Sex-related difference in intergenerational expansion of myotonic dystrophy gene

Genetic and Epigenetic Interplay Define Disease Onset and Severity in Repeat Diseases

Repeat diseases, such as fragile X syndrome, myotonic dystrophy, Friedreich ataxia, Huntington disease, spinocerebellar ataxias, and some forms of amyotrophic lateral sclerosis, are caused by repetitive DNA sequences that are expanded in affected individuals. The age at which an individual begins to experience symptoms, and the severity of disease, are partially determined by the size of the repeat. However, the epigenetic state of the area in and around the repeat also plays an important role in determining the age of disease onset and the rate of disease progression. Many repeat diseases share a common epigenetic pattern of increased methylation at CpG islands near the repeat region. CpG islands are CG-rich sequences that are tightly regulated by methylation and are often found at gene enhancer or insulator elements in the genome. Methylation of CpG islands can inhibit binding of the transcriptional regulator CTCF, resulting in a closed chromatin state and gene down regulation. The downregulation of these genes leads to some disease-specific symptoms. Additionally, a genetic and epigenetic interplay is suggested by an effect of methylation on repeat instability, a hallmark of large repeat expansions that leads to increasing disease severity in successive generations. In this review, we will discuss the common epigenetic patterns shared across repeat diseases, how the genetics and epigenetics interact, and how this could be involved in disease manifestation. We also discuss the currently available stem cell and mouse models, which frequently do not recapitulate epigenetic patterns observed in human disease, and propose alternative strategies to study the role of epigenetics in repeat diseases.

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.

Molecular genetics of congenital myotonic dystrophy

Myotonic Dystrophy type 1 (DM1) is a neuromuscular disease showing strong genetic anticipation, and is caused by the expansion of a CTG repeat tract in the 3'-UTR of the DMPK gene. Congenital Myotonic Dystrophy (CDM1) represents the most severe form of the disease, with prenatal onset, symptoms distinct from adult onset DM1, and a high rate of perinatal mortality. CDM1 is usually associated with very large CTG expansions, but this correlation is not absolute and cannot explain the distinct clinical features and the strong bias for maternal transmission. This review focuses upon the molecular and epigenetic factors that modulate disease severity and might be responsible for CDM1. Changes in the epigenetic status of the DM1 locus and in gene expression have recently been observed. Increasing evidence supports a role of a CTCF binding motif as a cis-element, upstream of the DMPK CTG tract, whereby CpG methylation of this site regulates the interaction of the insulator protein CTCF as a modulating trans-factor responsible for the inheritance and expression of CDM1.

CpG Methylation, a Parent-of-Origin Effect for Maternal-Biased Transmission of Congenital Myotonic Dystrophy

CTG repeat expansions in DMPK cause myotonic dystrophy (DM1) with a continuum of severity and ages of onset. Congenital DM1 (CDM1), the most severe form, presents distinct clinical features, large expansions, and almost exclusive maternal transmission. The correlation between CDM1 and expansion size is not absolute, suggesting contributions of other factors. We determined CpG methylation flanking the CTG repeat in 79 blood samples from 20 CDM1-affected individuals; 21, 27, and 11 individuals with DM1 but not CDM1 (henceforth non-CDM1) with maternal, paternal, and unknown inheritance; and collections of maternally and paternally derived chorionic villus samples (7 CVSs) and human embryonic stem cells (4 hESCs). All but two CDM1-affected individuals showed high levels of methylation upstream and downstream of the repeat, greater than non-CDM1 individuals (p = 7.04958 × 10⁻¹²). Most non-CDM1 individuals were devoid of methylation, where one in six showed downstream methylation. Only two non-CDM1 individuals showed upstream methylation, and these were maternally derived childhood onset, suggesting a continuum of methylation with age of onset. Only maternally derived hESCs and CVSs showed upstream methylation. In contrast, paternally derived samples (27 blood samples, 3 CVSs, and 2 hESCs) never showed upstream methylation. CTG tract length did not strictly correlate with CDM1 or methylation. Thus, methylation patterns flanking the CTG repeat are stronger indicators of CDM1 than repeat size. Spermatogonia with upstream methylation may not survive due to methylation-induced reduced expression of the adjacent SIX5, thereby protecting DM1-affected fathers from having CDM1-affected children. Thus, DMPK methylation may account for the maternal bias for CDM1 transmission, larger maternal CTG expansions, age of onset, and clinical continuum, and may serve as a diagnostic indicator.

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.

CTG-repeat length in distal and proximal leg muscles of symptomatic and non-symptomatic patients with myotonic dystrophy: relation to muscle strength and degree of histopathological abnormalities

Myotonic dystrophy (DM) is an autosomal dominant, multisystemic disorder with a variable phenotypic expression including muscle weakness and myotonia. The muscle wasting is most marked in distal limbs and in facial and neck muscles, although proximal limb muscles become affected as the disease progresses. The CTG-trinucleotide-repeat expansion associated with myotonic dystrophy is usually larger in muscle tissue than in leukocytes. It is unclear whether the repeat length itself bears any relation to the differences in the degree of weakness and atrophy between different muscles. We therefore analysed CTG-repeat lengths in blood and in proximal (m. vastus lateralis) and distal (m. tibialis anterior) muscles of patients with DM (n = 4) and non-symptomatic carriers of the mutant DM allele (n = 2) using conventional Southern blot hybridization. Muscle strength and histopathological abnormalities were evaluated for each muscle. In patients with clinical symptoms, the degree of paresis and morphological abnormalities was markedly more pronounced in m. tibialis anterior than in m. vastus lateralis. In these individuals, the CTG-repeat length was larger in muscles than in leukocytes, whereas in the two non-symptomatic carriers no difference could be detected. Furthermore, there was no clear difference in the repeat length between the two muscles in any of the patients. In conclusion, the selective muscular weakness and atrophy in DM do not seem to be related to differences in CTG-repeat length between different muscles.

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.

Homozygous myotonic dystrophy: clinical and molecular studies of three unrelated cases

We report the clinical and molecular study of three unrelated homozygous myotonic dystrophy patients. In the first family, the homozygous patient shows the classical form of the disease with two DM alleles of very different expansion sizes (1000 and 60 repeats). In the second family, the homozygous patient is mildly affected and carries a minimally expanded allele (64 repeats) and a "normal" allele (38 repeats) that increases in size when transmitted. Such an intergenerational expansion of an allele in this range of repeats has not been reported to date. The third homozygous case has late onset bilateral cataracts as the only symptom. She has two minimally expanded alleles (51 and 120 repeats) that showed different intergenerational enlargement during transmission to the next generation.

Frequency of intergenerational contractions of the CTG repeats in myotonic dystrophy

Myotonic dystrophy (MD), an autosomal dominant multisystemic disorder with a high phenotypic variability, is the most common muscular dystrophy in adult life. The mutation underlying DM has been characterized as an expanded CTG trinucleotide repeat sequence in the 3 untranslated region of a protein kinase gene on chromosome 19q13.2-13.3. We have analyzed the presence of CTG intergenerational variations on transmission in parent-child pairs affected with DM. The series includes 90% of all living affected descendants (symptomatic or asymptomatic) from a given myotonic dystrophy (DM) patient. A contraction of the CTG repeat size was observed in ten parent-child pairs (14.1%) and remained unchanged in five (7%) pairs. The number of CTG repeats decreased in 2/30 maternal transmissions (6.7%) and in 8/41 paternal transmissions (19.5%). We found 14 asymptomatic individuals carrying the CTG expansion among the offspring. In six of them, a contraction of the CTG repeat was observed, and in all six cases, the DM allele was paternally transmitted. Since nearly all the asymptomatic family members of DM patients were analyzed in this series, the observed percentage of contractions can be considered more realistic, even though the number of parent-child pairs is small.

Localization of myotonic dystrophy protein kinase in skeletal muscle and its alteration with disease

Myotonic dystrophy (DM) is an autosomal dominant disorder which affects skeletal muscle, heart, eye lens, brain, and endocrine functions. The disease-causing mutations are expansions of the triplet repeat CTG in the 3' untranslated region of a locus which encodes a serine/threonine protein kinase that represents a new family of protein kinases. A monoclonal antibody to a recombinant DM protein kinase (mAb DM-1) reacts specifically with the 64 kDa isoform of DM protein kinase in type I fibers in skeletal muscle, the fiber type which characteristically atrophies in the disease. Within type I fibers of normal muscle the isoform may be localized with mAb DM-1 to the triad region. In the DM disease state, the enzyme is redistributed to the pathologically characteristic peripheral sarcoplasmic masses. In markedly affected human distal myotonic muscle, the levels of the 64 kDa DM kinase isoform are elevated relative to slow skeletal myosin heavy chain. These results suggest that, consistent with the dominant clinical phenotype, the localization and accumulation of the 64 kDa isoform are altered in the heterozygous disease state.

Influence of the sex of the transmitting grandparent in congenital myotonic dystrophy

To analyse the influence of the sex of the transmitting grandparents on the occurrence of the congenital form of myotonic dystrophy (CDM), we have studied complete three generation pedigrees of 49 CDM cases, analysing: (1) the sex distribution in the grandparents' generation, and (2) the intergenerational amplification of the CTG repeat, measured in its absolute and relative values, between grandparents and the mothers of CDM patients and between the latter and their CDM children. The mean relative intergenerational increase in the 32 grandparent-mother pairs was significantly greater than in the 56 mother-CDM pairs (Mann-Whitney U test, p < 0.001). The mean expansion of the grandfathers (103 CTG repeats) was also significantly different from that seen in the grandmothers' group (154 CTG repeats) (Mann-Whitney U test, p < 0.01). This excess of non-manifesting males between the CDM grandparents' generation with a smaller CTG length than the grandmothers could suggest that the premutation has to be transmitted by a male to reach the degree of instability responsible for subsequent intergenerational CTG expansions without size constraints characteristic of the CDM range.

Postzygotic instability of the myotonic dystrophy p[AGC] in repeat supported by larger expansions in muscle and reduced amplifications in sperm

We have analysed the [AGC] expansion in leucocytes, muscle and sperm from 17 individuals affected by myotonic dystrophy (DM). Skeletal muscle showed a larger repeat number than leucocytes in the same patient. A similar degree of expansion was detected in differently affected muscles of a single patient. The germline mutation (< or = 350 repeats) was expanded in somatic cells of the progeny in all patients examined. Our results provide evidence of an early postzygotic instability of the [AGC] repeat in DM.

Discordant clinical outcome in myotonic dystrophy relatives showing (CTG)n > 700 repeats

A myotonic dystrophy (DM) family is described in which discordant DM phenotypes were found in the children of two affected sisters with similar CTG expansion and clinical manifestations. In this family, congenital as well as early severe childhood and later childhood onset DM coexist. This observation strengthens the limited ability of lymphocytes CTG repeat number analysis in predicting genotype-phenotype correlations in DM patients.

Contribution of molecular analyses to the estimation of the risk of congenital myotonic dystrophy

A molecular analysis of the maternal and child CTG repeat size and intergenerational amplification was performed in order to estimate the risk of having a child with congenital myotonic dystrophy (CMD). In a study of 124 affected mother-child pairs (42 mother-CMD and 82 mother-non-CMD) the mean maternal CTG allele in CMD cases was three times higher (700 repeats) than in non-CMD cases (236 repeats). When the maternal allele was in the 50-300 repeats range, 90% of children were non-CMD. In contrast, when the maternal allele was greater than 300 repeats, 59% inherited the congenital form. Furthermore, the risk of having a CMD child is also related to the intergenerational amplification, which was significantly greater in the mother-CMD pairs than in the mother-non-CMD pairs. Although the risk of giving birth to a CMD child always exists for affected mothers, our data show that such a risk is considerably higher if the maternal allele is greater than 300 repeats.

Triplet repeats in neuromuscular disorders

SBMA, SCA1 and DM are all neuromuscular disorders which have very different presentation with varying degrees of severity but with a common feature of muscular weakness. In addition they all show anticipation correlating with the length of an unstable trinucleotide repeat. It is possible that trinucleotide repeat expansions will underlie other hereditary neuromuscular disorders, for example spinocerebellar ataxia type 2 (mapped to 12q and with similar symptoms to SCA1 including anticipation) is a prime candidate (30). The biochemical effects of these expansions have not yet been elucidated and therefore effective therapeutic interventions cannot currently be designed for affected individuals. However, the relative ease of diagnosis and the availability of accurate prenatal testing have already had a significant impact on the patient groups.

Characteristics of dynamic mutation in Japanese myotonic dystrophy

To study the characteristics, if any, of unstable CTG repeat sequence in Japanese myotonic dystrophy (DM), we analyzed DNA from 351 at risk individuals (including affected and non-affected carriers and their descendants) from 105 families in Japan. A total of 93 DM families (196 affected and 116 unaffected individuals), including 84 DM parent-child pairs (44 father-child and 40 mother-child pairs), were examined, many of which had been previously tested by linkage analysis. We detected unstable CTG repeat mutations between 0.15 kb and 8.7 kb in size. The size of the mutation correlated with the age of onset of symptoms. There was a significant difference in DM allele size among the four groups (congenital, juvenile onset, classical, and minimal). Congenital DM had on average the largest repeat sizes. Comparison of parent-child pairs showed that most offspring had a larger repeat size than their parents, with only 2 of 84 showing a definite decrease in repeat size. The correlation coefficients for maternal and paternal transmission were 0.41 and 0.15, respectively. The parental age (maternal and paternal) did not correlate with intergenerational change of repeat. These observations are similar to those reported in Caucasians.

A case of paternally inherited congenital myotonic dystrophy

We report two sisters with congenital myotonic dystrophy (CDM) born to a normal mother and an affected father. The congenitally affected daughters had symptoms from birth. The age of onset of DM in the father was 39 years. Analysis of the CTG trinucleotide expansion in this family showed increase in the repeat length with increasing severity, with the smallest expansion in the grandfather and the largest expansion in the younger of the two CDM sisters. This family shows that exceptionally it is possible for CDM to be inherited paternally and refutes the hypothesis that CDM is exclusively of maternal origin. This contradicts several of the previous hypotheses concerning the mechanisms by which the CDM phenotype arises.

Myotonic dystrophy patients have larger CTG expansions in skeletal muscle than in leukocytes

The genetic basis of myotonic dystrophy is an unstable expansion of CTG repeats located in a gene on chromosome 19 that encodes a putative serine/threonine protein kinase. We studied the somatic mosaicism of the (CTG)n expansion in myotonic dystrophy patients. (CTG)n expansions were 2- to 13-fold greater in DNA isolated from skeletal muscle than in DNA from leukocytes in 10 of 11 patients with myotonic dystrophy. Different muscles of the same individual showed similar (CTG)n expansions. In postmortem tissues from an adult patient, (CTG)n expansions in brain, skeletal muscle, cardiac muscle, testes, and liver were all greater than in leukocytes. Normal myotonic dystrophy gene alleles from 7 healthy subjects had the same number of CTG repeats in leukocytes and muscle. The myotonic dystrophy mutation displays pronounced heterogeneity in somatic cells. The (CTG)n expansion observed in peripheral blood leukocytes is not necessarily representative of the repeat expansion in affected tissues, such as skeletal muscle and myocardium. In some patients with myotonic dystrophy, the predictive value of genetic analysis based on leukocyte DNA may be limited.