Learning Objectives:
- Understand the pathophysiology of DMD
- Determine the sequence of genetic testing for carrier status and prenatal diagnosis of DMD
- Understand the usefulness of CK levels as a screening tool
- Define “nondirective” for genetic counseling use
- Be able to generate a pedigree and recognize X linked recessive inheritance patterns
- Be able to answer the specific questions following the case
Pretest Questions:
1. Which statement best describes the mutation causing Duchenne Muscular Dystrophy?
- It is usually a single base pair substitution of valine for glutamic acid at the sixth position of the dystrophin gene.
- DMD can be caused by mutations in any number of scarolemma scaffolding proteins.
- DMD is usually caused by a large, out-of-frame deletion on the dystrophin gene.
- In order for DMD to manifest in disease, both copies of the dystrophin gene on chromosome 6 must have a mutation.
2. Choose the correct statement
- Young males with DMD frequently walk well into their late teens.
- Early diagnosis of DMD is crucial because there is a strict standard of care for patients with DMD that must begin as early as possible for best results.
- A high serum level of Creatine Kinase is diagnostic for DMD in a young boy.
- Use of steroids has been show to increase function in males with DMD.
3. Which of the following techniques can be used at the earliest gestational age to gather DNA from a fetus?
- Ultrasound
- Anmniocentesis
- Chorionic Villus Sampling
- Percutaneous Umbillical Blood Sampling
4. Choose the correct statement.
- Practitioners providing genetic counseling therapies should provide patients with education and facts, not definitive recommendations for reproductive choices.
- In the Washignton, DC area, a female desiring carrier testing for DMD rarely (< 25% of the time) has DNA available from her affected male relative for mutation testing.
- Linkage analysis is less invasive and more accurate than traditional molecular diagnostic testing and should be the first choice for testing to determining carrier status.
- DMD predominantly affects boys of Asian descent.
5. Choose the false statement:
- • X linked disorders manifest themselves in multiple generations on a pedigree
- • Autosomal Dominant inheritance patterns typically skip generations while it is unusual for an X linked recessive disorders to skip a generation in a pedigree.
- • Females are very rarely affected with X linked disorders
- • On a pedigree, the proband is denoted by an arrow
Case Study :
The C's present to the MFA Ob-Gyn Clinic to discuss pre-conception genetic counseling. Mr. C is a 32 yr old with no significant family history of genetic disorders in his family. His wife, Dr. C, is a 27yr old with a 17 yr old brother with Duchenne Muscular Dystrophy. They want to know the risks that their children could be affected with the disease.
Family History :
Dr. C is the oldest of five children, four girls and one boy (who is affected with DMD). Her mother had one brother, who also had DMD and died at 20 years of age. No other members of her extended family have DMD.
Questions:
What is the inheritance pattern of DMD?
What is the pathophysiology of DMD?
What is the significance of Creatine Kinase in affected males? In carrier females?
What genetic testing could you offer this couple?
What is the significance of a maternal uncle with the disease? How would genetic testing change if there was no other incidence of DMD in Dr. C's family?
Draw a pedigree of Dr. C's family.
Dr. C and her husband come back to your office to hear the results of her carrier testing and tells you that to her surprise she is 11 weeks pregnant. During the visit, you tell her that she is a carrier of the same dystrophin mutation as her affected brother. She tells you that she wants to know the mutation status of her fetus.
Questions :
What prenatal testing is available for this fetus?
What prenatal diagnosis would you perform first?
What does “nondirective” mean in the context of genetic counseling?
What web based resources and patient support groups are available for this patient?
For Discussion : Knowing that Dr. C's brother is living with DMD, how would you discuss her reproductive options if her fetus carried a DMD mutation?
Hyperlinks:
OMIM
genetest
www.mdausa.org
www.parentprojectmd.org
Reference articles:
Tantravahi, U. Molecular Genetics Testing for Prenatal Diagnosis . Clinics in Laboratory Medicine 23:2. June 2003. www.mdconsult.com
Matthews, KD. Muscular Dystrophy Overview: genetics and diagnosis. Neurology Clinics: 21(4) November 2003 . www.mdconsult.com
Questions:
What is the inheritance pattern of DMD?
DMD is an X linked recessive disorder. As with all XLR disorders, it is usually diagnosed in males, and rarely manifests clinically in females. Females are usually carriers of the mutation, which they can pass along to their offspring. 50% of the males born to a female carrier will have DMD, and 50% of males will be unaffected. Similarly, any female born to a carrier has a 50% chance of being a carrier herself.
Unlike some genetic diseases like Sickle Cell Anemia, DMD is caused by many different mutations. Thus, there is no single site on the Dystrophin gene that molecular diagnostic labs can look at to determine if an individual carries a mutation. This makes testing for DMD mutations more extensive than that for Sickle Cell Anemia.
What is the pathophysiology of DMD?
DMD is caused by a mutation in the dystrophin gene -- usually a large, out-of-frame deletion. Dystrophin is a structural protein associated with the support scaffolding of plasma membrane in muscle cells. Absent dystrophin causes a weakening of muscles, particularly skeletal and cardiac muscle. With contractions, the muscle cells burst, leaking their contents to the interstitium. Accordingly, the muscles respond with so-called “pseudo-hypertrophy” to maintain muscle mass.
Clinically, DMD typically manifests between 3-4 years of age with a positive Gower's sign and pseudohypertrophy of the calves. DMD is a progressive neuropathy. Males affected with DMD typically utilize a wheelchair full time by middle school, and continue to deteriorate physically until their demise, which usually occurs in their early twenties due to cardiac or respiratory failure.
Beckers Muscular Dystrophy (BMD) is a less sever variant of DMD. In these cases, the mutation usually involves an in frame deletion or insertion that codes for a dystrophin protein with some functional capacity. These men typically present later in childhood and have a slower, less severe progression of disease.
Currently, there is no cure for DMD or BMD. Current studies indicate that corticosteroid use can increase functional ability in males affected with DMD. However, the side effects associated with steroid use frequently dissuade physicians and patients from using this medication. In addition to steroids, current therapies are aimed at maintaining functionability for as long as possible in males with DMD by using wheel chairs, respirators, and pharmacotherapies to keep heart and lung function as high as possible.
What is the significance of Creatine Kinase in affected males? In carrier females?
CK levels are usually extremely high in young men affected with DMD – usually 50-100 times normal. This is due to leakage of muscle enzymes into the interstitum when weakened sarcolemmas burst. Definitive diagnosis is made via muscle biopsy or genetic testing. Young women with elevated CKs have a presumptive diagnosis as a DMD carrier. While these young women are functionally normal and have no abnormal biopsy results, approximately 50% of their muscle cells will have abnormal dystrophin in childhood. By the time they are young adults, theses muscle cells will have died, and the remaining functional 50% will remain. Thus, it is only during childhood that CK levels in carrier females will be elevated. Of note, some carrier females will not have grossly elevated CK levels. Therefore, a low CK test result lowers, but does not eliminate, risk for carrier status in females.
What genetic testing could you offer this couple?
Testing carrier status in Dr. C is the appropriate action at this time. DMD is definitively diagnosed with a muscle biopsy. Thus, the specific mutation affecting each individual is typically known. If her brother's mutation is known, PCR evaluation or DNA sequencing testing is an option. If the specific mutation has not been worked out but a biopsy is still available, the first step would be to determine the mutation in her brother, then screen Dr. C's DNA for the same mutation.
Sometimes a patient in Dr. C's situation wants to determine carrier status after her affected family member has passed away without a muscle biopsy available. In this instance, linkage analysis can be used to determine her risk of carrier status.
What is the significance of a maternal uncle with the disease? How would genetic testing change if there was no other incidence of DMD in Dr. C's family?
A living maternal uncle makes it overwhelmingly likely that Dr. C's mother is a carrier of the disease. If her brother were the first and only incidence of DMD in this pedigree, things get trickier. Haldane's rule states that 1/3 of XLR mutations arise spontaneously. Thus, it is imperative to establish where the mutation began. If the first incidence of mutation were in Dr. C's brother's DNA only, her risk for carrier status is that of the normal population. However, if the mutation first emerged in the P1 generation, with Dr. C's mother as a carrier, Dr. C has a 50% chance of being a carrier. Finally, even a negative test result for carrier status in the P1 female does not reduce Dr. C's risk to that of the normal population. Gonadal mosiacism is a phenomenon where a population of ova in Dr. C's mother's ovaries all carry the same dystrophin mutation. This occurs 15% of the time. In these instances, Dr. C has greater than population risk for being a carrier of this mutation.
Draw a pedigree of Dr. C's family
Please see last page.
Questions:
What prenatal test would you perform first?
First, it is important to establish the sex of the fetus. If the fetus is female, she has a vanishingly small chance of manifesting DMD. If the fetus is male, things get more interesting (see below)
What prenatal testing is available for this fetus?
Fetal DNA testing is the best option here. Retrieval of fetal DNA may come from an amniocentesis, PUBS, or CVS. Each of these three tests has a different time frame for when during gestation they may be performed. Chorionic Villus Sampling (CVS) can occur between 10-12 weeks gestational age. Amniocentesis can occur after 15-16 weeks gestational age, and Percutaneous Umbilical Cord Sampling can occur after 18 weeks gestational age. Once the fetal cells have been cultured, they maybe tested via sequencing or PCR or blotting techniques to determine if the fetus has a dystrophin mutation.
Would it be appropriate to determine carrier status if the fetus were a female? What about after the baby girl is born? When would you recommend carrier testing for Dr. C's daughter?
No and no. Current genetic testing guidelines for carrier status testing indicate that these tests should be performed when the female is considering her own reproductive choices.
What does “nondirective” mean in the context of genetic counseling?
Genetic counseling involves presenting patients with facts, educating them about risks of procedures and mutation/disease status, and allowing the patient to make an informed choice. Nondirective methods mean that genetic counselors do not make recommendations.
What web based resources and patient support groups are available for this patient?
The Muscular Dystrophy Association has a website with easy to understand explanations of DMD as well as BMD: www.mdausa.org This resource contains online chats, information about current clinical trials, as well as contact information about specialized neurology clinics for adults and children at local sites across the country. MDA provides funding for clinic visits, wheelchairs, local support groups and other services for patients with neuromuscular disease. In the Washington , DC area, these health care service representatives can be reached at washingtondcservices@mdausa.org
In addition, Parent Project Muscular Dystrophy has information about DMD and BMD, as well as a more research focused website at www.parentprojectmd.org .
Pedigree:
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