Recent studies suggest that the molecular basis of congenital myotonic dystrophy (CDM) differs from that of myotonic dystrophy (DM) type 1 (DM1). Epigenetic changes upstream of the DMPK locus appear to be a co-requirement, along with a threshold repeat expansion length, as a trigger for CDM. Yet, the basis for the considerable phenotypic differences between DM1 and CDM, downstream of genotypes, is poorly understood.

Understanding the divergence of the CDM and DM1 phenotypes may be found in the timing of the critical molecular events—while DM1 is driven by MBNL depletion and reversion to developmentally-regulated alternative splicing events, the severe phenotype of CDM may be linked to disruption of prenatal transitions in alternative splicing essential to normal muscle tissue development. However, little information has been available to support that hypothesis.

Thomas and colleagues (University of Florida and Osaka University Graduate School of Medicine) tested the hypothesis that prenatal depletion of MBNL and disruption of RNA alternative processing pathways critical to myogenesis (and likely other tissue-specific events) explains the severity of CDM. An MDF fellow, Łukasz Sznajder, contributed to this work.

These investigators utilized RNAseq to compare pre-mRNA processing in skeletal muscle biopsies of CDM, DM1, and individuals carrying DM1 pre-mutations. Their data show that alternative splicing events were highly conserved between DM1 and CDM, but consistently showed greater severity in CDM. Similarly, polyAseq identified a pattern of alternative polyadenylation in CDM samples that was similar to DM1, but also more severe.

Working from the model that in utero alternative splicing contributes to the severity of CDM, the team used existing RNAseq data sets to conduct in silico evaluations of RNA processing during in vitro differentiation of human primary myoblasts. They found that RNAs relevant to CDM showed prenatal isoform transitions that were predicted by the models of in utero consequences of expanded CUG repeats.

To extend their in silico findings, the investigators tested (a) the role MBNL plays in regulating RNA processing during myogenesis and (b) the linkage between RNA processing defects and CDM-like phenotypes using double (Mbnl1, Mbnl2) and triple MBNL (Mbnl1, Mbnl2, Mbnl3) knockout mice. In aggregate, these studies showed that double knockout mice developed a severe splicopathy and congenital myopathy, while data from the triple knockout suggests that Mbnl1 and Mbnl2 loss represents the primary cause of the spliceopathy, but the deletion of Mbnl3 is responsible for more subtle alterations in hundreds of additional splicing events. Both models also showed dramatic changes in gene expression profiles (particularly in stress-related pathways that have been linked to CDM), with, again, greater severity in the triple knockout. 

Taken together, these studies provide important insights into how molecular pathogeneic mechanisms may distinguish CDM and DM1, specifically that the breadth and timing of expanded CUG repeat toxicity and the resulting RNA processing defects contribute to the severity of CDM. Splicing changes in RNAs essential for the development of skeletal muscle were shown to be both MBNL-dependent and to occur in utero, and thus were linked to perturbations of myogenesis and the ensuing congenital myopathy. The novel mouse models developed here provide an important framework for future mechanistic studies to understand the divergence of CDM and DM1 phenotypes and to inform therapy development strategies.

This peer-reviewed research article was accompanied by an editorial by Drs. Jagannathan and Bradley, appearing in the same issue of the journal. This editorial is also referenced below.

References:

Disrupted prenatal RNA processing and myogenesis in congenital myotonic dystrophy.
Thomas JD, Sznajder ŁJ, Bardhi O, Aslam FN, Anastasiadis ZP, Scotti MM, Nishino I, Nakamori M, Wang ET, Swanson MS.
Genes Dev. 2017 Jul 11. doi: 10.1101/gad.300590.117. [Epub ahead of print]

Congenital myotonic dystrophy-an RNA-mediated disease across a developmental continuum.
Jagannathan S, Bradley RK.
Genes Dev. 2017 Jun 1;31(11):1067-1068. doi: 10.1101/gad.302893.117.

Inheritance of congenital myotonic dystrophy (CDM) is almost exclusively maternal and, while typically associated with large CTG expansions, is not always genetically differentiated from myotonic dystrophy type 1 (DM1) by repeat tract length. Correlations between CDM/DM1 genotype and phenotype can be improved through evaluation of somatic expansions. Yet it is clear that factors other than germ line repeat length underlie the bias toward maternal inheritance and the heterogeneity of CDM.

The laboratories of Drs. Karen Sermon (Vrije Universiteit Brussel) and Chris Pearson (Hospital for Sick Children) recently collaborated on an epigenetic analysis of the DM1 genetic locus in a cohort of DM1 and CDM patients. Prior reports showed that the DM1 locus resides in a 3.5 kB CpG island with putative CTCF sites, suggesting an epigenetic mechanism for DM1 regulation and disease phenotypes that diverge from CTG length assessments. Earlier reports also established variability in methylation status at that locus in both DM1 patients and DM1 transgenic mice. Drs. Sermon and Pearson hypothesized that CTG expansion might alter CpG methylation status and that a consequent regulatory dysfunction contributes to the severity of the CDM phenotype.

Drs. Sermon and Pearson and team evaluated multiple generations of several families, including 20 individuals with CDM. Results showed nearly an absolute correlation between the methylation status upstream of the expanded CTG repeat and the occurrence of CDM (19/20 cases). By contrast, this pattern of methylation was rarely found among DM1 patients (2/59 cases). The authors suggest that CpG site methylation is an important contributing factor, with the development of CDM not being determined by CTG repeat length alone.

Analysis of human embryonic stem cells (hESC) and chorionic villus samples from the study cohort identified upstream CpG site methylation only in maternally-derived samples; paternal samples never showed methylation upstream of expanded DMPK alleles.

Generational increases in both methylation and CTG expansion length were seen in each CDM family studied. Yet since CTG repeat lengths overlapped in DM1 and CDM, while upstream methylation was almost exclusive to CDM, the authors concluded that methylation status is a stronger indicator of CDM than absolute repeat length. Moreover, they speculate that the maternal inheritance bias of CDM may be a consequence of a failed survival of spermatogonia carrying the pathogenic methylation upstream of DMPK. Importantly, while their data suggests that it is rare, the authors do not exclude paternal inheritance for CDM.

Reference:

CpG Methylation, a Parent-of-Origin Effect for Maternal-Biased Transmission of Congenital Myotonic Dystrophy.
Barbé L, Lanni S, López-Castel A, Franck S, Spits C, Keymolen K, Seneca S, Tomé S, Miron I, Letourneau J, Liang M, Choufani S, Weksberg R, Wilson MD, Sedlacek Z, Gagnon C, Musova Z, Chitayat D, Shannon P, Mathieu J, Sermon K, Pearson CE.
Am J Hum Genet. 2017 Mar 2;100(3):488-505. doi: 10.1016/j.ajhg.2017.01.033.

MDF is pleased to announce that Dr. Melissa (“Missy”) Dixon, a Research Associate in the Deptartment of Neurology at the University of Utah, has been awarded a 2016-2017 postdoctoral fellowship.

Dr. Dixon’s research proposal is titled “Evaluation of Functional Connectivity as a Brain Biomarker in Congenital Myotonic Dystrophy.” In this study, she and her colleagues will use magnetic resonance imaging (MRI) to evaluate connectivity networks in the brains of children with congenital-onset myotonic dystrophy (CDM) to see if they differ from those of children without CDM, whether they change over a one-year time period, and whether the MRI results correlate with data from neuropsychological testing.

Dr. Dixon has an extensive background in clinical psychology, neuropsychological testing and clinical trial coordination. She received her doctorate in counseling psychology from the University of Utah in December 2015. We recently talked with Dr. Dixon to learn more.

MDF: There was a time when most children with CDM didn’t survive very long. Do they have a better prognosis now?

MD: Oh, absolutely. I would say that kids with this disorder have a better prognosis than they did a decade ago. We now know that respiratory and feeding problems can be life-threatening, and we’re better equipped to work with those issues from the start.

MDF: What is known so far about the neuropsychology and the brain abnormalities in children with congenital-onset myotonic dystrophy?

MD: Networks in the brain are kind of like a highway system. You can get from Illinois to Colorado by taking Interstate 80, but if there’s a block in that road or a piece of the road that’s missing, you have to take a different route to go around it. 

I don’t know if there are fewer “interstates” in the brains of children with CDM compared to those of children without CDM, but it may be that they’re using more roundabout pathways for getting from one place to another. I think the networks are different [from those in unaffected children.]  

People have used resting-state functional MRI [fMRI] during the resting state [without an attentional focus] to look at brain connectivity in kids who have autism, and they’ve found that it’s sensitive enough to show that there are differences in their connectivity networks. 

Earlier DM studies relied on structural imaging techniques. These can demonstrate a wide range of changes, but they’re not well correlated with clinical outcomes, such as IQ.

We think that by using fMRI we’ll be able to look at connectivity differences in these brain networks and see if they change over time in kids with CDM. 

MDF: Will this study be helpful in telling parents what to expect as their child matures?

MD: We’re hoping to be able to demonstrate changes over time by looking at blood flow in the brain using resting-state fMRI, at baseline and then at a year from baseline.

We’ll be tracking neuropsychological measures, such as executive function and IQ assessment. We’ll also look at adaptive behavior, at how a child is functioning, through a questionnaire that a parent or caregiver will fill out.  At the completion of the study, we would hope to tell parents where a child may have the most learning difficulty, and design interventions to approach those learning difficulties.  

MDF: If you do see abnormalities in connectivity in the brain, what are the possible implications?

MD: We know that cognition is impacted in CDM, but there is not a very sensitive way to see how this changes during the course of a short period of time, like during a drug trial. This technique could become an endpoint for a clinical trial to test the effect of a potential drug or therapy.

MDF: Is it possible that something like, say, DMPKRx, which is being developed by Ionis Pharmaceuticals, could have an effect on the brain, if it could be made to cross the blood-brain barrier?

MD: If not that particular therapeutic, then perhaps another, could potentially slow or halt the progression of the brain changes in this disease as we learn more about them.

MDF: Can you say more about the fMRI study?

MD: We’ll be enrolling 20 participants with CDM, ages 7 to 14, and we already have a control group for comparison. We’re not recruiting yet for the fMRI study; we’re still at the IRB [institutional review board] stage, applying for study approval. We hope to start recruiting in June 2016, and we’ll be posting that on the MDF site when we do. [Studies are listed at the MDF Study and Trial Resource Center under the Current Studies and Trials tab.]

We do, however, have an ongoing study of the natural history of CDM here at the University of Utah, and we hope to recruit some participants for the fMRI study from that group. [The natural history study, which is open, is Health Endpoints and Longitudinal Progression in Congenital Myotonic Dystrophy. Read more about it at the MDF Study and Trial Resource Center under Current Studies and Trials.]

We’ve done neuropsychological testing with the children in the natural history study. The children are all different, and that’s what makes CDM so interesting. The cognitive piece is fascinating. That profile for them is definitely varied, and I think that some of the measures that we’ve been using are just not sensitive enough to understand what’s going on.

I chose 7-year-olds as the lower cut-off age, because they have to stay in the fMRI scanner for 20 to 30 minutes. That can be difficult or scary for any child, and children with CDM sometimes have sensory issues. They’ll have something that’s like a fish tank for them to watch during the scan. It’s not like a movie, where they’d be actively thinking about things, but they’ll be looking at something.

MDF: You have a broad background in clinical psychology. What led you to study children with myotonic dystrophy?

MD: In 2013, when [neuromuscular disease specialist] Dr. Nicholas Johnson came here from the University of Rochester, I became interested in people who have myotonic dystrophy, particularly congenital myotonic dystrophy. I’m interested in understanding the neuropsychological differences in this population. There isn’t a whole lot of known information about congenital myotonic dystrophy and neuropsychological function.

MDF: Did you know that another MDF research fellow, Dr. Ian DeVolder, is doing a study of fMRI in adults with type 1 DM?

MD: Yes, I saw that. That was great to see that someone is doing something very similar in adults. I’m sure our paths will cross as we move forward in our careers. Hopefully, both of our efforts will better define the neuropsychological dysfunction throughout the life spectrum.

Community members discuss daily living strategies for motivating their adult children living with juvenile-onset DM1.

Please note: This is an audio recording only. To view the accompanying presentation, please click here.

Community members Penni Warford, Sarah Clarke and Ann Spaulding discuss best practices for planning and implementing your child's Individualized Education Plan.

This is an audio recording only. To see the handout that accompanied this presentation, please click here.

Dr. Craig Campbell, MD, of Western University in Ontario, Canada, has a discussion with audience members about congenital DM. In the video, Dr. Campbell does not follow a set slide show, but he created slides for additional viewing information. Please click here to see his slides.

Community-led session led by Sarah Berman, Erica Kelly, and Catherine Wycoff, DPT, GCFP, ABMCP. Parents of children living with DM and a hippotherapy specialist discuss the benefit that this type of therapy can often have.

View related web resources from this presentation

The Myotonic Dystrophy Family Registry currently has over 1,900 participants, making it one of the largest and most up-to-date myotonic dystrophy (DM) registries in the world. If you’ve been diagnosed with DM1 or DM2, including congenital or juvenile onset, or are the primary caregiver for some who has, and you haven’t already joined the Registry, we need you!

By participating in the Registry you can help researchers from industry and academia identify potential clinical trial participants and research study subjects, and increase understanding of the impact and complexity of this disease.

The Registry is patient-driven, which means you’re in charge of your information. You can opt out of the Registry at any time, and you can also visit the Registry website to review de-identified (anonymous) data and information the same way that registered researchers do. Your individual information is kept completely confidential. Data in patient registries is typically considered out of date and less useful if it is not updated at least every 18 months, so we’ll remind you to log back into the Registry to review and update your survey occasionally.

Click here to go to the Registry website, read and sign the consent form, and get started.

Questions? Contact the Registry Coordinator for more information.

08/20/2014