Duchennes Muscular Dystrophy - Coursework Example

Case study/genetic counseling (MOLECULAR MEDICINE) Task 1 Duchenne’s Muscular Dystropy (DMD) , a common form of muscular dystrophy ,as known is an fatal disease, characterized by progressive muscle weakness and atropy of muscle tissues starting from the legs and pelvic, eventually affecting the whole body. DMD is a fatal X- linked recessive disorder, with the ratio of occurrence of two in every 10,000 young men. This disease is caused by the mutation of Xp21 gene that codes for the protein Dystropin, in DMD the dystropin is absent, and in Becker’s muscular dystropy it is in lesser amount. In cellular level, DMD, the result of mutation of dystropin gene, whose protein are responsible for the connection of muscle fibers to extra cellular matrix through a protein complex that contains many subunit. The absence of dystropin permits excess calcium penetrate the cell wall- the sarcolemma, excess of which leads to a cascade of reaction resulting in the production of Reactive oxygen species resulting in oxidative stress ending in the death of sarcolemma- followed by the necrosis of muscle fiber that is ultimately replaced by the adipose and connective tissue. In genetic level, Duchenne muscular dystrophy is an inherited disorder , in an X-linked recessive pattern. In this case, Brenda is reported to have an half brother (same maternal lineage) to be affected with DMD and no case history in James family. Now considering James as unaffected father and Brenda as a carrier mother, the possibility is that they can have a boy or girl. Boys carry an X and a Y chromosome. The disorder is carried on the X Chromosome, which means that boys have only one copy of the gene available (X gene from the mother). Girls have two copies of the gene (one on each X chromosome). If one gene is affected, girls still have the other gene on the other X-chromosome that can be used to make dystrophin. So presuming that, The boy gets a normal X from mother he would be unaffected Girl gets normal X from mother is unaffected. Girl gets mutated X from mother still unaffected and is a carrier. Boy gets an mutated X from mother –affected. The chances are one in four that this couple will have a child, a boy, with DMD. In general the mutation in the X chromosome causes a premature translation termination( nonsense or frameshift mutation ), about 2/3 of mutation causes deletion of one or many exons in the dystropin gene. Some of this causes lethal condition where there is no production of dystropin while in some is deletion in same gene with less or poor quality protein production as BMD. This protects the muscles of BMD from deteoriating rapidly compared to DMD The DMD manifest between 2 to 6 years and it severely progresses causing weakness in skeletal and voluntary muscle ,by early teens heart and respiratory muscle are affected, where as the BMD is milder with late onset in teens or early adulthood course slower and far less predictable. To further know about the information on risk, the following test and diagnosis can be done on the couple and families before starting the family: This is done by recording the family history. The mother is a confirmed as obligate carrier if the mothers brothers also have DMD. By assay of creatine kinase in the mother. This should ideally be done when the mother is not pregnant as during pregnancy the value of CK tends to be low. At least three estimations of CK should be done on three different days. The person to be tested should not have engaged in exercise or exertion before the test e.g. climbing many stairs etc. The CK is raised in only 2/3 of the carriers. By doing a DNA diagnostic test by either looking for deletions or by linkage studies. DNA analysis shows up the missing exon. Brenda’s mothers any normal brother or Brenda’s any normal brother needs to undergo DNA analysis to identify which is the chromosome of the two that carries the mutated gene Task 2: With the next visit it is known that Brenda’s mother has a brother who was also affected by DMD who died of it. So now we know that two male member in the family are affected individual confirming both Brenda and her mother to be carriers. This can be easily explained by the pedigree analysis. With an unaffected father and carrier mother, since it is a X linked disorder, the chances are Unaffected son , presuming he gets mother’s normal X gene Unaffected daughter normal X gene from mother Carrier daughter as daughters for sure get an normal X gene from father and if gets the mutated gene from mother still is unaffected and is a carrier. Affected son, with Y gene coming from father and mutated X gene from mother-affected by DMD. Here , Brenda’s mother generation (1) one boy affected Brenda’s generation: (2) one boy affected So again there is 50% risk of Brenda having an affected boy or carrier girl. So there is very mild chance of a daughter getting affected- mild degree of muscular dystropy rarely manifesting as a carrier, with 50% chance of being a carrier exhibiting no symptom and 50% risk chance for an affected son. Exception as ,in 1/3 of cases the disease is manifested in offspring’s with no family history due to random, spontaneous mutation owing to the large size of the gene in the mother egg and not in blood cell hence undetectable by normal carrier test . Also in girls , skewed X inactivation is seen where though normal X gene is present, for currently unknown reason, flawed X manifest itself resulting in Mosaic DMD whose effect in long term is under study. With the next visit it is recognized that Brenda is pregnant and I have provided to her the summary about DMD, the prenatal tests, prognosis and treatement available. Task 3 Duchenne muscular dystropy is an inherited disorder , characterized by rapidly worsening muscle, the weakness that starts from the leg and pelvis that latter affects the whole body eventually affecting the heart and lungs resulting in death before teens. The disease is named after Guillaume Benjamin Amand Duchenne, who discovered that the disease is due to mutation in dystropin gene that encodes a essential protein in myocytes affecting 2 among 10,000. DMD mainly affecting boys with girls being the carriers 2/3 is by genetic inheritance from mother while remaining is mutation in egg gene. It is prevalent widely in boys than girls as it is X linked recessive disorder, the defective gene found in X chromosome. Women are generally carrier as they have two X- chromosome so even if they have a defective X chromosome , other gene produces the protein enough to mask any symptoms, but the boys have only one X from other so if it is defective he is affected. So sons of carrier woman have 50% chance to be affected and daughters 50% chance to be carrier. The patho physiology is gene Xp 21 is mutated resulting in deletion of exon required for the production of the protein dystropin that connects muscle fiber to extra cellular matrix, absence of which causes excess calcium influx that results in oxidative stress by production of ROS that eventually leads to death of sarcolemma followed by cell necrosis that results in replacement of muscle fiber by adipose and connective tissue. The main symptom is progressive muscle weakness followed by muscle wasting that starts in the proximal muscle first. Symptoms usually occurs age of 6 ,at times as early as pregnancy. Thus the legs, hip, pelvic, thigh and shoulders are affected in order, calves often enlarged, featuring awkward gait, frequent fall , difficult motor skill, fatigue ,mental retardation, skeletal and muscular deformity and hypertrophy and muscular contracture. Thus muscle wasting, calf muscle enlargement followed by cardio myopathy and respiratory problem may occur. The test include the walking gait -the Gower’s sign, increased Creatine kinase in blood , electromyopathy showing weakness caused by destruction of heart muscle, dna testing, and muscle biopsy to know the presence and level of dystropin. In case of preganant women, the prenatal diagnosis are available .Prenatal testing and aminocentesis for pregnancies at risk in identified family embers suffering with DMD. When one or both parents are 'carriers' , there is a risk that their unborn child will be affected by that condition. So the 'Prenatal tests' are carried out during pregnancy, to try to find out whether the fetus (unborn child) is affected. Different types of prenatal tests can be carried out after about 10 weeks of pregnancy. Chorion villus sampling (CVS) can be done at 10-12 weeks, and amniocentesis at about 14-16 weeks, while placental biopsy and foetal blood sampling can be done at about 18 weeks. In CVS ,a small piece of placental tissue (chorionic villi) the cord by which the baby is attached to the womb is collected. This tissue is derived from the fetus. By studying its DNA one can determine whether the unborn child has DMD or not. The Chorionic villus sampling is done at 10-12 weeks of pregnancy through a catheter, inserted from below and beyond 11 weeks by a catheter inserted through the abdomen. Chorionic villus sampling is processed to extract the DNA. This is examined to see whether there is abnormality of the dystrophin gene. If the fetus is affected no treatment is possible at stage, in that event to avoid delivery of an affected child the option of abortion is available Women and/or couples need to consider carefully which test to have and to discuss this with their genetic counsellor. Earlier testing would allow early termination which would probably be less traumatic for the couple, but it carries a slightly higher risk of miscarriage than later testing (about 2%, as opposed to 0.5%).Genetic counseling is advised if there is a family history of the disorder. Duchenne muscular dystrophy can be detected with about 95% accuracy by genetic studies performed during pregnancy. The commonly used DMD test looks for missing portions of the dystrophin gene, called exons. An exon is the region within a gene that contains important parts of the genetic code. About 60 percent of DMD cases are caused by deletions in one or more exons within the dystrophin gene. The Prognosis is fatal, with the Duchenne muscular dystrophy eventually affecting all voluntary muscles, and the heart and breathing muscles, the Survival is rare beyond the early 30s. Recent advancements in medicine are extending the lives of those afflicted but of less effect in increasing the lifespan. Death typically occurs from respiratory failure or heart disorders. The treatment (till date as no cure is yet identified), is usually to control the symptom as steroids to increase strength and energy, physical activity and exercise, physiotherapy, and orthopedic appliances. In Molecular level as medicine, Stem cell research that had shown some way to replace damaged muscle tissue Researches to up regulate Utropin as a substitute for dystropin Another research as U7 gene transfer technique , a combination of exon skipping and transfer of gene that produce Antisense oligonucleotide (AON) themselves without need to be injected regularly . the AON’s are potential drugs that modify genetic information in such a way that DMD is converted into BMD. This successful in animals is yet to be tested in human. A PTC124 treatment that skips stop instruction and allows complete dystropin production is under research. Recent research that Losartan that reduce hypertension halts disease progression in mice. Protrandim an non nutritional supplement increase antioxidant level. Thus many research are undertaken to alleviate the patients and families misery. The only available prevention is genetic counselling. Appendix: 1. McClorey G, Moulton H, Iversen P, Fletcher S, Wilton S (2006). "Antisense oligonucleotide-induced exon skipping restores dystrophin expression in vitro in a canine model of DMD". Gene Ther 13 (19): 1373-1381. PMID 16724091.  2. McClorey G, Fall A, Moulton H, Iversen P, Rasko J, Ryan M, Fletcher S, Wilton S (2006). "Induced dystrophin exon skipping in human muscle explants". Neuromuscul Disord 16 (9-10): 583-590. PMID 16919955 3. Flanigan KM, von Niederhausern A, Dunn DM, Alder J, Mendell J, Weiss, RB, 2003."Rapid Direct Sequence Analysis of the Dystrophin Gene." American Journal of Human Genetics, , vol. 72, no. 4, pp. 931-939 4. Facts About Duchenne and Becker Muscular Dystrophies (DMD and BMD) Muscular Dystrophy Association http://www.mdausa.org/disease/dmd.html 5. Duchenne muscular dystrophy http://www.nlm.nih.gov/medlineplus/ency/article/000705.htm 6. Molecular Aspect of DMD http://research.i2r.a-star.edu.sg/Templar/DB/Dystrophin/intro.html 7. Sherri Garcia, CDC’s 2005 Science Ambassador Program, Muscular Dystrophy Web Walk Answers http://www.sgrh.com/dept/gene/gene_g.htm Read More