While most students don’t fully grasp this idea, there are a few examples we can use that can help to explain this.  I have always used examples of giraffes and the development of long necks, or antibiotic resistance in bacteria, but these seem to be a bit out of the realm of many 5^th grade students.  So what better example than ourselves!

During another lesson, I introduced the development of lactase persistence, or having the ability to drink and eat dairy products past infancy.  Digesting the sugar in milk (lactose) is dependent on whether or not the cells of your small intestine are producing the enzyme lactase.  Lactase is responsible for breaking lactose into smaller components that then get absorbed into the bloodstream.  For mammals that get milk from mother early in life, this enzyme is essential.  Would a mutation in the DNA that would allow a cell to continue to make lactase past infancy be beneficial?  It all depends on which population of humans you ask.

If it is a population of humans that began drinking the milk of other animals after the development of agriculture, like those of Northern European descent, it would be selected for.  These populations now show the highest frequency of lactase persistence among all human populations.  If dairy was not a part of your diet after infancy, this mutation wouldn’t be considered beneficial and would not have been selected for, such as in African, Asian and South American populations.

So, when teaching evolution and the changes that we see in species over time, it is nice to be able to give an example that we can see in humans.  Using an example that is a recent development in humans over the last 10,000 years, may help students to understand this concept better, and apply it across any species.

This has recently been reported for patients with Hemophilia B.  Hemophilia B, also known as Christmas disease, is due to a deficiency of the clotting factor IX (FIX).  The first reported case of Hemophilia B due to a decrease in FIX was in 1952, and was called “Christmas Disease” after the first patient diagnosed was named Stephen Christmas.  Without this clotting factor, the blood does not form clots and results in severe bleeding episodes, especially in the joints and muscles.

Bettert reatment for this disorder began back in the 1960’s where they would inject FIX concentrates into the blood of patients with hemophilia B.  This increased the average age of death of 24 to a median lifespan of 63 years of age.  So with the success of the protein therapy, why try to fix the genes?  With each treatment costing $150,000 to $300,000, a patient needing clotting factors for hemophilia could incur a lifetime cost of $20 million.

So there needs to be a way that a patient can have a more effective treatment option that will cost less.  This new treatment option offers some hope.  Using a new virus for the administration of the gene, patients have seen an increased production of FIX protein for longer periods of time, and were able to stop or decrease the amount of concentrate injections they would need.  With one injection of the virus only costing about $30,000, dramatic cost savings have already been seen.  While this does offer new hope for the treatment of clotting disorders, follow-up with a larger number of patients and for longer periods of time will be needed to fully weigh the benefits and risks of this technique.  Once this has been done, hopefully we will see gene therapy used more in practice and maybe even for more than just clotting disorders.

Ponder, Katherine P.  Merry Christmas for Patients with Hemophilia B. The New England Journal of Medicine 10.1056; December 10, 2011.  Nathwani A.C., Tuddenham E.G.D., Rangarajan S., et al.

The underlying cause for FXTAS is a mutation in the gene for Fragile X Mental Retardation Protein (FMRP), which is located on the X-chromosome; its protein product, Pur-α is essential for normal neural function. Scientists have recently determined the three-dimensional structure for this protein, a first step in the potential identification of an effective treatment that would address the cause and not just the symptoms of FXTAS.

As described in Your Genes, Your Health, the gene contains a region that consists of repeated CGG DNA triplets. In healthy people, both copies of their FMRP gene contain 5-54 copies of the CGG triplet. Disorder manifests itself if the repeat numbers exceed 54 in both gene copies (the disorder is recessive): 55-200 repeats lead to FXTAS, as described above. Repeat numbers exceeding 200 lead to Fragile X Syndrome (FXS), the second most common cause of heritable mental impairment after Down’s syndrome.

Given that the symptoms of FXTAS include shaking hands, it is interesting that the authors describe the shape of the FMRP protein, Pur-α as: “The crystal structure of Pur-α … looks like a hand: four so-called beta-strands, corresponding to four fingers, form a beta-sheet, and an adjacent alpha-helix resembles a thumb.” Apparently, pairs of PUR repeats bind to each other in a configuration that is reminiscent of a handshake, forming a functional unit that binds to RNA.