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HelpTest Code LAB1386 Myotonic Dystrophy Type 1 Repeat Expansion Analysis
Clinical System Name
Myotonic Dystrophy Type 1 Repeat Expansion Analysis
Synonyms
Myotonic Dystrophy Type 1
DM1
DMPK
Sample Requirements
Preferred Specimen: Whole Blood
Container(s): Lavender Top Tube, Lavender Microtainer
Preferred Vol: 5.0 mL
Minimum Vol: 3.0 mL
Alternative Specimen: DNA
Container(s): Sterile Plastic Tube
Preferred Vol: 15 ug, concentration: 50 ng/ul
Minimum Vol:
Alternative Specimen: Swab
Container(s): Buccal Swab
Preferred Vol: 1 Buccal Swab
Minimum Vol: 1 Buccal Swab
Alternative Specimen: Saliva
Container(s): Saliva Kit
Preferred Vol: 1 Saliva Kit
Minimum Vol: 1 Saliva Kit
Processing Instructions
Reject due to:
Spin: N
Aliquot: N
Temp: RT
Storage location: Affix large Epic labels to tubes and place in room temperature Send Outs rack.
Off-site collection: Send whole blood refrigerated.
Stability
| Specimen Type | Temperature | Time |
|---|---|---|
| Whole Blood | Room temp | 3 d |
| Whole Blood | Refrigerated | 7 d |
| Whole Blood | Frozen | N |
| Extracted DNA | Room temp | 3-4 d |
| Extracted DNA | Refrigerated | 1 y |
| Extracted DNA | Frozen | Indefinitely |
Availability
| STAT | TAT |
|---|---|
| No | 21 days |
Performing Laboratory
Baylor College of Medicine
Baylor Genetics Laboratories
2450 Holcombe - Grand Blvd Dock
Houston, TX 77021-2024
Phone: (800) 411-4363
Department
Department: Send Outs/Genetic
Phone Number: (206) 987-2563
Methodology
Method: PCR-amplification, Southern Blot with densitometry
Analytical Volume:
Limitations:
Reference Range
DMPK alleles with >49 repeats are abnormal; alleles with <35 repeats are normal. individuals with 35-49 repeats (premutation alleles) do not have symptoms but their children are at increased risk of inheriting larger repeats and having symptoms.
Send Out Instructions
| Reference Lab Test Name: | Myotonic Dystrophy Type 1 Repeat Expansion Analysis |
| Reference Lab Test Number: | 6041 |
| Instructions: | Ship at room temperature, avoid freezing sample. Send overnight via FedEx. |
Description
The Myotonic Dystrophy Type 1 Repeat Expansion Analysis is performed to determine the number of CTG repeats in the DMPK gene.
Myotonic dystrophy type 1 is one of a group of muscular dystrophies and is the most common adult muscular dystrophy. The onset of symptoms is variable and ranges from congenital to adulthood. The congenital form typically presents with hypotonia, severe intellectual disability, and potentially lethal respiratory distress. In older individuals, the characteristic findings include progressive weakness, muscle wasting, myotonia, and cataracts; in males, frontal balding and testicular atrophy are common. Premutation carriers of myotonic dystrophy may be at risk for cardiac irregularities.
Unaffected: 5-34 repeats
Premutation carrier: 35-49 repeats
Full mutation, myotonic dystrophy: ≥ 50 repeats
Pregnancies with congenital myotonic dystrophy often show decreased fetal movement, talipes, and severe polyhydramnios on ultrasonography; diagnostic testing for myotonic dystrophy and/or carrier screening for the pregnant individual should be considered (1,2).
References:
1. Zaki M, Boyd PA, Impey L, Roberts A, Chamberlain P. Congenital myotonic dystrophy: prenatal ultrasound findings and pregnancy outcome. Ultrasound Obstet Gynecol. 2007 Mar;29(3):284-8. doi: 10.1002/uog.3859. PMID: 17238150.
2. Yee C, Choi SJ, Oh SY, Ki CS, Roh CR, Kim JH. Clinical characteristics of pregnancies complicated by congenital myotonic dystrophy. Obstet Gynecol Sci. 2017 Jul;60(4):323-328. doi: 10.5468/ogs.2017.60.4.323. Epub 2017 Jul 14. PMID: 28791262; PMCID: PMC5547078.
Clinical Utility
Indications for testing include:
- Confirm a clinical diagnosis of myotonic dystrophy
- Evaluate suspected myotonic dystrophy
- Presymptomatic testing of adults at-risk for myotonic dystrophy (in conjunction with genetic counseling)
- Prenatal diagnosis of an at-risk pregnancy
- Differential diagnosis of neonates with unexplained hypotonia and/or respiratory distress