PKU, a genetic disease, illustrating the principle:

Transcription

1 PKU, a genetic disease, illustrating the principle: DNA RNA Protein What happens when a job doesn t get done? I. PKU, illustrates the relevance of chemistry to human health. PKU stands for phenylketonuria. ref. Matalon, KM Top Clin Nutr 16(4):41-50 (2001) A. We are constantly being tested (from birth). 1) Why? Test newborns to check for congenital health problems. 2) What does congenital means The U.S. National Screening Status Report lists the status of newborn screening in the United States. B. What is PKU? A genetic disease caused by insufficient phenylalanine hydroxylase (PAH) activity. 1) PAH rxn.: 2) PAH is an enzyme. 3) The PAH of PKU patients is absent or doesn t work as well as normal PAH because its structure is different from wild type PAH. Wild type means This is 1 of ~ 10 4 jobs you must do to stay alive, healthy. C. Why does lack of PAH activity cause problems in PKU? 1) Phenylalanine (Phe) levels increase. Some of this Phe is converted to phenylketones, which is excreted in urine. PAH phenylalanine + oxidant tyrosine phenylketones 2) Part of the PKU pathology appears to be caused by elevated Phe. How Phe does this is not completely clear, (ex. :see Shefer, et al., J Neurosci Res. 61(5): , 2000). 3) Tyrosine (Tyr) is not formed via the PAH pathway in PKU patients, so Tyr is an essential amino acid for them (dietary supplements needed!). 1

2 Aside on PKU and diet Meat probably comes to mind 1 st when you think of protein in your diet, but most foods contain some protein. Some food additives also provide a risk for PKU patients. The structure of the artificial sweetener aspartame (found in some diet drinks) is shown. Why Aspartame might be a problem for PKU patients? Structure! D. Back to Why test newborns? PKU can be treated! See: Irene, spouse, sister, (& Stephanie) (Pheno, Genotypes?) This photo was taken at my wedding a few years ago. I have PKU. I was diagnosed on the 3rd day of my life when treatment with the diet began.... The other woman in the picture is my older sister. She also has PKU but when she was born there was no newborn screening program for PKU. The diagnosis of her disease was made late in her first year of life when she developed convulsions. It was too late for treatment and she has never known the benefits of early diagnosis and treatment as I know them...(irene, May 1998) Note: The pictures at right and above are from The Montreal Children=s Hospital Hyperphenylalaninemia (PKU) Resource Booklet for Families, by Peter M. Nowacki at The Biochemical Genetics Unit; updated by Annie Capua, Margaret Lilly, David Cé, Manyphong Phommarinh and Shannon Ryan. Used with permission, Dr. C. Scriver. 1. Set diet with low levels of 2. Supplement w/ Tyr, which leads to healthy Irene & eventually to healthy 2 yr old Stephanie, 2

3 E. What happens to PKU patients that are not treated or are not compliant with (or didn t have knowledge of) the treatment? i.e., What is the pathology? See Irene=s sister, above. 1. Mental retardation 2. Microencephaly (means? ) 3. Congenital heart disease F. Genetics ALoss of function@ genetic diseases & diploidy. 0. A linear sequence of bases in your DNA that codes for a specific function is called a gene. 1. Genotype vs. phenotype (Look up if unfamiliar.) genotype phenotype 2. Why doesn=t genotype = phenotype? Ploidy!!! a) Are you haploid or diploid? b) Better: i) Which of your cells are haploid? ii) Which of your cells are diploid? 3. What can you say about genes of a PKU patient? Nice web site (from McGill Univ.) with lots of information about specific PKU mutations: also see: 4. What can you say about genes of the parents of a PKU patient? G. Remember The Central Dogma: DNA RNA Protein H. How does the Central Dogma relate to PKU & the PAH gene? 1. The Human Genome Project View of a haploid karyotype. 3

4 2. Now search PKU It turns out that the gene that codes for PAH is on chromosome #12. Remember: A gene is a linear sequence of bases in DNA that codes (information) for a specific function (job). Based on: Dr. C. Scriver, Human Mutation 28(9): (2007). 4,932 is ~ 2.9% of 171,266! Why have the extra 97%? 4

5 4. The thin vertical lines in the mutation map (see link) ( are put together to form the mature mrna (short for? ). The open blocks labeled 1-12 are cut (spliced) out. 5. View the PAH gene like a paper you have written that has 171, 266 letters. (Are spelling errors possible?). Spelling error a mutation (DNA sequence change). a) Clearly a spelling error could result in less effective communication. (like a harmful mutation) b) Can you imagine a spelling error resulting in more effective communication. (a helpful mutation?) different mutations (as of )! PAHdb website, Dr. C. Scriver I. If we understand, structure tells us about function and helps develop treatments. 1. The structure of DNA: a double helix (this piece is only 20 bp long!) Space-filling model wireframe model ribbon model And finally: 2. Structure of the protein PAH. (from PDB structure 1PAH. Hydrogen atoms are not shown.) Space-filling model, CPK atom coloring Ribbon model. α-helices? β-sheet? 5