR5.  Scientists noticed that people with a defect in the CCR5 gene (a 32-bp deletion) are incapable of making the CCR5 receptor and are highly resistant to HIV infection.  Unlike with other genetic defects, these people appear otherwise healthy.  If an HIV patient’s own white blood cells could be removed, their CCR5 genes mutated, and the cells returned to the body, then the patient would be protected from further infection.  The virus would not be eradicated from the patient’s body, but the HIV would be unable to attack more white blood cells so that the patient’s immune system would remain intact to fight off other infections.

The primary problem with such a technique, however, has been an issue of gene targeting: how do you inactivate the CCR5 gene without disrupting the rest of the genome?  Scientists have found  a pair of proteins that can help them do just that.  Zinc finger proteins are exceptionally good at binding to DNA.  In addition, they can be designed to bind to very specific sequences of DNA.  A type of enzyme, called a restriction endonuclease, can cut DNA.  Scientists have hitched these two protein types together to create the molecular equivalent of a heat-seeking missile:  the zinc fingers home in on and attach to a sequence in the CCR5 gene while the restriction enzyme damages the DNA in that gene only.  Cells come armed with machinery for patching damaged DNA, but the repair process eliminates some of the information needed to make the CCR5 protein.  The end result is a shorter, defective CCR5 protein which cannot guide HIV into the cell.

Scientists at Sangamo BioSciences have used this approach to inactivate the CCR5 gene in a type of differentiated white blood cell called a T cell.  The company has progressed to clinical trials with HIV patients and has so far found that the genetically altered T cells survive and function normally while the amount of virus detected in patients decreases.  Because T cells eventually die, however, the company is also in the process of modifying blood stem cells, called Hematopoietic Stem Cells (HSCs), in much the same way.  If the company succeeds in modifying HSCs, then any new white blood cell made in a patient’s body would be immune to attack—the treatment would need to occur only once for life-long protection to be realized.

Clostridium sporogenes are anaerobic soil dwellers which cannot survive in the presence of oxygen.   Researchers have genetically modified these bacteria so that they produce an enzyme that activates a cancer drug.  It turns out that the centers of solid cancer tumors contain very little oxygen.  Researchers hope to inject cancer patients’ tumors with the engineered Clostridium spores, which would not survive in the rest of the oxygen-rich body.   After a tumor is infected with the Clostridium, a patient would also be injected with a cancer drug.  The drug would circulate throughout the body in an inactive, “pro-drug” form, and would become active only inside of the tumor.  This would allow for targeted attack of the tumor with fewer healthy cell casualties than conventional chemotherapy.  If successful, this treatment would be especially useful for hard to reach tumors, such as in the brain.

One of the scientists involved in the research, Professor Nigel Minton, explains, “Clostridia are an ancient group of bacteria that evolved on the planet before it had an oxygen-rich atmosphere and so they thrive in low oxygen conditions. When Clostridia spores are injected into a cancer patient, they will only grow in oxygen-depleted environments, i.e. the centre of solid tumours. This is a totally natural phenomenon, which requires no fundamental alterations and is exquisitely specific. We can exploit this specificity to kill tumour cells but leave healthy tissue unscathed.”

The bacteria are slotted to be tested in clinical trials starting in 2013.

While the genetic modification of plants for human consumption is common in the United States (think corn and soybeans), genetically modified (GM) animals have yet to be approved.  But now, a Massachusetts-based company, AquaBounty, is petitioning the FDA to sell genetically modified Atlantic Salmon to consumers.

Thanks to some genetic mix and match, the salmon, dubbed AquAdvantage, reach full size faster and with less food than non-modified salmon.  Typically, Atlantic Salmon produce growth hormone during warm months only.  AquaBounty scientists borrowed a growth hormone gene from another salmon species, the Chinook, and hitched it to a molecular “on switch,” called a promoter, from the eel-like ocean pout.  Combined, these additions to the Atlantic Salmon genome allow the creatures to produce growth hormone year-round, shortening maturation time from three years to two while reducing feed consumption by 10%.  The genetic manipulation is essentially analogous to feeding cows and poultry growth hormones, a regular American farming practice.

AquaBounty insists that the GM fish are identical in composition and taste to their non-modified counterparts, making them safe for human consumption.  The reduced maturation time would ostensibly protect wild fish stocks and stabilize the salmon supply as seafood demand rises with the awakening of the American health conscience.  AquaBounty’s assurances haven’t, however, prevented consumer advocacy groups from maligning the product as a repulsive “Frankenfish” whose approval would open a Pandora’s Box of genetic misfits that would make it onto our dinner tables.

Dear reader, you likely already consume GM crops; would it bother you to eat genetically modified animals too?

Dr. Charles Arntzen (Co-Director of the Center for Infectious Diseases and Vaccinology, The Biodesign Institute and Florence Ely Nelson Presidential Chair, Arizona State University) has spearheaded the genetic modification of plants to enable food crops to produce antigenic proteins.  So far, he has experimented with bananas that protect against Norwalk Virus and potatoes that promote antibody production against Hepatitis B and pathogenic strains of E.coli.  Dr. Arntzen cautions that vaccine containing crops are medicines, not food that would show up at a local grocery store, and would be regulated as pharmaceuticals.  You can read an interview with Dr. Arntzen here.

Plant based, edible vaccines would not only make vaccine-aged children happier, they would also simplify global vaccine distribution. Currently, vaccines require refrigeration and skilled health workers to inject them.  Fruits and vegetables can be freeze dried and pulverized into powders that would be heat stable: vaccine powders and pills could then be shipped around the globe without concern for a “cold chain” to preserve them, and could be eaten by populations without access to trained health workers.

The US government has also acknowledged that plant based vaccines would be faster and cheaper to produce than conventional vaccines, which are made in eggs over the course of several months.  In early 2010, the research arm of the Department of Defense, the Defense Advanced Research Projects Agency (DARPA), awarded the Texas Plant-Expressed Vaccine Consortium a $40 million grant for Project Green Vax, an initiative to develop vaccines in tobacco plants.  The Project’s first task is to show that tobacco can yield 10 million effective doses of the H1N1 vaccine per month.  It would be incredibly ironic if tobacco redeems its reputation for being a public health scourge by becoming a public health staple.

In a bone marrow transplant, a patient’s own marrow is destroyed and replaced with tissue from a donor. The donor marrow contains healthy hematopoietic stem cells (HSCs, adult stem cells in the blood) which repopulate the patient’s body with healthy red and white blood cells for oxygen transport and immune defense. Just as with other varieties of organ donation, tissue-type matches are critical. In the case of the AIDS patient, another screen was also applied: his doctors searched for donors whose cells were also resistant to HIV infection.

In order to infect a white blood cell, HIV must latch onto 2 receptors on the cell’s surface: the cd4 receptor and the CCR5 receptor. Some people have no CCR5 receptors due to mutations in the genes encoding the protein—those people are highly resistant to HIV infection. The AIDS patient received bone marrow from a donor whose HSCs (and subsequent white blood cells) could not produce the CCR5 receptor: his new cells cannot be infected by the HIV that decimated his old cells. The AIDS patient was, rather ironically, cured by receiving “defective” cells. The absence of a CCR5 receptor does not appear to affect normal physiology.

Bone marrow transplants are, unfortunately, not a feasible treatment option for the 33.4 million people infected with HIV worldwide. Patients receiving the transplant are at major risk for infection as they wait for their immune systems to regenerate: between 10 and 30% of patients die from the procedure. In addition, there are very few HIV resistant donors relative to the number of infected individuals. On average, 1 of every 1,000 Europeans carries 2 copies of the defective gene while the mutation (a 32 base pair deletion) is very rare in people of Asian and African descent.

The tidings are not all together grim, however. This case shows that HIV can be treated by inhibiting CCR5 expression. If a patient’s HSC could be removed, rehabilitated via gene therapy, and returned to the patient, future AIDS treatment might not require life-long drug courses or dangerous transplants.

According to the United Nations, between 15,000 and 20,000 people are injured or killed by landmines that litter more than 80 countries including Sudan, Somalia, Iraq, and Afghanistan. The bacteria would likely fail to identify those mines that do not leak, and would detect chemical remnants where mines were once, but are no longer, active. But as the students’ advisor, Alistair Elfick points out, “this anti-mine sensor is a great example of how innovation in science can be of benefit to wider society.”

The nucleus, where chromosomal DNA is stored and protected, is not the only source of DNA in a mammalian cell.  Mitochondria have their own 16,569 bp genomes encoding 37 genes involved in the production of biological energy, ATP.  Mutations in these genes have been linked with human diseases including diabetes, cancer, infertility, and neurodegeneration.  Women who carry a mixture of normal and mutated mitochondrial DNA, a condition called heteroplasmy, may be physiologically healthy but risk having seriously ill children.  The birth of Mito and Tracker, at the Oregon Health and Science University (OHSU), suggests that there may be a way for heteroplasmic women to bear healthy, (mostly) biological children.

When one female’s chromosomal DNA is transferred into another’s enucleated egg cell, the first female’s mitochondrial genome is replaced.  The resulting child features mom’s and dad’s chromosomal DNA, but also carries a donor female’s mitochondrial DNA.  Should the DNA transfer technique work in human cells, “who’s my other mommy” could be a question for the courts.

In the emerging field of pharmacogenetics, scientists study genome variations and correlate them with drug treatment response.  For example, variations (also called polymorphisms) in genes encoding enzymes involved in drug metabolism have been found to affect the activation, deactivation, and toxicity of drugs used to treat cancer, heart disease, and psychiatric disorders.  Recently, scientists found that DNA sequence can also be used to predict responsiveness to current Hepatitis C treatment (a 48-week course of peginterferon-α-2b combined with ribavarin).

The sequence at a single DNA position (single nucleotide polymorphism, or SNP) on chromosome 19, close to the gene encoding the interferon-λ-3 protein, has a significant effect on a patient’s ability to clear Hepatitis C infection with treatment.  Patients with a C nucleotide at the critical position on both copies of chromosome 19 (CC genotype) are two to three times more likely to respond to treatment than those patients with the T nucleotide at the same position (TT genotype).

In the case of Hepatitis C, the mechanism by which one sequence is more therapeutic than the other is not yet understood.  However, sequence information can still assist doctors in selecting appropriate treatments: as alternative Hepatitis C treatments become available, doctors may bypass the current treatment for those patients with the TT genotype.

As DNA sequencing becomes cheaper and easier, and genome information becomes elucidated, the personalization of medicine may become a reality.