Comparison of CTG repeat length expansion and clinical progression of myotonic dystrophy over a five year period

Immortalized human myotonic dystrophy type 1 muscle cell lines to address patient heterogeneity

Historically, cellular models have been used as a tool to study myotonic dystrophy type 1 (DM1) and the validation of therapies in said pathology. However, there is a need for in vitro models that represent the clinical heterogeneity observed in patients with DM1 that is lacking in classical models. In this study, we immortalized three DM1 muscle lines derived from patients with different DM1 subtypes and clinical backgrounds and characterized them at the genetic, epigenetic, and molecular levels. All three cell lines display DM1 hallmarks, such as the accumulation of RNA foci, MBNL1 sequestration, splicing alterations, and reduced fusion. In addition, alterations in early myogenic markers, myotube diameter and CTCF1 DNA methylation were also found in DM1 cells. Notably, the new lines show a high level of heterogeneity in both the size of the CTG expansion and the aforementioned molecular alterations. Importantly, these immortalized cells also responded to previously tested therapeutics. Altogether, our results show that these three human DM1 cellular models are suitable to study the pathophysiological heterogeneity of DM1 and to test future therapeutic options.

Overview of the Complex Relationship between Epigenetics Markers, CTG Repeat Instability and Symptoms in Myotonic Dystrophy Type 1

Among the trinucleotide repeat disorders, myotonic dystrophy type 1 (DM1) is one of the most complex neuromuscular diseases caused by an unstable CTG repeat expansion in the DMPK gene. DM1 patients exhibit high variability in the dynamics of CTG repeat instability and in the manifestations and progression of the disease. The largest expanded alleles are generally associated with the earliest and most severe clinical form. However, CTG repeat length alone is not sufficient to predict disease severity and progression, suggesting the involvement of other factors. Several data support the role of epigenetic alterations in clinical and genetic variability. By highlighting epigenetic alterations in DM1, this review provides a new avenue on how these changes can serve as biomarkers to predict clinical features and the mutation behavior.

Longitudinal increases in somatic mosaicism of the expanded CTG repeat in myotonic dystrophy type 1 are associated with variation in age-at-onset

In myotonic dystrophy type 1 (DM1), somatic mosaicism of the (CTG)n repeat expansion is age-dependent, tissue-specific and expansion-biased. These features contribute toward variation in disease severity and confound genotype-to-phenotype analyses. To investigate how the (CTG)n repeat expansion changes over time, we collected three longitudinal blood DNA samples separated by 8-15 years and used small pool and single-molecule PCR in 43 DM1 patients. We used the lower boundary of the allele length distribution as the best estimate for the inherited progenitor allele length (ePAL), which is itself the best predictor of disease severity. Although in most patients the lower boundary of the allele length distribution was conserved over time, in many this estimate also increased with age, suggesting samples for research studies and clinical trials should be obtained as early as possible. As expected, the modal allele length increased over time, driven primarily by ePAL, age-at-sampling and the time interval. As expected, small expansions <100 repeats did not expand as rapidly as larger alleles. However, the rate of expansion of very large alleles was not obviously proportionally higher. This may, at least in part, be a result of the allele length-dependent increase in large contractions that we also observed. We also determined that individual-specific variation in the increase of modal allele length over time not accounted for by ePAL, age-at-sampling and time was inversely associated with individual-specific variation in age-at-onset not accounted for by ePAL, further highlighting somatic expansion as a therapeutic target in DM1.

Myotonic dystrophy type 1 (DM1) clinical sub-types and CTCF site methylation status flanking the CTG expansion are mutant allele length-dependent

Myotonic dystrophy type 1 (DM1) is a complex disease with a wide spectrum of symptoms. The exact relationship between mutant CTG repeat expansion size and clinical outcome remains unclear. DM1 congenital patients (CDM) inherit the largest expanded alleles, which are associated with abnormal and increased DNA methylation flanking the CTG repeat. However, DNA methylation at the DMPK locus remains understudied. Its relationship to DM1 clinical subtypes, expansion size and age-at-onset is not yet completely understood. Using pyrosequencing-based methylation analysis on 225 blood DNA samples from Costa Rican DM1 patients, we determined that the size of the estimated progenitor allele length (ePAL) is not only a good discriminator between CDM and non-CDM cases (with an estimated threshold at 653 CTG repeats), but also for all DM1 clinical subtypes. Secondly, increased methylation at both CTCF sites upstream and downstream of the expansion was almost exclusively present in CDM cases. Thirdly, levels of abnormal methylation were associated with clinical subtype, age and ePAL, with strong correlations between these variables. Fourthly, both ePAL and the intergenerational expansion size were significantly associated with methylation status. Finally, methylation status was associated with ePAL and maternal inheritance, with almost exclusively maternal transmission of CDM. In conclusion, increased DNA methylation at the CTCF sites flanking the DM1 expansion could be linked to ePAL, and both increased methylation and the ePAL could be considered biomarkers for the CDM phenotype.

The Contribution of Somatic Expansion of the CAG Repeat to Symptomatic Development in Huntington's Disease: A Historical Perspective

The discovery in the early 1990s of the expansion of unstable simple sequence repeats as the causative mutation for a number of inherited human disorders, including Huntington's disease (HD), opened up a new era of human genetics and provided explanations for some old problems. In particular, an inverse association between the number of repeats inherited and age at onset, and unprecedented levels of germline instability, biased toward further expansion, provided an explanation for the wide symptomatic variability and anticipation observed in HD and many of these disorders. The repeats were also revealed to be somatically unstable in a process that is expansion-biased, age-dependent and tissue-specific, features that are now increasingly recognised as contributory to the age-dependence, progressive nature and tissue specificity of the symptoms of HD, and at least some related disorders. With much of the data deriving from affected individuals, and model systems, somatic expansions have been revealed to arise in a cell division-independent manner in critical target tissues via a mechanism involving key components of the DNA mismatch repair pathway. These insights have opened new approaches to thinking about how the disease could be treated by suppressing somatic expansion and revealed novel protein targets for intervention. Exciting times lie ahead in turning these insights into novel therapies for HD and related disorders.

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.

Instability of TCF4 Triplet Repeat Expansion With Parent-Child Transmission in Fuchs' Endothelial Corneal Dystrophy

Purpose

Fuchs' endothelial corneal dystrophy (FECD) caused by the CTG triplet repeat expansion in the TCF4 gene (CTG18.1 locus) is the most common repeat expansion disorder. Intergenerational instability of expanded repeats and clinical anticipation are hallmarks of other repeat expansion disorders. In this study, we examine stability of triplet repeat allele length and FECD disease severity in parent–child transmission of the expanded CTG18.1 allele.

Methods

We studied 44 parent–child transmissions of the mutant expanded CTG18.1 allele from 26 FECD families. The CTG18.1 polymorphism was genotyped using short tandem repeat analysis, triplet repeat primed PCR assay, and Southern blot analysis. FECD severity was assessed using modified Krachmer grading (KG) system. Triplet repeat length of mutant allele and KG severity were compared between generations.

Results

Instability of the expanded allele was seen in 14 of 44 (31.8%) parent–child transmissions, and the likelihood of an unstable event increased with the size of the parental allele (\begin{document}\newcommand{\bialpha}{\boldsymbol{\alpha}}\newcommand{\bibeta}{\boldsymbol{\beta}}\newcommand{\bigamma}{\boldsymbol{\gamma}}\newcommand{\bidelta}{\boldsymbol{\delta}}\newcommand{\bivarepsilon}{\boldsymbol{\varepsilon}}\newcommand{\bizeta}{\boldsymbol{\zeta}}\newcommand{\bieta}{\boldsymbol{\eta}}\newcommand{\bitheta}{\boldsymbol{\theta}}\newcommand{\biiota}{\boldsymbol{\iota}}\newcommand{\bikappa}{\boldsymbol{\kappa}}\newcommand{\bilambda}{\boldsymbol{\lambda}}\newcommand{\bimu}{\boldsymbol{\mu}}\newcommand{\binu}{\boldsymbol{\nu}}\newcommand{\bixi}{\boldsymbol{\xi}}\newcommand{\biomicron}{\boldsymbol{\micron}}\newcommand{\bipi}{\boldsymbol{\pi}}\newcommand{\birho}{\boldsymbol{\rho}}\newcommand{\bisigma}{\boldsymbol{\sigma}}\newcommand{\bitau}{\boldsymbol{\tau}}\newcommand{\biupsilon}{\boldsymbol{\upsilon}}\newcommand{\biphi}{\boldsymbol{\phi}}\newcommand{\bichi}{\boldsymbol{\chi}}\newcommand{\bipsi}{\boldsymbol{\psi}}\newcommand{\biomega}{\boldsymbol{\omega}}P = 5.9 \times {10^{ - 3}}\end{document}). A tendency for contraction was seen in transmission of large alleles (repeat length > 120), whereas intermediate alleles (repeat length between 77 and 120) had predilection for further expansion (\begin{document}\newcommand{\bialpha}{\boldsymbol{\alpha}}\newcommand{\bibeta}{\boldsymbol{\beta}}\newcommand{\bigamma}{\boldsymbol{\gamma}}\newcommand{\bidelta}{\boldsymbol{\delta}}\newcommand{\bivarepsilon}{\boldsymbol{\varepsilon}}\newcommand{\bizeta}{\boldsymbol{\zeta}}\newcommand{\bieta}{\boldsymbol{\eta}}\newcommand{\bitheta}{\boldsymbol{\theta}}\newcommand{\biiota}{\boldsymbol{\iota}}\newcommand{\bikappa}{\boldsymbol{\kappa}}\newcommand{\bilambda}{\boldsymbol{\lambda}}\newcommand{\bimu}{\boldsymbol{\mu}}\newcommand{\binu}{\boldsymbol{\nu}}\newcommand{\bixi}{\boldsymbol{\xi}}\newcommand{\biomicron}{\boldsymbol{\micron}}\newcommand{\bipi}{\boldsymbol{\pi}}\newcommand{\birho}{\boldsymbol{\rho}}\newcommand{\bisigma}{\boldsymbol{\sigma}}\newcommand{\bitau}{\boldsymbol{\tau}}\newcommand{\biupsilon}{\boldsymbol{\upsilon}}\newcommand{\biphi}{\boldsymbol{\phi}}\newcommand{\bichi}{\boldsymbol{\chi}}\newcommand{\bipsi}{\boldsymbol{\psi}}\newcommand{\biomega}{\boldsymbol{\omega}}P = 1.3 \times {10^{ - 3}}\end{document}). Although we noted increased KG severity in the offspring in three pairs, none of these transmissions were associated with allele instability.

Conclusions

We observed instability of the TCF4 triplet repeat expansion in nearly a third of parent–child transmissions. Large mutant CTG18.1 alleles are prone to contraction, whereas intermediate mutant alleles tend to expand when unstably transmitted. Intergenerational instability of TCF4 repeat expansion has implications on FECD disease inheritance.

FXS-Like Phenotype in Two Unrelated Patients Carrying a Methylated Premutation of the FMR1 Gene

Fragile X syndrome (FXS) is mostly caused by two distinct events that occur in the FMR1 gene (Xq27.3): an expansion above 200 repeats of a CGG triplet located in the 5′UTR of the gene, and methylation of the cytosines located in the CpG islands upstream of the CGG repeats. Here, we describe two unrelated families with one FXS child and another sibling presenting mild intellectual disability and behavioral features evocative of FXS. Genetic characterization of the undiagnosed sibling revealed mosaicism in both the CGG expansion size and the methylation levels in the different tissues analyzed. This report shows that in the same family, two siblings carrying different CGG repeats, one in the full-mutation range and the other in the premutation range, present methylation mosaicism and consequent decreased FMRP production leading to FXS and FXS-like features, respectively. Decreased FMRP levels, more than the number of repeats seem to correlate with the severity of FXS clinical phenotypes.

Unusual association of a unique CAG interruption in 5' of DM1 CTG repeats with intergenerational contractions and low somatic mosaicism

Myotonic dystrophy type 1 (DM1) is a dominant multisystemic disorder associated with high variability of symptoms and anticipation. DM1 is caused by an unstable CTG repeat expansion that usually increases in successive generations and tissues. DM1 family pedigrees have shown that ∼90% and 10% of transmissions result in expansions and contractions of the CTG repeat, respectively. To date, the mechanisms of CTG repeat contraction remain poorly documented in DM1. In this report, we identified two new DM1 families with apparent contractions and no worsening of DM1 symptoms in two and three successive maternal transmissions. A new and unique CAG interruption was found in 5' of the CTG expansion in one family, whereas multiple 5' CCG interruptions were detected in the second family. We showed that these interruptions are associated with maternal intergenerational contractions and low somatic mosaicism in blood. By specific triplet-prime PCR, we observed that CTG repeat changes (contractions/expansions) occur preferentially in 3' of the interruptions for both families.

Functional impairment in patients with myotonic dystrophy type 1 can be assessed by an ataxia rating scale (SARA)

Myotonic dystrophy type 1 (DM1) is not characterised by ataxia per se; however, DM1 and ataxia patients show similar disturbances in movement coordination often experiencing walking and balance difficulties, although caused by different underlying pathologies. This study aims to investigate the use of a scale previously described for the assessment and rating of ataxia (SARA) with the hypothesis that it could have utility in DM1 patients as a measure of disease severity and risk of falling. Data from 54 DM1 patients were pulled from the PHENO-DM1 natural history study for analysis. Mean SARA score in the DM1 population was 5.45 relative to the maximum score of eight. A flooring effect (score 0) was observed in mild cases within the sample. Inter-rater and test-retest reliability was high with intraclass coefficients (ICC) of 0.983 and 1.00, respectively. Internal consistency was acceptable as indicated by a Cronbach's alpha of 0.761. Component analysis revealed two principle components. SARA correlated with: (1) all measures of muscle function tested, including quantitative muscle testing of ankle dorsiflexion (r = -0.584*), the 6 min walk test (r = -0.739*), 10 m walk test (r = 0.741*), and the nine hole peg test (r = 0.602*) and (2) measures of disease severity/burden, such as MIRS (r = 0.718*), MDHI (r = 0.483*), and DM1-Activ (r = -0.749*) (*p < 0.001). The SARA score was predicted by an interaction between modal CTG repeat length and age at sampling (r = 0.678, p = 0.003). A score of eight or above predicted the use of a walking aid with a sensitivity of 100% and a specificity of 85.7%. We suggest that further research is warranted to ascertain whether SARA or components of SARA are useful outcome measures for clinical trials in DM1. As a tool, it can be used for gathering information about disease severity/burden and helping to identify patients in need of a walking aid, and can potentially be applied in both research and healthcare settings.

Effects of Replication and Transcription on DNA Structure-Related Genetic Instability

Many repetitive sequences in the human genome can adopt conformations that differ from the canonical B-DNA double helix (i.e., non-B DNA), and can impact important biological processes such as DNA replication, transcription, recombination, telomere maintenance, viral integration, transposome activation, DNA damage and repair. Thus, non-B DNA-forming sequences have been implicated in genetic instability and disease development. In this article, we discuss the interactions of non-B DNA with the replication and/or transcription machinery, particularly in disease states (e.g., tumors) that can lead to an abnormal cellular environment, and how such interactions may alter DNA replication and transcription, leading to potential conflicts at non-B DNA regions, and eventually result in genetic stability and human disease.

A longitudinal physical profile assessment of skeletal muscle manifestations in myotonic dystrophy

Objectives: To develop an assessment that describes the skeletal muscle manifestations in myotonic dystrophy subjects and then use it to quantify the presentation of skeletal muscle disability and to show change over time.

Design: A quantified skeletal muscle assessment was developed and applied three times over a two-year period at intervals around 12 months. Thirty-six subjects with myotonic dystrophy and 20 subjects without neuromuscular disability were evaluated. The assessment comprised manual muscle testing of five pairs of muscles, measuring neck flexor strength with a strain gauge, respiratory function tests, power and lateral pinch grip strength, all tests of impairment. Assessment of the ability to move from sitting to standing and fasten buttons tested disability.

Results: Results from subjects with myotonic dystrophy were compared to the normal data. The subjects with myotonic dystrophy were significantly weaker in proximal upper limb muscles, quadriceps, tibialis anterior muscles and neck flexor muscles as well as power and lateral pinch grips. There was also significant reduction in forced expiratory volume at one second (FEV1) and forced vital capacity (FVC). Significant disability was seen in the myotonics in moving from sitting to standing and in fastening buttons. Over the two-year study period proximal upper limb and lower limb muscle strength, FVC and sit-to-stand ability declined significantly. Power grip declined but lateral pinch grip and FEV1 improved significantly. Button fastening ability improved significantly.

Conclusion: The test developed was shown to be reliable and sensitive to the change in skeletal muscle manifestations in subjects with myotonic dystrophy who were shown to be significantly weaker than normal subjects.

Impact of alternative DNA structures on DNA damage, DNA repair, and genetic instability

Repetitive genomic sequences can adopt a number of alternative DNA structures that differ from the canonical B-form duplex (i.e. non-B DNA). These non-B DNA-forming sequences have been shown to have many important biological functions related to DNA metabolic processes; for example, they may have regulatory roles in DNA transcription and replication. In addition to these regulatory functions, non-B DNA can stimulate genetic instability in the presence or absence of DNA damage, via replication-dependent and/or replication-independent pathways. This review focuses on the interactions of non-B DNA conformations with DNA repair proteins and how these interactions impact genetic instability.

CGG allele size somatic mosaicism and methylation in FMR1 premutation alleles

BACKGROUND

Greater than 200 CGG repeats in the 5'UTR of the FMR1 gene lead to epigenetic silencing and lack of the FMR1 protein, causing fragile X Syndrome. Individual carriers of a premutation (PM) allele with 55-200 CGG repeats are typically unmethylated and can present with clinical features defined as FMR1-associated conditions.

METHODS

Blood samples from 17 male PM carriers were assessed clinically and molecularly by Southern blot, western blot, PCR and QRT-PCR. Blood and brain tissue from an additional 18 PM males were also similarly examined. Continuous outcomes were modelled using linear regression and binary outcomes were modelled using logistic regression.

RESULTS

Methylated alleles were detected in different fractions of blood cells in all PM cases (n=17). CGG repeat numbers correlated with percent of methylation and mRNA levels and, especially in the upper PM range, with greater number of clinical involvements. Inter-tissue/intra-tissue somatic instability and differences in percent methylation were observed between blood and fibroblasts (n=4) and also observed between blood and different brain regions in three of the 18 PM cases examined. CGG repeat lengths in lymphocytes remained unchanged over a period of time ranging from 2 to 6 years, three cases for whom multiple samples were available.

CONCLUSIONS

In addition to CGG size instability, individuals with a PM expanded allele can exhibit methylation and display more clinical features likely due to RNA toxicity and/or FMR1 silencing. The observed association between CGG repeat length and percent of methylation with the severity of the clinical phenotypes underscores the potential value of methylation in affected PM to further understand penetrance, inform diagnosis and expand treatment options.

Methods to detect replication-dependent and replication-independent DNA structure-induced genetic instability

DNA can adopt a variety of alternative secondary (i.e., non-B DNA) conformations that play important roles in cellular metabolism, including genetic instability, disease etiology and evolution. While we still have much to learn, research in this field has expanded dramatically in the past decade. We have summarized in our previous Methods review (Wang et al., Methods, 2009) some commonly used techniques to determine non-B DNA structural conformations and non-B DNA-induced genetic instability in prokaryotes and eukaryotes. Since that time, we and others have further characterized mechanisms involved in DNA structure-induced mutagenesis and have proposed both replication-dependent and replication-independent models. Thus, in this review, we highlight some current methodologies to identify DNA replication-related and replication-independent mutations occurring at non-B DNA regions to allow for a better understanding of the mechanisms underlying DNA structure-induced genetic instability. We also describe a new web-based search engine to identify potential intramolecular triplex (H-DNA) and left-handed Z-DNA-forming motifs in entire genomes or at selected sequences of interest.

Endocrine function in 97 patients with myotonic dystrophy type 1

The aim of this study was to investigate the endocrine function and its association to number of CTG repeats in patients with myotonic dystrophy type 1 (DM1). Concentration of various hormones and metabolites in venous blood was used to assess the endocrine function in 97 patients with DM1. Correlation with CTG(n) expansion size was investigated with the Pearson correlation test. Eighteen percent of the DM1 patients had hyperparathyroidism with increased PTH compared with 0.5% in the background population. Of these, 16% had normocalcemia and 2% had hypercalcemia. An additional 3% had hypercalcemia without elevation of PTH; 7% had abnormal TSH values (2% subnormal and 5% elevated TSH levels); 5% of the patients had type 2 diabetes mellitus; 17% of the male DM1 patients had increased LH and low levels of plasma testosterone indicating absolute androgen insufficiency. Another 21% had increased LH, but normal testosterone levels, indicating relative insufficiency. Numbers of CTG repeats correlated directly with plasma PTH, phosphate, LH, and tended to correlate with plasma testosterone for males. This is the largest study of endocrine dysfunction in a cohort of Caucasian patients with DM1. We found that patients with DM1 have an increased risk of abnormal endocrine function, particularly calcium metabolism disorders. However, the endocrine dysfunction appears not to be of clinical significance in all of the cases. Finally, we found correlations between CTG(n) expansion size and plasma PTH, phosphate, and testosterone, and neck flexion strength.

Expanded CTG repeat demarcates a boundary for abnormal CpG methylation in myotonic dystrophy patient tissues

Myotonic dystrophy (DM1) affects multiple organs, shows age-dependent progression and is caused by CTG expansions at the DM1 locus. We determined the DM1 CpG methylation profile and CTG length in tissues from DM1 foetuses, DM1 adults, non-affected individuals and transgenic DM1 mice. Analysis included CTCF binding sites upstream and downstream of the CTG tract, as methylation-sensitive CTCF binding affects chromatinization and transcription of the DM1 locus. In humans, in a given foetus, expansions were largest in heart and smallest in liver, differing by 40-400 repeats; in adults, the largest expansions were in heart and cerebral cortex and smallest in cerebellum, differing by up to 5770 repeats in the same individual. Abnormal methylation was specific to the mutant allele. In DM1 adults, heart, liver and cortex showed high-to-moderate methylation levels, whereas cerebellum, kidney and skeletal muscle were devoid of methylation. Methylation decreased between foetuses and adults. Contrary to previous findings, methylation was not restricted to individuals with congenital DM1. The expanded repeat demarcates an abrupt boundary of methylation. Upstream sequences, including the CTCF site, were methylated, whereas the repeat itself and downstream sequences were not. In DM1 mice, expansion-, tissue- and age-specific methylation patterns were similar but not identical to those in DM1 individuals; notably in mice, methylation was present up- and downstream of the repeat, but greater upstream. Thus, in humans, the CpG-free expanded CTG repeat appears to maintain a highly polarized pattern of CpG methylation at the DM1 locus, which varies markedly with age and tissues.

Models for chromosomal replication-independent non-B DNA structure-induced genetic instability

Regions of genomic DNA containing repetitive nucleotide sequences can adopt a number of different structures in addition to the canonical B-DNA form: many of these non-B DNA structures are causative factors in genetic instability and human disease. Although chromosomal DNA replication through such repetitive sequences has been considered a major cause of non-B form DNA structure-induced genetic instability, it is also observed in non-proliferative tissues. In this review, we discuss putative mechanisms responsible for the mutagenesis induced by non-B DNA structures in the absence of chromosomal DNA replication.

Prenatal diagnosis in myotonic dystrophy type 1. Thirteen years of experience: implications for reproductive counselling in DM1 families

OBJECTIVES

To analyse the results obtained from prenatal diagnoses in myotonic dystrophy type 1 (DM1) performed in our hospitals during the last 13 years.

METHODS

Molecular analyses were conducted on chorionic villi or cultured amniotic fluid samples obtained for prenatal diagnosis of DM1. CTG expansion was analyzed by polymerase chain reaction (PCR) and Southern blot techniques.

RESULTS

From 154 prenatal diagnoses performed in 13 years, 51% were found to be healthy and 49% affected. Considering the 75 carriers of the mutation, in 65.3% of the cases, the mother was the transmitting parent versus 36.5% of fathers. From these female transmissions, 31/49 foetuses had expansion in the neonatal form range, namely, congenital myotonic dystrophy (CMD).

CONCLUSIONS

In our series, no significant deviation of the 50% expected frequency of transmission in autosomal dominant disorder was seen. We show that when the disease is transmitted by a male, the mean intergenerational variation is minimal (mean = 56 CTG, SD = 177 CTG). However, this does not occur in the affected mothers, where the mean intergenerational expansion is very high (mean = 948 CTG, SD = 815 CTG) and the difference is statistically significant (t-Student p < 0.0001). Our data have important implications for the genetic counselling of DM1 families.

Instability in the transmission of the myotonic dystrophy CTG repeat in human oocytes and preimplantation embryos

OBJECTIVE

To elucidate the timing and variability of CTG repeat expansion within the human dystrophia myotonica protein kinase (DMPK) gene in early development.

DESIGN

Triplet-primed polymerase chain reaction was used to amplify the expanded CTG repeat in oocytes and embryos obtained from myotonic dystrophy 1 (DM1) patients, and a heminested polymerase chain reaction approach was used to amplify the normal CTG repeats in supernumerary IVF embryos.

SETTING

University hospital laboratory.

PATIENT(S)

Two DM1-affected females undergoing preimplantation genetic diagnosis who carried different CTG repeats. Also, 61 IVF patients who carried a (CTG)(5-18) and (CTG)(19-37) normal DMPK repeat.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

The degree of expansion of the repeat in the oocytes and embryos compared with the DM1-affected maternal repeat and the size of the (CTG)(19-37) repeat compared with the parental size in IVF embryos.

RESULT(S)

The degree of repeat expansion was greater than the DM1 maternal lymphocyte for two of four oocytes, including a germinal vesicle-stage oocyte and 17 of 20 three-cell to blastocyst stage embryos. A change in the (CTG)(19-37) repeat was seen in 7 (7%) of 95 paternal transmissions but in no maternal transmissions.

CONCLUSION(S)

Because the repeat was already expanded in the immature oocyte, the initial expansion most likely occurs during oogenesis. A variable degree of DMPK(CTG)(n) expansion in the embryo is seen from different mothers. In addition, instability in paternal transmission of normal-range (CTG)(19-37) repeats occurs at the level of the embryo.

Repeat instability: mechanisms of dynamic mutations

Disease-causing repeat instability is an important and unique form of mutation that is linked to more than 40 neurological, neurodegenerative and neuromuscular disorders. DNA repeat expansion mutations are dynamic and ongoing within tissues and across generations. The patterns of inherited and tissue-specific instability are determined by both gene-specific cis-elements and trans-acting DNA metabolic proteins. Repeat instability probably involves the formation of unusual DNA structures during DNA replication, repair and recombination. Experimental advances towards explaining the mechanisms of repeat instability have broadened our understanding of this mutational process. They have revealed surprising ways in which metabolic pathways can drive or protect from repeat instability.

Clinical and molecular aspects of the myotonic dystrophies: a review

Type 1 myotonic dystrophy or DM1 (Steinert's disease), which is the commonest muscular dystrophy in adults, has intrigued physicians for over a century. Unusual features, compared with other dystrophies, include myotonia, anticipation, and involvement of other organs, notably the brain, eyes, smooth muscle, cardiac conduction apparatus, and endocrine system. Morbidity is high, with a substantial mortality relating to cardiorespiratory dysfunction. More recently a second form of multisystem myotonic disorder has been recognized and variously designated as proximal myotonic myopathy (PROMM), proximal myotonic dystrophy (PDM), or DM2. For both DM1 and DM2 the molecular basis is expansion of an unstable repeat sequence in a noncoding part of a gene (DMPK in DM1 and ZNF9 in DM2). There is accumulating evidence that the basic molecular mechanism is disruption of mRNA metabolism, which has far-reaching effects on many other genes, in part through the induction of aberrant splicing, explaining the multisystemic nature of the disease. The unstable nature of the expansion provides a molecular explanation for anticipation. This review emphasizes the clinical similarities and differences between DM1 and DM2. It examines current views about the molecular basis of these disorders, and contrasts them with other repeat expansion disorders that have increasingly been recognized as a cause of neurological disease.

Role of replication and CpG methylation in fragile X syndrome CGG deletions in primate cells

Instability of the fragile X CGG repeat involves both maternally derived expansions and deletions in the gametes of full-mutation males. It has also been suggested that the absence of aberrant CpG methylation may enhance repeat deletions through an unknown process. The effect of CGG tract length, DNA replication direction, location of replication initiation, and CpG methylation upon CGG stability were investigated using an SV40 primate replication system. Replication-dependant deletions with 53 CGG repeats were observed when replication was initiated proximal to the repeat, with CGG as the lagging-strand template. When we initiated replication further from the repeat, while maintaining CGG as the lagging-strand template or using CCG as the lagging-strand template, significant instability was not observed. CpG methylation of the unstable template stabilized the repeat, decreasing both the frequency and the magnitude of deletion events. Furthermore, CpG methylation slowed the efficiency of replication for all templates. Interestingly, replication forks displayed no evidence of a block at the CGG repeat tract, regardless of replication direction or CpG methylation status. Templates with 20 CGG repeats were stable under all circumstances. These results reveal that CGG deletions occur during replication and are sensitive to replication-fork dynamics, tract length, and CpG methylation.

Mutagenic stress modulates the dynamics of CTG repeat instability associated with myotonic dystrophy type 1

The molecular basis of the myotonic dystrophy type 1 is the expansion of a CTG repeat at the DMPK locus. The expanded disease-associated repeats are unstable in both somatic and germ lines, with a high tendency towards expansion. The rate of expansion is directly related to the size of the pathogenic allele, increasing the size heterogeneity with age. It has also been suggested that additional factors, including as yet unidentified environmental factors, might affect the instability of the expanded CTG repeats to account for the observed CTG size dynamics over time. To investigate the effect of environmental factors in the CTG repeat instability, three lymphoblastoid cell lines were established from two myotonic dystrophy patients and one healthy individual, and parallel cultures were concurrently expanded in the presence or absence of the mutagenic chemical mitomycin C for a total of 12 population doublings. The new alleles arising along the passages were analysed by radioactive small pool PCR and sequencing gels. An expansion bias of the stepwise mutation was observed in a (CTG)124 allele of a cell line harbouring two modal alleles of 28 and 124 CTG repeats. Interestingly, this expansion bias was clearly enhanced in the presence of mitomycin C. The effect of mitomycin C was also evident in the normal size alleles in two cell lines with alleles of 13/13 and 12/69 repeats, where treated cultures showed new longer alleles. In conclusion, our results indicate that mitomycin C modulates the dynamics of myotonic dystrophy-associated CTG repeats in LBCLs, enhancing the expansion bias of long-pathogenic repeats and promoting the expansion of normal length repeats.

Middle latency auditory-evoked potentials in myotonic dystrophy: relation to the size of the CTG trinucleotide repeat and intelligent quotient

OBJECTIVE

Major components of MLAEPs are thought to originate in the temporal lobe. Absence of the Pb potential has been demonstrated in MLAEPs in Alzheimer's disease and demented Parkinson's disease patients. To validate usefulness of middle latency auditory-evoked potentials (MLAEPs) in evaluating the central nervous system (CNS) involvement of myotonic dystrophy (MyD).

METHODS

MLAEPs were recorded in eight patients with MyD and nine normal control subjects. In the patient group, the size of the CTG triplet repeat expansion within the dystrophia myotonica protein kinase (DMPK) gene and the revised Wechsler Adult Intelligence Scale (WAIS-R) were also assessed.

RESULTS

The latency of the Nb potential showed a significant correlation with the size of the CTG repeat expansion (r=0.734, P=0.036). The Pb latency also tended to prolong according to CTG amplification (r=0.644, P=0.087). The amplitudes of Na and Pa significantly increased compared with those of normal control subjects (P=0.024 and 0.016, respectively). However, they did not correlate with IQ or CTG amplification.

CONCLUSIONS

Abnormal MLAEPs may indicate CNS involvement in MyD. Although the precise generating mechanisms of Nb are unclear, the correlation of Nb latency with CTG amplification suggests that MLAEPs can reflect the extent of genetic abnormality.

A biologia molecular contribuindo para a compreensão e a prevenção das doenças hereditárias

O fim do seqüenciamento do genoma humano levanta inúmeras questões: Como o projeto genoma humano vai influenciar nossas vidas? Como a medicina tem se beneficiado do estudo dos genes? Quais são as aplicações práticas imediatas e o que se espera para o futuro? Quais são as implicações éticas? Este capítulo ilustra como as doenças genéticas têm contribuído para a compreensão do genoma humano. Ajuda-nos a entender como nossos genes funcionam quando normais e por que causam doenças quando alterados. Do ponto de vista prático, o estudo dos genes tem permitido o diagnóstico molecular para um número crescente de patologias, o que é fundamental para evitar outros exames invasivos, identificar casais em risco, e prevenir o nascimento de novos afetados. Além disso, discute-se quais são as perspectivas futuras em relação ao tratamento destas e de outras patologias genéticas incluindo a clonagem para fins terapêuticos e a utilização de células-tronco. Finalmente aborda as implicações éticas relacionadas ao uso de testes genéticos. Os benefícios de cada teste, principalmente para doenças de início tardio para as quais ainda não há tratamento, têm que ser discutidos exaustivamente com os consulentes antes de sua aplicação.

Myotonic dystrophy and myotonic dystrophy protein kinase

Myotonic dystrophy protein kinase (DMPK) was designated as a gene responsible for myotonic dystrophy (DM) on chromosome 19, because the gene product has extensive homology to protein kinase catalytic domains. DM is the most common disease with multisystem disorders among muscular dystrophies. The genetic basis of DM is now known to include mutational expansion of a repetitive trinucleotide sequence (CTG)n in the 3'-untranslated region (UTR) of DMPK. Full-length DMPK was detected and various isoforms of DMPK have been reported in skeletal and cardiac muscles, central nervous tissues, etc. DMPK is localized predominantly in type I muscle fibers, muscle spindles, neuromuscular junctions and myotendinous tissues in skeletal muscle. In cardiac muscle it is localized in intercalated dises and Purkinje fibers. Electron microscopically it is detected in the terminal cisternae of SR in skeletal muscle and the junctional and corbular SR in cardia muscle. In central nervous system, it is located in many neurons, especially in the cytoplasm of cerebellar Purkinje cells, hippocampal interneurons and spinal motoneurons. Electron microscopically it is detected in rough endoplasmic reticulum. The functional role of DMPK is not fully understood, however, it may play an important role in Ca2+ homeostasis and signal transduction system. Diseased amount of DMPK may play an important role in the degeneration of skeletal muscle in adult type DM. However, other molecular pathogenetical mechanisms such as dysfunction of surrounding genes by structural change of the chromosome by long trinucleotide repeats, and the trans-gain of function of CUG-binding proteins might be responsible to induce multisystemic disorders of DM such as myotonia, endocrine dysfunction, etc.

"Shake hands"; diagnosing a floppy infant--myotonic dystrophy and the congenital subtype: a difficult perinatal diagnosis

Myotonic dystrophy is a multi-organ disease inherited in a complicated way. Congenital myotonic dystrophy is a distinct entity with severe symptoms leading to a high rate of perinatal morbidity and mortality. The occurrence of congenital myotonic dystrophy often allows a subsequent diagnosis in the mother with important implications for her life, her further pregnancies and offspring. Genetic principles of anticipation and somatic mosaicism are involved and hamper the prenatal diagnostic possibilities. A family is presented in which maternal myotonic dystrophy and congenital myotonic dystrophy were diagnosed after the third pregnancy. The key features leading to the diagnosis were obstetric history, neonatal hypotonia and asphyxia, facial abnormalities in the mother together with the inability to bury eyelashes and delayed release of grip after shaking hands. The disorder is reviewed with respect to clinical symptoms, pathogenesis and genetics.

Clinical and genetic heterogeneity in myotonic dystrophies

This review of myotonic dystrophies primarily concentrates on the clinical and genetic findings that can distinguish a novel form of myotonic dystrophy, myotonic dystrophy type 2 (DM2); proximal myotonic myopathy (PROMM); and proximal myotonic dystrophy (PDM) from myotonic dystrophy type 1 (DM1). The multisystemic nature of these disorders leads to a spectrum of symptoms and signs. Careful clinical evaluation of patients with DM2/PROMM shows that the similarities among the multisystemic myotonic disorders outweigh the differences. An important point in the comparison of the phenotypes of DM1 and DM2/PROMM is that no severe congenital type of DM2/PROMM has yet been described. Genetic linkage analyses show that myotonic dystrophies can be divided into three types: the conventional Steinert type linked to chromosome 19q13.3 (DM1); DM2/PROMM and PDM linked to chromosome 3q21.3; and families not linked to either chromosomal site. Although the diagnosis may be clinically suspected, it depends on DNA analysis.

Lens epithelial changes and mutated gene expression in patients with myotonic dystrophy

AIMS

Examination of the expression of the mutated allele of myotonic dystrophy protein kinase gene and lens epithelial cell changes in patients with myotonic dystrophy.

METHODS

Six eyes from three patients with myotonic dystrophy underwent cataract surgery. The lens epithelium was photographed to examine the morphological changes. mRNAs were extracted to determine myotonic dystrophy protein kinase gene expression in the lens epithelium and peripheral blood. Age matched lens epithelial cells from senile cataracts were used as controls.

RESULTS

All eyes showed iridescent or posterior subcapsular lens opacity. The expression of the myotonic dystrophy protein kinase gene with trinucleotide repeat expansion was evaluated by reverse transcriptase polymerase chain reaction, Southern blotting, and sequence analysis. Lens epithelial cell densities were extremely reduced in the patients compared with the control group.

CONCLUSION

To the authors' knowledge, this is the first report to describe the relation between lens epithelial cell changes and mutated gene expression in patients with myotonic dystrophy. The gene may be mitotically unstable in the lens epithelial cells; it may influence cell density and lens epithelial function, and it may lead to the development of typical subcapsular lens opacity.

A longitudinal physical profile assessment of skeletal muscle manifestations in myotonic dystrophy

OBJECTIVES

To develop an assessment that describes the skeletal muscle manifestations in myotonic dystrophy subjects and then use it to quantify the presentation of skeletal muscle disability and to show change over time.

DESIGN

A quantified skeletal muscle assessment was developed and applied three times over a two-year period at intervals around 12 months. Thirty-six subjects with myotonic dystrophy and 20 subjects without neuromuscular disability were evaluated. The assessment comprised manual muscle testing of five pairs of muscles, measuring neck flexor strength with a strain gauge, respiratory function tests, power and lateral pinch grip strength, all tests of impairment. Assessment of the ability to move from sitting to standing and fasten buttons tested disability.

RESULTS

Results from subjects with myotonic dystrophy were compared to the normal data. The subjects with myotonic dystrophy were significantly weaker in proximal upper limb muscles, quadriceps, tibialis anterior muscles and neck flexor muscles as well as power and lateral pinch grips. There was also significant reduction in forced expiratory volume at one second (FEV1) and forced vital capacity (FVC). Significant disability was seen in the myotonics in moving from sitting to standing and in fastening buttons. Over the two-year study period proximal upper limb and lower limb muscle strength, FVC and sit-to-stand ability declined significantly. Power grip declined but lateral pinch grip and FEV1 improved significantly. Button fastening ability improved significantly.

CONCLUSION

The test developed was shown to be reliable and sensitive to the change in skeletal muscle manifestations in subjects with myotonic dystrophy who were shown to be significantly weaker than normal subjects.

Fragile-X syndrome and myotonic dystrophy: parallels and paradoxes

Fragile-X syndrome and myotonic dystrophy are caused by triplet repeat expansions embedded in CpG islands in the transcribed non-coding regions of the FMR1 and the DMPK genes, respectively. Although initial reports emphasized differences in the mechanisms by which the expanded triplet repeats caused these diseases, results published in the past year highlight remarkable parallels in the likely molecular etiologies. At both loci, expansion is associated with altered chromatin, aberrant methylation, and suppressed expression of the adjacent FMR1 and DMAHP genes, implicating epigenetic mediation of these genetic diseases.

Myotonic dystrophy: molecular windows on a complex etiology

Myotonic dystrophy (DM) is the most common form of adult onset muscular dystrophy, with an incidence of approximately 1 in 8500 adults. DM is caused by an expanded number of trinucleotide repeats in the 3'-untranslated region (UTR) of a cAMP-dependent protein kinase (DM protein kinase, DMPK). Although a large number of transgenic animals have been generated with different gene constructions and knock-outs, none of them faithfully recapitulates the multisystemic and often severe phenotype seen in human patients. The transgenic data suggest that myotonic dystrophy is not caused simply by a biochemical deficiency or abnormality in the DM kinase gene product. Emerging studies suggest that two novel pathogenetic mechanisms may play a role in the disease: the expanded repeats appear to cause haploinsufficiency of a neighboring homeobox gene and also abnormal DMPK RNA appears to have a detrimental effect on RNA homeostasis. The complex, multisystemic phenotype may reflect an underlying multifaceted molecular pathophysiology: the facial dysmorphology may be due to pattern defects caused by haploinsufficiency of the homeobox gene, while the muscle disease and endocrine abnormalities may be due to both altered RNA metabolism and deficiency of the cAMP DMPK protein.

The neuropathology of CAG repeat diseases: review and update of genetic and molecular features

Classification of inherited neurodegenerative diseases is increasingly based on their genetic features, which supplement, clarify, and sometimes replace the older clinical and pathologic schemata. This change has been particularly rapid and impressive for the CAG repeat disorders. In Huntington's disease, X-linked spinobulbar muscular atrophy, dentatorubropallidoluysian atrophy, and a series of autosomal dominant cerebellar atrophies, genetic advances have resolved many nosologic issues, and opened new avenues for exploration of pathogenesis. In this review, we summarize classic and current concepts in neuropathology of these CAG repeat diseases.

Somatic instability of the myotonic dystrophy (CTG)n repeat during human fetal development

Myotonic dystrophy is characterised by the striking level of somatic heterogeneity seen between and within tissues of the same patient, which probably accounts for a significant proportion of the pleiotropy associated with this disorder. The congenital form of the disease is associated with the largest (CTG)n repeat expansions. We have investigated the timing of instability of myotonic dystrophy (CTG)n repeats in a series of congenitally affected fetuses and neonates. We find that during the first trimester the repeat is apparently stable and that instability only becomes detectable during the second and third trimesters. In our series repeat instability is apparent only after 13 weeks gestational age and before 16 weeks. The appearance of heterogeneity shows some tissue specificity, with heart most commonly having the largest expansion. The degree of heterogeneity is not correlated with initial expansion size as gauged by chorionic villus and blood (CTG)n repeat sizes.

Prediction of myotonic dystrophy clinical severity based on the number of intragenic [CTG]n trinucleotide repeats

We carried out a genotype-phenotype correlation study, based on clinical findings in 465 patients with myotonic dystrophy (DM), in order to assess [CTG] repeat number as a predictive test of disease severity. Our analysis showed that the DM subtypes defined by strict clinical criteria fall into three different classes with a log-normal distribution. This distribution is useful in predicting the probability of specific DM phenotypes based on triplet [CTG] number. This study demonstrates that measurement of triplet expansions in patients' lymphocyte DNA is highly valuable and accurate for prognostic assessment.

Myotonic dystrophy: will the real gene please step forward!

The mutation underlying myotonic dystrophy (DM) was identified at the end of 1991 amidst great rejoicing from the patients supporting the research and, not least, from those who spent so long searching for it. Subsequently, the molecular genetic phenomena associated with DM have been clearly explained by the transmission behaviour of the expanding repeat, which remains the only mutation that has been described in patients. We understand the molecular basis of anticipation, why the severe congenital form is almost exclusively transmitted by affected mothers and we have widely accepted models of the population genetics of DM. Yet, despite all these clearly explained molecular events, we appear to be hardly any closer to understanding the molecular pathology of DM than when the mutation was first identified. To understand the reason for this, we have to look in detail at the mutation itself, and in particular at the locus and its complex nuances. In doing so, we begin to realise that DM is unique amongst the Mendelianly inherited disorders, in that the mutation, because of its location in a very gene-rich region of the genome, probably simultaneously renders several genes dysfunctional. The somatic heterogeneity of the repeat, coupled with the involvement of several genes, accounts for the pleiotropy observed in the phenotype. Added to this complexity is the uncertainty of the level at which gene dysfunction or gain of function is occurring. It is possibly at the level of DNA/chromatin structure and/or RNA regulation/processing, and all of these pathways may, in different tissues, contribute to the final phenotype.

Monozygotic twins with 22q11 deletion and discordant phenotypes

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CTG repeat length in muscle from patients affected with myotonic dystrophy (DM)

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