Case 7 - myotonic dystrophy type 1

Question 1

epidemiology

Answer 1

• Most frequent dystrophy in adults
• Multisystemic disorder
• Prevalence 1/2100

Question 2

pathophysiology

Answer 2

• DM1 is caused by expansion of cytosine-thymine-guanine (CTG) repeat in the 3’-untranslated region of the DM1 protein kinase (DMPK) gene on chromosome 19q13.3
  o More repeats correlating with more severe disease
  o Those with less than 35 repeats are considered normal, and those that manifest with clinical symptoms typically have greater than 50.
  o Maternal transmissions will often result in greater CTG expansion.
  o Age-dependent growth of expansion occurs

Question 3

pathogenic mechanism

Answer 3

• The main pathogenic mechanism of this disease involves the RNA transcribed from the expanded allele containing long tracts of (CUG)
• The RNA results in a toxic effect through two RNA-binding proteins: MBNL1 (muscleblind-like 1) and CUGBP1 (CUG-binding protein 1).
  o MBNL1 is sequestered on CUG repeat-containing RNA resulting in its loss-of-function
  o CUGBP1 is up-regulated through a signalling pathway.
• The downstream effects include disrupted regulation of alternative splicing, mRNA translation and mRNA stability, which contribute to the multiple features of DM1.
• Other pathogenic mechanisms include reduced DMPK protein synthesis (haploinsufficiency) and reduced transcription of neighbouring genes

Question 4

symptoms

Answer 4

• Symptoms of myotonic dystrophy might include difficulty releasing one’s grip (myotonia), weakness of muscles in the hands and feet, difficulty swallowing and abnormal heart rhythms.
• Non-muscle symptoms may also include learning difficulties, daytime sleepiness, infertility and early cataracts.

Question 5

genetics

Answer 5

• Autosomal dominant hereditary repeat mutation of DMPK gene
• In DM1, repeat mutations are dynamic gene defects and show instability with variation in different tissue and cell types causing somatic mosaicism
• The size of the CTG repeat appears to increase over time
• Children may inherit repeat lengths considerably longer than those present in the transmitting parent.
  o This phenomenon is known as genetic anticipation
  o Germ-cell instability is possibly the major determining factor underlying the pronounced anticipation in DM1

Question 6

diagnosis

Answer 6

• The diagnosis of DM1 is suspected in individuals with characteristic muscle weakness and is confirmed by molecular genetic testing of DMPK
• Two-step molecular diagnostic procedure is used in DM1 genetic testing.
  o The first step is to analyse whether an individual is heterozygous for alleles within normal size range by using PCR and fragment-length analysis.
  o If only one normal allele is detected triplet-repeat primed PCR (TP-PCR) will be done
• TP-PCR is based on the use of locus-specific PCR primers in combination with a primer designed across the repeated sequence
  o After PCR and fragment analysis, products of different sizes are visible as continuous ladder with a 3-base-pair periodicity.
  o In the presence of DM1 expansion, a continuous ladder exceeds the normal size range

Question 7

treatment

Answer 7

• Antisense oligonucleotides (ASO) are single strands of DNA or RNA that are complementary to a chosen sequence.
• In the case of antisense RNA they prevent protein translation of certain messenger RNA strands by binding to them, in a process called hybridization.
  o Antisense oligonucleotides can be used to target a specific, complementary (coding or non-coding) RNA.
  o If binding takes place this hybrid can be degraded by the enzyme RNase H.
   RNase H is an enzyme that hydrolyses RNA, and when used in an antisense oligonucleotide application results in 80-95% down-regulation of mRNA expression
• Targeting DMPK mRNA abolished CUG-expanded foci, enabled nuclear redistribution of MBNL1/2, and corrected aberrant splicing