Table 1.

Summary of Molecular Genetic Testing Used in Myotonic Dystrophy Type 2

Gene 1Test MethodMutations Detected 2Mutation Detection Frequency by Gene and Test Method 3
CNBPTargeted mutation analysis 4CCTG tetranucleotide repeat expansion in intron 199% 5
Sequence analysis 6Sequence variantsNo reported cases 7
1.
2.

See Molecular Genetics for information on allelic variants.

3.

The ability of the test method used to detect a mutation that is present in the indicated gene.

4.

May involve:
(a) Routine PCR, which detects normal-sized alleles but not abnormal-sized alleles because it cannot amplify across the expansion;
(b) Southern blot analysis, which detects approximately 80% of expansions;
(c) PCR repeat-primed assay, which aids in the detection of the CCTG repeat expansion. This assay, in which the primers are adjacent to and within the elongated CCTG repeat, differentially detects expanded alleles as a smear with varying repeat sizes but shows control alleles as a discrete band. The PCR repeat-primed assay products are probed with an internal probe to assure the necessary specificity.

5.

Detection frequency varies by method used. When routine PCR analysis, Southern blot analysis, and PCR repeat-primed assay are all used, the mutation detection frequency is greater than 99%.

6.

Examples of mutations detected by sequence analysis may include small intragenic deletions/insertions and missense, nonsense, and splice site mutations; typically, exonic or whole-gene deletions/duplications are not detected. For issues to consider in interpretation of sequence analysis results, click here.

7.

Because of variation in the number of flanking TG and TCTG repeats and variation in the number of interruptions within the CCTG repeat tract, the precise number of CCTGs can only be determined by sequence analysis. Although sequencing of some expanded alleles was necessary to demonstrate that only the CCTG portion of the complex repeat expands in affected individuals, sequencing to determine the specific CCTG repeat length is not useful for diagnostic purposes because most expansions are too long for efficient sequence analysis.

From: Myotonic Dystrophy Type 2

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