Cognitive impairment is a characteristic feature of adult-onset myotonic dystrophy type 1 (DM1). While some progress has been made in understanding the molecular mechanisms underlying the CNS changes, it has been more difficult to assess DM1-associated cognitive deficits. Barriers to clarifying the patient cognitive profile include disease heterogeneity, difficulties in assembling a sufficiently powered cohort, uncertainty about optimal endpoint selection, and the little information available on the temporal window required to detect meaningful change. Key unresolved questions in DM1 include (a) is the cognitive impairment stable or progressive and (b) if progressive, is impairment restricted to specific functional domains or is there a more global pattern of decline?

The prominence of neuromuscular symptoms, and the accessibility of muscle for molecular, cellular, and functional evaluations, has focused DM1 therapy development on skeletal muscle endpoints. Yet pharmaceutical and biotechnology companies are now recognizing that cognitive changes represent substantial burden of disease, and thus are essential endpoints for patient-focused drug development. Just as was the case for skeletal muscle-focused efforts, a better understanding of the nature, variability, and progression of cognitive impairment in DM1 is needed to facilitate clinical assessment of CNS-targeted candidate therapies and improved patient care.

Cognitive Longitudinal Study

Dr. Benjamin Gallais, a Wyck Foundation/MDF Postdoctoral Research Fellowship recipient, and his colleagues at the University of Sherbrooke and University of Quebec at Chicoutimi have reported a 9-year longitudinal natural history study of cognitive function in a cohort of genetically confirmed, adult- (90 subjects) and late-onset (25 subjects) DM1 patients. The study assessed a battery of neuropsychological tests, including cognition (language, memory, visual attention, processing speed, visuoconstructive abilities, and executive functions) and intellect (WAIS-R 7) at two time points. Careful steps were taken to avoid or minimize unintended bias from subject selection, subjects lost to follow-up, and data collection and analysis.

The overall group of adult- and late-onset DM1 subjects showed major (significant in ≥ 2/3 of subjects) declines over the study period in verbal memory, visual attention, and processing speed, while improvement was noted in verbal fluencies and global intelligence. In addition, the percentage of all subjects with cognitive impairment increased between the initial and final evaluations. Both age and duration of disease at time of study entry negatively correlated with all cognitive domains studied. By contrast, the number of CTG repeats, degree of skeletal muscle impairment (MIRS), and education level all poorly correlated with cognitive and intellect scores. Finally, the rate of cognitive decline over the 9-year period was higher in late- than adult-onset subjects, although overall cognitive performance level at study end was worse for the adult-onset group.

Study Results

Gallais and colleagues concluded that cognitive impairment in adult- and late-onset DM1 is prevalent, moderate in severity, progressive, and global in the range of functions impacted. They suggest that the CNS phenotype seen in this DM1 cohort can be interpreted, at least in part, as an acceleration of normal aging, a hypothesis they support with reference to neuronal accumulation of a neurodegenerative protein (tau), white matter changes, and occurrence of aging-linked signs and symptoms in other organ systems (e.g., cataracts, baldness, erectile dysfunction, and endocrine dysfunction) in DM1.

These findings are important from the perspectives of size of and care in cohort selection, thoroughness of evaluations, and length of study. A caveat is that two time points, 9 years apart, do not provide sufficient resolution of the natural history of CNS impairment to guide clinical trial design. The authors do indicate that the study is continuing with shorter intervals between assessments. While recognizing the value that CNS imaging would have added, the authors note the challenges of including MRI measures in studies of cohorts that are adequately powered for neuropsychological assessments.

The lessons learned here must not be lost, but rather used to impact subsequent natural history studies of cognitive function in DM1. Taken together, the results from this study better characterize the disease and represent an important step forward for the design and selection of DM1 clinical trial endpoints to assess cognition, as well as for further guidance in recognizing and managing cognitive symptoms in patients with adult- and late-onset DM1.

Reference:

Cognitive Decline Over Time in Adults with Myotonic Dystrophy Type 1: A 9-year Longitudinal Study.
Gallais B, Gagnon C, Mathieu J, Richer L.
Neuromuscul Disord. 2016 Oct 14. pii: S0960-8966(16)30846-X. doi: 10.1016/j.nmd.2016.10.003. [Epub ahead of print]

The multi-system involvement and heterogeneity that characterize myotonic dystrophy (DM) have fostered several efforts to design patient-reported outcome measures (PROMs) for clinical studies and trials. The intent of PROMs is to use patient feedback in design and implementation of validated questionnaires that can simultaneously capture changes across the challenging symptomology of DM while obtaining clinically meaningful information to support regulatory approval. While PROMs can prove insightful as an analytic tool for complex disorders, the potential barriers to PROM design, development, and interpretation are such that the Food and Drug Administration (FDA) developed a Guidance for Industry document (pdf) to aid in development of PROMs. Choice of PROMs for use in interventional trials must then be carefully informed.

Dr. Tara Symonds and her colleagues at Clinical Outcomes Solutions (COS) recently reported out a literature review of available PROMs that focus on type 1 myotonic dystrophy (DM1). COS a is health economics and outcomes research consulting group with considerable experience in understanding PROM design and implementation in clinical studies. Disclosure: The COS project was funded by Biogen, a company engaged in therapy development in DM1.

Dr. Symonds and her group evaluated a health status measure (MDHI), three activities of daily living scales (DM1-Activ, DM-Activc, and Life-H), two health related quality of life measures (INQoL & INQoL Serbian), and five sleep and fatigue measures (ESS, DSS, CFS, FSS, and FDSS) comparing their validity, reliability, and ability to detect change of each to guide choice of PROMs for use in DM1 studies.

MDHI was viewed as the only measure that attempted to capture all aspects of a DM1 patient's life that were impacted. Design of MDHI specifically for DM, internal consistency of the tool across domains assessed, test-retest reliability, and design in compliance with FDA’s Guidance for Industry were viewed as favorable traits. It was also noted that construct validity had been established for MDHI via comparison with a variety of existing functional measures (e.g., MMT, grip testing, and timed function tests).

DM1-Activ also was considered to have good validity and reliability and showed construct validity when compared to various manual testing measures and the Muscular Impairment Rating Scale (MIRS).

Most other PROMs assessed by the authors were viewed as more limited in capability and performance, and all PROMs were thought to require further assessment of responsiveness and meaningful change thresholds in interventional clinical trials. Dr. Symonds and team concluded that MDHI is arguably the best measure for use in clinical studies and trials provided that the critical areas of responsiveness and definition of meaningful change to patient are addressed. DM1-Activ also was deemed to have potential as a PROM in interventional trials, as long as content validity is explored further and the issues of responsiveness and meaningful change prove acceptable. Other measures were considered acceptable in evaluation of specific domains of the symptomatology of DM1.

FDA’s Guidance for Industry supports the use of well-designed and implemented PROMs as putative primary endpoint measures for clinical trials. Even as a secondary measure, a carefully selected PROM can bring considerable value to clinical trials, not the least is insight into meaningful benefit to the patient. The publication by Dr. Symonds and colleagues provides an evaluation of currently available tools by experts outside of the DM community. While current drug discovery and development efforts have focused on skeletal muscle function, desired treatments for DM will have to address a much wider disease burden. Validated and reliable PROMs with an ability to capture changes in multiple symptoms important to DM1 patients may prove to be a valuable tool in natural history studies and definitive clinical trials.

Reference:

A Review of Patient Reported Outcome Measures for Use in DM1 Patients
Symonds T, Randall JA, Campbell P.
Muscle Nerve. 2016 Nov 11. doi: 10.1002/mus.25469.

Heterogeneity in the presentation and course of DM1 is a well-recognized feature that has, thus far, complicated the management of patients and design of clinical trials. Gene discovery in other muscular dystrophies (e.g., CMD or LGMD) provided the means, not only for better classification, but of supporting patient care and therapy development. While we understand the relevant genes in DM, knowledge of modifiers of the DM1 clinical spectrum is lacking. In the absence of genetic or environmental disease modifiers, or of diagnostic or predictive biomarkers, the French Myotonic Dystrophy Clinical Network has evaluated a large cohort of DM1 patients in the DM-Scope registry and developed discrete disease profiles to support a five-grade model of DM1.

Although we understand the existence of heterogeneity in DM1 (differences in onset age, organ systems affected, and severity and order of appearance of symptoms), the curious dilemma for the field is what lies behind the substantial clinical variability. As yet, no modifier genes have been discovered that provide molecular links to DM1 molecular mechanisms and the observed heterogeneity, nor have any biomarkers been validated that can predict time of onset and severity of the multi-organ system consequences of DM1.

The French DM Clinical Network sought to better understand the heterogeneity of DM1, with the rationale that better characterized phenotypes would support improvements in mechanistic research, patient care, and clinical phases of drug development. Members of the Network recently published findings from a cohort of 2,167 adult DM1 patients evaluated across the 28 member neuromuscular centers.

Basing analysis on CTG repeat length and detailed evaluation of the occurrence, onset time and order of occurrence, and severity of symptoms, their data support a classification scheme with five types of DM1: congenital, infantile, juvenile, adult-onset, and late-onset. While CTG repeat lengths overlapped, the investigators showed differences in repeat length distribution among the five categories. CTG length alone did not appear to be a valid marker for disease prognosis.

The investigators established that co-varying patterns in CTG length distribution and age of onset and frequency of systems define five disease grades and suggest that patient care should be aligned with disease grade. Several of the clinical features of DM1 appeared to exhibit specific onset times that were linked to disease grade. Disease grade, for example, then could be used to predict the timing of need for specialty care, as the authors noted that cardiac and aging-associated features (e.g., cataracts, endocrine symptoms) can develop very early in specific DM1 grades. The classification scheme also appears to better refine understanding of patterns of symptomatology in childhood DM1.

Taken together, the five-grade classification system published by the French DM Research Network provides an important framework to guide patient care and, potentially, to stratify patients in interventional clinical trials. Clearly, taking the next step to better understand and track the mechanistic factors behind the heterogeneity in DM1 will be essential to characterization of patients, refinement of the five-grade model, and improvement of patient care and the design/stratification of clinical studies and trials.

Reference:

Unravelling the myotonic dystrophy type 1 clinical spectrum: A systematic registry-based study with implications for disease classification.
De Antonio M, Dogan C, Hamroun D, Mati M, Zerrouki S, Eymard B, Katsahian S, Bassez G.
French Myotonic Dystrophy Clinical Network. Rev Neurol (Paris). 2016 Sep 21. pii: S0035-3787(16)30205-3. doi: 10.1016/j.neurol.2016.08.003.

Skeletal Muscle Transplants and the Pathobiology of Muscular Dystrophy

Transplantation of muscle precursor cells—to regenerate myofibers not compromised by the patient’s mutation--has attracted considerable attention as a candidate therapy for multiple forms of muscular dystrophy. While direct injection of stem cells into the target organ is feasible for some diseases, the sheer volume and body-wide distribution of skeletal muscle compromise direct injection strategies for muscular dystrophy. Translation to clinical trials using systemically delivered cells in humans then would face considerable difficulties including delivery, survival in the environment of dystrophic/regenerating muscle, and a host of regulatory issues regarding preparation, properties, and safety of a cell therapeutic. Unfortunately, the field is fraught with controversy as multiple international clinics offer stem cell ‘therapies’ of questionable efficacy and safety. Muscle transplant experiments in model organisms, however, may have value for identifying new aspects of the pathogenesis of muscular dystrophies, including myotonic dystrophy, and a recent study appears to have done that.

Intracellular Fate of Toxic RNA

A new study has utilized a novel mouse model, combining expression of pathogenic expanded CUG repeat RNA (HSA^LR) with an immunodeficient strain (NSG), thereby allowing study of muscle stem cell transplantation in a DM1 model (designated NSG-HSA^LR; Mondragon-Gonzalez et al., 2019). The research team’s findings may improve understanding of the trafficking of DM1-related toxic RNA within the unique environment of multinuclear skeletal myofibers.

Muscle progenitor cells (sourced from human iPAX7 PLZ iPS line, mouse satellite cells, or Pax3-inducible mES cells) were injected into pre-injured tibialis anterior muscles of NSG-HSA^LR mice, as well as native HSA^LR and NSG controls. Endpoint analyses (presence/absence of nuclear foci/MBNL sequestration and splicing patterns for selected transcripts known to be mis-spliced in HSA^LR) were performed 4 weeks after injection. Transplanted myonuclei could be identified by markers specific to each of the three cell sources.

The central finding of the study was identification of nuclear foci/MBNL sequestration in myonuclei of each category of transplanted muscle stem cells, indicating the translocation of toxic RNA transcripts from host myonuclei to mutation-free transplanted myonuclei. The team attributed this to toxic RNA released into and acquired from shared cytoplasm following formation of chimeric myofibers. RT-PCR using primers specific to transplanted human myoblasts showed DM1-related splicopathy in SERCA1, LDB3, and CACNA1S transcripts, thereby supporting the notion of internuclear transfer of toxic RNA. Controls fit expected patterns and were negative for nuclear foci and splicopathy.

Conclusions on the Mobility of Expanded Repeat RNA

Fusion of mutation-free donor precursor cells into existing NSG-HSA^LR myofibers was accompanied by translocation of toxic RNA, originating from host myonuclei, into donor myonuclei. This resulted in a DM1-like phenotype in donor cells. This finding suggests that (a) expanded repeat RNA is not confined to the nuclear domain it originated from and (b) toxic RNA then can migrate from cytoplasm into myonuclei other than where it originated, and result in formation of nuclear foci and the ensuing mis-splicing.

One implication of these findings, potentially broadly involving putative therapy development strategies in DM1, is that those myonuclei not exposed to a given therapy could spread pathology to treated myonuclei within the same myofiber and negate a positive effect. Thus, further exploration of the occurrence and mechanism of internuclear RNA transfer identified here is essential.

Reference:

Transplantation studies reveal internuclear transfer of toxic RNA in engrafted muscles of myotonic dystrophy 1 mice.
Mondragon-Gonzalez R, Azzag K, Selvaraj S, Yamamoto A, Perlingeiro RCR.
EBioMedicine. 2019 Aug 21. pii: S2352-3964(19)30553-5. doi: 10.1016/j.ebiom.2019.08.031. [Epub ahead of print]

New Review Article Series on DM

A special issue of the on-line International Journal of Molecular Sciences (edited by Prof. Lubov Timchenko) has been publishing a series of review articles on DM. To date, these articles have focused on the role of short tandem repeat expansions in RNA toxicity in DM1 and DM2 (Sznajder and Swanson, 2019) and on experiences with the development of CRISPR/Cas genome editing for DM1 (Raaijmakers et al., 2019). MDF's Research News recently highlighted one of these reviews. The latest piece in this series reviews small molecule drug development efforts aimed at DNA, RNA, and protein stages in the pathogenesis of DM1 (Reddy et al., 2019). The lead author of this review, Dr. Kaalak Reddy (University of Albany SUNY), is a former MDF Research Fellow.

Small Molecule Drugs for DM1

Small molecule compounds offer considerable advantages as putative, orally delivered drugs, a delivery route likely to be essential for systematically addressing multi-organ system diseases like DM. Knowledge of druggable chemical space (the depth and breadth of compounds with drug-like properties defined by Lipinski rule of 5 and beyond), and the analoging possible via medicinal chemistry, collectively allows: (a) high-throughput identification of parent compounds with activity at any one of multiple levels of the disease mechanisms operative in DM and (b) iterative compound optimization via analysis of Structure-Activity Relationships (SAR). Academic efforts toward discovery and development of small molecule drugs have improved in recent years, although considerable need for industry’s very large compound libraries, high-throughput capacity, and more rapid medicinal chemistry capability remains.

Dr. Reddy and colleagues frame their discussion around the molecular targets that are available to stem the pathogenesis of DM1, noting that much (but not all; e.g., AMO Pharma’s Tideglusib) progress has been made in targeting mechanisms downstream of either the expanded DNA repeats or toxic RNA. They proceed to document how that picture is changing.

The authors review, in detail, efforts for small molecule drug development for several targets/strategies, including targeting toxic RNA strategies based upon knowledge of target crystal structure (i.e., affinity for DM1 or DM2 expanded repeats), small molecule screens for toxic RNA targeting (including traditional screens, repurposed drug library screens, combinatorial chemistry screens, and specific target screens to disrupt toxic RNA-MBNL binding or nuclear foci), upregulation of MBNL protein, mis-spicing as a readout for high throughput screens, targeting CUGBP1, blocking toxic RNA transcription, targeting RAN translation, and modulating DNA expanded repeat instability. Taken together, the review serves as a digestible compendium of small molecule drug efforts in DM.

Potential for Small Molecule Drugs for DM

The authors have highlighted the breadth and depth of current efforts to bring candidate small molecule therapies into the clinic for DM. The potential for success is optimized by both the range of targets in the mainstream of established molecular mechanisms and the diversity of strategies applied to those targets. The oral bioavailability that can be achieved for small molecule drugs and their potential cost profile (versus recent pricing of biologics in other neuromuscular disease indications) also makes these efforts attractive. Finally, synergistic value may be obtained if two or more molecules receive marketing approval to address the primary pathogenic mechanisms in DM.

References:

Short Tandem Repeat Expansions and RNA-Mediated Pathogenesis in Myotonic Dystrophy.
Sznajder ŁJ, Swanson MS.
Int J Mol Sci. 2019 Jul 9;20(13). pii: E3365. doi: 10.3390/ijms20133365. Review.

CRISPR/Cas Applications in Myotonic Dystrophy: Expanding Opportunities.
Raaijmakers RHL, Ripken L, Ausems CRM, Wansink DG.
Int J Mol Sci. 2019 Jul 27;20(15). pii: E3689. doi: 10.3390/ijms20153689. Review.

Mitigating RNA Toxicity in Myotonic Dystrophy using Small Molecules.
Reddy K, Jenquin JR, Cleary JD, Berglund JA.
Int J Mol Sci. 2019 Aug 17;20(16). pii: E4017. doi: 10.3390/ijms20164017. Review.

The Problem of Clinical Outcome Measures in DM1

Despite much work being done to develop knowledge, clinical development tools, and relationships to de-risk the entire therapeutic development pipeline for DM1, the status of clinical trial outcome measures has remained a substantive hurdle. The intersection of the slow progression of most aspects of DM with the need for timely go/no decision-making on safety and efficacy of candidate therapeutics highlights the difficulties in arriving at a pragmatic battery of outcome measures. Moreover, the heterogeneity of presentation and progression of DM1 means that outcome measure development and validation can be driven only by sufficiently powered natural history studies using optimized operating procedures across multiple clinical sites.

Progress Toward DM1 Outcome Measures

Dr. Aura Cecilia Jimenez-Moreno (Newcastle University) and colleagues, working within the international Outcome Measures for Myotonic Dystrophy (OMMYD) group, have published a large cohort, cross-sectional and longitudinal analysis of a battery of tests aimed at identifying functional outcome measures for DM1 (Jimenez-Moreno et al., 2019). Measures included in the study were 6-minute walk test, 30-second sit and stand test, timed 10-minute walk test, timed 10-minute walk/run test, and 9-hole peg test, following specified operating procedures.

The OMMYD study was initiated with cross-sectional analysis of the five outcome measures in a cohort of 213 subjects recruited at two sites (University College London and Newcastle University; 98 subjects then were followed longitudinally at the NU site). The battery of tests selected here appears feasible for use in interventional clinical trials as 96% of the participants completed all outcome measures in a single test session. Both body mass index and disease severity associate with functional measures. Gender differences in outcome measure performance were a bit complex, with their relationship to established phenotypic variations in DM1 by gender (Dogan et al., 2016) unclear. Intra-session reliability analysis showed that two trials of each of the functional tests were sufficient to provide reliable/valid scores. Average scores of most functional tests significantly correlated with subject quantitative muscle strength, SARA score, and results of PROMs (MDHI and DM1-ActivC).

Longitudinal analysis of the Newcastle cohort involved a second follow-up at 12 months. This cohort included both adult- and late-onset phenotypic DM1. Except for the 9-hole peg test, all the functional tests and SARA showed significant changes from the first evaluation—changes were similar in both phenotypic groups.

Limitations and Path Forward

This study did not, however, report out on study subject perception of the meaningfulness of the functional changes in outcome measures (i.e., potential changes in burden of disease) detected across the 12-month longitudinal study period. This information will be vital to how regulatory agencies view the impact of a candidate therapeutic. The authors also note that use of independent evaluations on two successive days would have provided important data on instrument validity and measurement error. Assessment of inter-rater variations also was not determined. Finally, it was suggested that larger cohorts will be needed to account for differences in subgroups identifiable within the study cohort.

Overall, the OMMYD report makes many valuable specific recommendations for subsequent natural history studies geared toward assessing and validating clinical outcome measures for interventional trials in DM1. Continued efforts by OMMYD, the U.S.-based Myotonic Dystrophy Clinical Research Network, and other teams, particularly coordination among these efforts, may be essential to establish a validated protocol that is both feasible and sufficient for decision-making in clinical trials.

References:

Analysis of the functional capacity outcome measures for myotonic dystrophy.
Jimenez-Moreno AC, Nikolenko N, Kierkegaard M, Blain AP, Newman J, Massey C, Moat D, Sodhi J, Atalaia A, Gorman GS, Turner C, Lochmüller H.
Ann Clin Transl Neurol. 2019 Aug;6(8):1487-1497. doi: 10.1002/acn3.50845. Epub 2019 Jul 22.

Gender as a Modifying Factor Influencing Myotonic Dystrophy Type 1 Phenotype Severity and Mortality: A Nationwide Multiple Databases Cross-Sectional Observational Study.
Dogan C, De Antonio M, Hamroun D, Varet H, Fabbro M, Rougier F, Amarof K, Arne Bes MC, Bedat-Millet AL, Behin A, Bellance R, Bouhour F, Boutte C, Boyer F, Campana-Salort E, Chapon F, Cintas P, Desnuelle C, Deschamps R, Drouin-Garraud V, Ferrer X, Gervais-Bernard H, Ghorab K, Laforet P, Magot A, Magy L, Menard D, Minot MC, Nadaj-Pakleza A, Pellieux S, Pereon Y, Preudhomme M, Pouget J, Sacconi S, Sole G, Stojkovich T, Tiffreau V, Urtizberea A, Vial C, Zagnoli F, Caranhac G, Bourlier C, Riviere G, Geille A, Gherardi RK, Eymard B, Puymirat J, Katsahian S, Bassez G.
PLoS One. 2016 Feb 5;11(2):e0148264. doi: 10.1371/journal.pone.0148264. eCollection 2016.

As a consequence of the retention of mutant DMPK transcripts in the nucleus in DM1, patients express baseline levels of DMPK protein that are already half those of unaffected individuals. Since a key therapeutic strategy relies upon degradation of DMPK transcripts using antisense oligonucleotides (ASO), there are concerns as to whether essential functions of DMPK may be comprised and thereby contribute to the pathogenesis in DM1.

A University of Rochester team led by Dr. Charles Thornton has addressed this issue using mouse models with constitutive (genetic deletion) or acquired (ASO reduction) reductions in Dmpk. The function of DMPK is currently unknown. Prior reports in genetic models have shown that mice with heterozygous deletion of Dmpk exhibit cardiac conduction system defects, while those with homozygous deletion show skeletal muscle myopathy and weakness. Thus optimization of the ability of ASOs to target and degrade DMPK transcripts could exacerbate cardiac and skeletal muscle dysfunction in DM1.

Dr. Thornton and colleagues reevaluated the impact of genetic and ASO-induced reductions in Dmpk in two mouse models. They saw no effect of genetic deletion or ASO knockdown on cardiac (heart rate, PR interval, QRS duration, left ventricular contractile parameters) or skeletal (grip strength) functional measures, despite the substantial reductions that were achieved in Dmpk protein levels. Current strategies for ASO knockdown in DM1 utilize an allele-selective approach by targeting and degrading the mutant DMPK transcripts that are retained in the nucleus. However, there is the possibility that wild-type DMPK transcripts that traffic to the cytoplasm may also be degraded, as the next generation ASO chemistries result in more effective delivery to skeletal and cardiac muscles.

Yet despite the concern that substantial reductions in Dmpk protein may impact these muscles, Dr. Thornton’s team did not uncover any pathophysiology associated with Dmpk knockdown, even when genetic and ASO strategies were combined to yield as much as 90% reduction. Differences between the results of the Thornton team and prior investigations may relate to technical differences in the studies, mouse background strain differences, or features of the genetic knockout alleles.

The levels of Dmpk silencing seen in this study most likely would exceed those that could be obtained in DM1 patients, even with a highly effective ASO drug. The level of reduction of Dmpk in the mouse models also would likely exceed the reductions that are necessary to effectively restore the DM1-linked changes in mRNA splicing, and thereby mitigate DM1 signs and symptoms.

Hence these findings support the notion that strategies to increase the effectiveness of ASO candidate therapies can be effective in DM1 without increasing the risk of cardiac and skeletal muscle events.

Reference:

Dmpk gene deletion or antisense knockdown does not compromise cardiac or skeletal muscle function in mice.
Carrell ST, Carrell EM, Auerbach D, Pandey SK, Bennett CF, Dirksen RT, Thornton CA.
Hum Mol Genet. 2016 Aug 13. pii: ddw266.

A recent study corroborated increased susceptibility to cancer in DM1, for women in particular, and linked the elevated risk to depressed levels of a tumor suppressor microRNA (miRNA). The association between DM1 and increased risk of certain types of cancer was first recognized in 1965. Recent studies have validated these initial findings and suggested that cancer risks in DM1 were greater in women, but the causative mechanisms remained unknown. 

Dr. Adolfo López de Munain (Donostia University Hospital, San Sebastián, Spain) and colleagues have now corroborated gender differences in susceptibility to cancer and identified potential molecular mechanisms behind the cancer risk in DM1. In a publication in Neurology, Dr. López de Munain’s team quantified cancer risk in a well-characterized cohort of 424 patients with DM1, representing > 18,000 patient years of data, and explored the potential molecular links between DM1 and cancer prevalence.

All patients in the study had molecular confirmation of DM1 and substantial longitudinal phenotypic data available. The observed numbers of cancers in the DM1 cohort were compared against the numbers that would be expected, as calculated from the Basque region’s overall prevalence numbers, in order to determine standardized cancer incidence ratios. The investigators also performed gene expression analyses as a first step to understand molecular mechanisms behind the cancer prevalence data.

When compared to a general, geographically controlled population, DM1 patients showed a 2-fold increased risk of developing cancer. Mean age of malignant cancer detection in the DM1 cohort was 47 years. Gastrointestinal, genitourinary, skin and thyroid were the most frequent sites for malignant tumors. Increased risk was stronger in women with DM1.

In the overall DM1 population evaluated in this study, cancer represented the third leading cause of death, after respiratory and circulatory diseases. Analyses of molecular factors that may differentiate the at-risk DM1 population included CTG repeat length and genome-wide expression analysis of blood leukocytes using Affymetrix microarrays. The authors did not find a correlation between expanded CTG repeat length and cancer risk. Genome wide expression analysis did show differential expression of several genes that were previously linked to cancer (e.g., PDK4, DAPK1, CASP5, and PLA2G7).

Moreover, female patients with DM1 displayed significant down-regulation of the miRNA-200c/141 tumor suppressor family, while levels of this miRNA were elevated in men with DM1. Prior studies, in non-DM cohorts, have shown an association between declines in miRNA-200c and tumor progression/poor prognosis. Although further studies will be needed to mechanistically link changes in the DM1 transcriptome to increased cancer risk, the data from the San Sebastián group supports a compelling hypothesis linking reduction in tumor suppressor genes to cancer risk in women with DM1. The gender-specific differences in susceptibility to cancer, and the linkage to reduced levels of miRNA-200c, are particularly compelling findings for validation in independent DM1 cohorts and further mechanistic analyses.

Reference:

"Cancer risk in DM1 is sex-related and linked to miRNA-200/141 downregulation."
Fernández-Torrón R, García-Puga M, Emparanza JI, Maneiro M, Cobo AM, Poza JJ, Espinal JB, Zulaica M, Ruiz I, Martorell L, Otaegui D, Matheu A, López de Munain A.
Neurology. 2016 Aug 24. pii: 10.1212/WNL.0000000000003124. [Epub ahead of print]

Building a Better Mouse

MDF has entered into a one-year, $90,000 partnership with Dr. Cat Lutz and Jackson Laboratory (Bar Harbor, ME) to develop a new mouse model of myotonic dystrophy type 1 (DM1).

The most commonly used DM1 mouse model, the HSALR mouse developed in Dr. Charles Thornton’s lab, has been an invaluable contributor to the understanding of pathogenic mechanisms in DM1 and has served in the development of preclinical rationale (proof of concept) to drive clinical trials in DM1. HSALR mice exhibit aberrant splicing of many genes that are mis-spliced in DM1, including Clcn1 and, consequently, show prominent myotonia. However, this model has limitations that include expression of the untranslated CUG repeat in an mRNA unrelated to DM1, tissue-limited CUG repeat expression (e.g., absent from critical brain and heart tissues, since regulation is driven by the HSA promoter), and the mouse lacks many of the multi-systemic features of DM1.

The deficiencies of the HSALR mouse model may be a consequence of the insertion site, length, developmental expression and/or flanking sequences of the CUG repeat. The partnership with Jackson Laboratory addresses these issues by seeking to develop a DM1 mouse model that more closely mimics the genetics of DM1. Dr. Lutz will develop a BAC transgenic mouse with insertion of an expanded repeat tract DMPK gene and flanking regions isolated from a DM patient BAC library. This new mouse should better replicate the molecular and cellular pathogenic mechanisms that operate in DM1, and may then better express the wider organ system involvement that is seen in DM1 patients.

By working with Jackson Laboratory, MDF intends to have the new DM1 model readily available to both academic researchers and drug developers at modest cost and without Intellectual Property restrictions within the next 12 months.

Creation and Distribution of DM Cell Lines for Research and Therapy Development

MDF is collaborating with the Human Cell and Data Repository (NHCDR), a joint venture involving the National Institute of Neurological Disorders and Stroke (NINDS) and RUCDR Infinite Biologics at Rutgers University. The partners in the collaboration are dedicated to the development of new fibroblast and iPSC lines, including isogenic iPSC lines for neurological disorders.

Over the next year, the collaboration will develop DM1 and DM2 fibroblast cell lines and at least four iPSC lines each from DM1 and DM2 patient cells. Through this collaboration, we will achieve unencumbered access and distribution of cell lines essential to mechanistic and drug discovery studies in academia and companies.

Availability of quality controlled, isogenic iPSC lines will mitigate, if not eliminate, an early stage barrier to entry of biotechnology and pharmaceutical companies into high-throughput screening programs for DM1 and DM2. Given the multi-system consequences of DM, availability of iPSC lines is of particular importance, as they provide the means to derive myoblast, cardiomyoblast, neuronal, or other cell types for use in studies of tissue-specific disease mechanisms and/or testing and optimizing specifically targeted candidate therapeutics.

When available, the DM patient-derived cell lines will be accessed through the NHCDR on-line catalog. The ensuing collaboration between MDF and academic and government partners will assure the availability of critical patient-derived resources at modest cost and without Intellectual Property restrictions that could hinder commercial drug development.

A variety of tools are being brought to bear in order to study one of the least understood consequences of DM: the impact upon brain cognitive function. Lead author, Dr. Dimitri Renard, recently reported his team's findings using 18F-deoxy-glucose positron emission tomography (FDG-PET) in 48 DM1 patients seen in their clinic in Nimes, France. Their findings are consistent with reduced activity in a brain area responsible for rapid eye movements (saccades) and are consistent with prior reports of eye movement abnormalities in DM1.

Several research groups in the U.S. and elsewhere are using brain imaging techniques, most commonly MRI, in conjunction with psychological evaluations, in order to understand what brain areas and what functions are impacted in DM. FDG-PET imaging provides a unique capability in that it assesses function through increases or decreases in metabolism within specific brain regions. FDG-PET works by measuring the use of a key energy source, glucose, by imaging differences in the uptake of a modified glucose molecule carrying a radioactive tag. Any differences in FDG uptake between impaired and normal tissue then are visualized by imaging the tagged glucose.

Dr. Renard and colleagues studied 48 symptomatic and genetically confirmed DM1 patients (including early and late onset DM1), in comparison to a matched control group, using FDG-PET brain imaging. The most striking observation was reduced FDG-uptake in Brodmann’s area 8, an area known as the “frontal eye fields.” Although patients exhibited a range of expanded CTG repeat lengths (83 to 2000), there was no significant correlation between repeat length and degree of impairment in FDG uptake. Since the authors determined CTG repeat length from blood samples, and repeat length can differ in blood, muscle, and brain of the same patient, the authors acknowledge that tissue differences may explain the lack of a correlation. Likewise, this study did not find differences between early and late onset patients, but the sample size may not have been sufficient to draw statistically meaningful conclusions on FDG-uptake versus age of onset.

Taken together, progress in understanding the central nervous system consequences of DM will require application of both modern brain imaging and neuropsychological testing. It is essential that we understand changes in the brain, both to improve clinical management of DM patients, as well as to develop targeted therapies for the brain. The increase in research on the DM brain in the last few years is encouraging.

Reference:

In myotonic dystrophy type 1 reduced FDG-uptake on FDG-PET is most severe in Brodmann area 8.
Renard D, Collombier L, Castelli C, Pouget JP, Kotzki PO, Boudousq V.
BMC Neurol. 2016 Jul 13.