Variability of multisystemic features in myotonic dystrophy type 1--lessons from Serbian registry

Molecular and Clinical Implications of Variant Repeats in Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is one of the most variable monogenic diseases at phenotypic, genetic, and epigenetic level. The disease is multi-systemic with the age at onset ranging from birth to late age. The underlying mutation is an unstable expansion of CTG repeats in the DMPK gene, varying in size from 50 to >1000 repeats. Generally, large expansions are associated with an earlier age at onset. Additionally, the most severe, congenital DM1 form is typically associated with local DNA methylation. Genetic variability of DM1 mutation is further increased by its structural variations due to presence of other repeats (e.g., CCG, CTC, CAG). These variant repeats or repeat interruptions seem to confer an additional level of epigenetic variability since local DNA methylation is frequently associated with variant CCG repeats independently of the expansion size. The effect of repeat interruptions on DM1 molecular pathogenesis is not investigated enough. Studies on patients indicate their stabilizing effect on DMPK expansions because no congenital cases were described in patients with repeat interruptions, and the age at onset is frequently later than expected. Here, we review the clinical relevance of repeat interruptions in DM1 and genetic and epigenetic characteristics of interrupted DMPK expansions based on patient studies.

DM1 Phenotype Variability and Triplet Repeat Instability: Challenges in the Development of New Therapies

Myotonic dystrophy type 1 (DM1) is a complex neuromuscular disease caused by an unstable cytosine thymine guanine (CTG) repeat expansion in the DMPK gene. This disease is characterized by high clinical and genetic variability, leading to some difficulties in the diagnosis and prognosis of DM1. Better understanding the origin of this variability is important for developing new challenging therapies and, in particular, for progressing on the path of personalized treatments. Here, we reviewed CTG triplet repeat instability and its modifiers as an important source of phenotypic variability in patients with DM1.

Body composition analysis in patients with myotonic dystrophy types 1 and 2

INTRODUCTION

To date, there are only several reports on body composition in myotonic dystrophy type 1 (DM1) and there are no data for myotonic dystrophy type 2 (DM2). The aim was to analyze body composition of patients with DM1 and DM2, and its association with socio-demographic and clinical features of the diseases.

METHODS

There were no statistical differences in sociodemographic features between 20 DM1 patients and 12 DM2 patients. Body composition was assessed by DEXA (dual-energy x-ray absorptiometry). A three-compartment model was used: bone mineral content (BMC), fat mass (FM), and lean tissue mass (LTM).

RESULTS

Patients with DM1 and DM2 had similar total body mass (TBM), BMC, FM, and LTM. Patients with DM1 had higher trunk-limb fat index (TLFI) in comparison to DM2 patients which indicates visceral fat deposition in DM1 (1.16 ± 0.32 for DM1 vs. 0.87 ± 0.23 for DM2, p < 0.05). Right ribs bone mineral density was lower in DM2 group (0.68 ± 0.07 g/cm² vs. 0.61 ± 0.09 g/cm², p < 0.05). Higher percentage of FM in legs showed correlation with lower strength of the upper leg muscles in DM1 (ρ = - 0.47, p < 0.05). Higher muscle strength in DM2 patients was in correlation with higher bone mineral density (ρ = + 0.62, p < 0.05 for upper arm muscles, ρ = + 0.87, p < 0.01 for lower arm muscles, ρ = + 0.72, p < 0.05 for lower leg muscles).

CONCLUSION

DM1 patients had visceral obesity, and percentage of FM correlated with a degree of muscle weakness in upper legs. In DM2 patients, degree of muscle weakness was in correlation with higher FM index and lower bone mineral density.

Prospective measurement of quality of life in myotonic dystrophy type 1

INTRODUCTION

Generic patient reported outcome measures have had varied success in tracking QoL in myotonic dystrophy type 1 (DM1).

AIM

To analyze changes of Individualized Neuromuscular Quality of Life questionnaire (INQoL) scores in clinic patients with DM1 over a 6-year period.

METHOD

Patients completed the INQoL at baseline and after a 6-year period through their attendance in a neurology outpatient clinic. Severity of muscular involvement in DM1 was analyzed using the Muscular Impairment Rating Scale (MIRS).

RESULTS

Ninety-nine DM1 patients completed a baseline visit. Sixty-seven of these patients were retested at an interval time. The overall INQoL score improved in our sample of patients (P<.05) as did the following subscales: myotonia (P<.05), pain (P<.05), activities (P<.01), social relationships (P<.01), and body image (P<.05). No changes were observed for the independence and emotions scales. There were no differences in mean change of INQoL scores between patients with worsened MIRS and those with no change in MIRS scale after follow-up (P>.05).

CONCLUSION

Individualized Neuromuscular Quality of Life questionnaire scores improved in our cohort of DM1 patients during a 6-year period. INQoL score did not correlate with progression of muscle weakness. This must be better understood before the selection of the instrument for use in trials to measure therapeutic benefit in DM1 patients.

Magnetic resonance imaging of leg muscles in patients with myotonic dystrophies

Magnetic resonance imaging (MRI) of muscles has recently become a significant diagnostic procedure in neuromuscular disorders. There is a lack of muscle MRI studies in patients with myotonic dystrophy type 1 (DM1), especially type 2 (DM2). To analyze fatty infiltration of leg muscles, using 3.0 T MRI in patients with genetically confirmed DM1 and DM2 with different disease durations. The study comprised 21 DM1 and 10 DM2 adult patients. Muscle MRI was performed in axial plane of the lower limbs using T1-weighted (T1w) sequence. Six-point scale by Mercuri et al. was used. Fatty infiltration registered in at least one muscle of lower extremities was found in 71% of DM1 and 40% of DM2 patients. In DM1 patients, early involvement of the medial head of gastrocnemius and tibialis anterior muscles was observed with later involvement of other lower leg muscles and of anterior and posterior thigh compartments with relative sparing of the rectus femoris. In DM2, majority of patients had normal MRI findings. Early involvement of lower legs and posterior thighs was found in some patients. Less severe involvement of the medial head of the gastrocnemius compared to other lower leg muscles was also observed, while involvement of proximal muscles was rather diffuse than selective. It seems that both in DM1 and DM2 some muscles may be affected before weakness is clinically noted and vice versa. We described characteristic pattern and way of progression of muscle involvement in DM1 and DM2.

The UK Myotonic Dystrophy Patient Registry: facilitating and accelerating clinical research

Myotonic dystrophy type 1 (DM1) is the most frequent muscular dystrophy worldwide with complex, multi-systemic, and progressively worsening symptoms. There is currently no treatment for this inherited disorder and research can be challenging due to the rarity and variability of the disease. The UK Myotonic Dystrophy Patient Registry is a patient self-enrolling online database collecting clinical and genetic information. For this cross-sectional “snapshot” analysis, 556 patients with a confirmed diagnosis of DM1 registered between May 2012 and July 2016 were included. An almost even distribution was seen between genders and a broad range of ages was present from 8 months to 78 years, with the largest proportion between 30 and 59 years. The two most frequent symptoms were fatigue and myotonia, reported by 79 and 78% of patients, respectively. The severity of myotonia correlated with the severity of fatigue as well as mobility impairment, and dysphagia occurred mostly in patients also reporting myotonia. Men reported significantly more frequent severe myotonia, whereas severe fatigue was more frequently reported by women. Cardiac abnormalities were diagnosed in 48% of patients and more than one-third of them needed a cardiac implant. Fifteen percent of patients used a non-invasive ventilation and cataracts were removed in 26% of patients, 65% of which before the age of 50 years. The registry’s primary aim was to facilitate and accelerate clinical research. However, these data also allow us to formulate questions for hypothesis-driven research that may lead to improvements in care and treatment.