Presymptomatic diagnosis of myotonic dystrophy

Neurocognitive Features of Motor Premanifest Individuals With Myotonic Dystrophy Type 1

Objective

The goal of the study was to identify brain and functional features associated with premanifest phases of adult-onset myotonic dystrophy type 1 (i.e., PreDM1).

Methods

This cross-sectional study included 68 healthy adults (mean age = 43.4 years, SD = 12.9), 13 individuals with PreDM1 (mean age: 47.4 years, SD = 16.3), and 37 individuals with manifest DM1 (mean age = 45.2 years, SD = 9.3). The primary outcome measures included fractional anisotropy (FA), motor measures (Muscle Impairment Rating Scale, Grooved Pegboard, Finger-Tapping Test, and grip force), general cognitive abilities (Wechsler Adult Intelligence Scales), sleep quality (Scales for Outcomes in Parkinson's Disease–Sleep), and apathy (Apathy Evaluation Scale).

Results

Individuals with PreDM1 exhibited an intermediate level of white matter FA abnormality, where whole-brain FA was lower relative to healthy controls (difference of the estimated marginal mean [EMM_difference] = 0.02, 95% confidence interval (CI) 0.01–0.03, p < 0.001), but the PreDM1 group had significantly higher FA than did individuals with manifest DM1 (EMM_difference = 0.02, 95% CI 0.009–0.03, p < 0.001). Individuals with PreDM1 exhibited reduced performance on the finger-tapping task relative to control peers (EMM_difference = 5.70, 95% CI 0.51–11.00, p = 0.03), but performance of the PreDM1 group was better than that of the manifest DM1 group (EMM_difference = 5.60, 95% CI 0.11–11.00, p = 0.05). Hypersomnolence in PreDM1 was intermediate between controls (EMM_difference = −1.70, 95% CI −3.10–0.35, p = 0.01) and manifest DM1 (EMM_difference = −2.10, 95% CI −3.50–0.60, p = 0.006).

Conclusions

Our findings highlight key CNS and functional deficits associated with PreDM1, offering insight in early disease course.

Predictors of prognosis in type 1 myotonic dystrophy (DM1): longitudinal 18-years experience from a single center

The aim of the study was to identify possible predictors of neurological worsening and need of non-invasive ventilation (NIV) in individuals affected by myotonic dystrophy type 1 (DM1), the most common form of adult-onset muscular dystrophy.

Core Clinical Phenotypes in Myotonic Dystrophies

Myotonic dystrophy type 1 (DM1) and type 2 (DM2) represent the most frequent multisystemic muscular dystrophies in adulthood. They are progressive, autosomal dominant diseases caused by an abnormal expansion of an unstable nucleotide repeat located in the non-coding region of their respective genes DMPK for DM1 and CNBP in DM2. Clinically, these multisystemic disorders are characterized by a high variability of muscular and extramuscular symptoms, often causing a delay in diagnosis. For both subtypes, many symptoms overlap, but some differences allow their clinical distinction. This article highlights the clinical core features of myotonic dystrophies, thus facilitating their early recognition and diagnosis. Particular attention will be given to signs and symptoms of muscular involvement, to issues related to respiratory impairment, and to the multiorgan involvement. This article is part of a Special Issue entitled “Beyond Borders: Myotonic Dystrophies—A European Perception.”

Myotonic dystrophy types 1 and 2

Myotonic dystrophies (dystrophia myotonica, or DM) are inherited disorders characterized by myotonia and progressive muscle degeneration, which are variably associated with a multisystemic phenotype. To date, two types of myotonic dystrophy, type 1 (DM1) and type 2 (DM2), are known to exist; both are autosomal dominant disorders caused by expansion of an untranslated short tandem repeat DNA sequence (CTG)(n) and (CCTG)(n), respectively. These expanded repeats in DM1 and DM2 show different patterns of repeat-size instability. Phenotypes of DM1 and DM2 are similar but there are some important differences, most conspicuously in the severity of the disease (including the presence or absence of the congenital form), muscles primarily affected (distal versus proximal), involved muscle fiber types (type 1 versus type 2 fibers), and some associated multisystemic phenotypes. The pathogenic mechanism of DM1 and DM2 is thought to be mediated by the mutant RNA transcripts containing expanded CUG and CCUG repeats. Strong evidence supports the hypothesis that sequestration of muscle-blind like (MBNL) proteins by these expanded repeats leads to misregulated splicing of many gene transcripts in corroboration with the raised level of CUG-binding protein 1. However, additional mechanisms, such as changes in the chromatin structure involving CTCN-binding site and gene expression dysregulations, are emerging. Although treatment of DM1 and DM2 is currently limited to supportive therapies, new therapeutic approaches based on pathogenic mechanisms may become feasible in the near future.

The expanding world of myotonic dystrophies: how can they be detected?

Myotonic dystrophy (DM) comprises at least two genetically distinct forms, both of which are caused by expansions of microsatellite repeats. The expansion of a CTG repeat in the DMPK gene leads to the first genetic form (DM type 1), and the expansion of a CCTG repeat in the ZNF9 gene causes the second genetic form of the disease (DM type 2). In both cases, the repeat units may expand to several thousand repeats, and the number of repeats in the expanded alleles shows a high degree of meiotic and somatic instability. The unprecedented size of expansions and their dynamic nature still represents a diagnostic challenge, which has been facilitated using different methods and modifications since the identification of the underlying mutations of these disorders. Here, we present an overview of the basic methods described for the purpose of identification of the DM type 1 and DM type 2 expansions and discuss particular modifications and improvements implemented to extend the detection ranges of these methods. Our review focuses on the advantages and disadvantages of the methods based on Southern blot analysis, polymerase chain reaction amplification, and in situ hybridization techniques and also on the possibilities of preimplantation and prenatal genetic testing.

CTG repeats at the myotonic protein kinase gene in a healthy Chilean population sample

OBJECTIVES

To study the variability at the myotonic dystrophy protein kinase (DMPK) gene in a Chilean sample of healthy people. DM1 is an autosomal dominant disorder caused by an expansion of a (CTG) repeat at the 3'-UTR of the gene DMPK. Healthy individuals have alleles under 35 repeats and diseased individuals have over 50.

METHODS

Genotyping the number of (CTG) repeats at this gene in a sample of healthy Chilean people.

RESULTS

Allele frequencies were significantly different from those of other populations. The most frequent allele was with five repeats. The frequency of larger alleles (>18 CTG repeats) was 11%, close to the European frequency (12%) and higher than the Japanese (8%) and Aboriginal Pehuenche samples (8%).

CONCLUSIONS

Allelic frequencies in the Chilean sample studied were intermediate between those of the two ancestral populations (European and Pehuenche).

Myotonic dystrophy associated with QT prolongation and torsade de pointes

A rare case of myotonic dystrophy (MD) with congestive heart failure, associated with QT prolongation and torsade de pointes (TdP) is reported. A 53-year-old woman was admitted to the hospital because of congestive heart failure. Electrocardiograph (ECG) showed first-degree atrioventricular block and QT prolongation. During hospitalization, TdP appeared but returned to sinus rhythm spontaneously. As the patient had quadriplegia, a myopathic face, cataracts, diabetes mellitus, and an increased number of cytosine-thymineguanine (CTG) repeats (760 repeats), she was diagnosed as having MD. Electrocardiographic analysis of her family also revealed abnormal QT(U) prolongation in her daughter and brother who both had MD, while ECG findings of other family members without MD were normal. Thus, the presence of QT(U) prolongation was associated with MD in this family.

DMPK-associated myotonic dystrophy and CTG repeats in Alabama African Americans

Myotonic dystrophy type 1 (DM1) is a result of a CTG expansion in the 3'-untranslated region of the DMPK gene. DM1 is rare among African blacks who have fewer large CTG repeats in the normal range than other racial/ethnic groups. Neither the prevalence of DM1 nor the relationship of CTG expansion to clinical status in African Americans (AAs) is well documented. We describe two AA brothers with DM1, each of whom had CTG repeats of 5/639; their father was reported to have DM1 and had CTG repeats of 5/60. Other family members had CTG repeats of 5-14. An unrelated AA patient from a second kinship also had DM1; an analysis revealed CTG repeats of 27/191. In 161 Alabama AA control subjects, we observed 18 CTG alleles from 5 to 28 repeats; the most common allele had five CTG repeats. The frequency of CTG repeats >or=15 were greater (p < 0.0003) in Pygmy, Amhara Ethiopian, Ashkenazi Jewish, North African Jewish, Israeli Muslim Arab, European white, and Japanese populations than in the Alabama AA population. These data suggest that the risk for DM1 in AAs is intermediate between that of African blacks and whites of European descent.

Patients with primary cataract as a genetic pool of DMPK protomutation

Myotonic dystrophy 1 (DM1) is known to diminish reproductive fitness in its severe form. Since no de novo mutations are known for this disease, it has the tendency to become extinct from a population. To explain the preservation of DM1 in a population, a hypothesis that a pool of subjects for the mutated gene exists in the apparently healthy (non-DM1) population was tested. In order to determine the (CTG) repeat number, PCR was performed in 274 patients found to have primary cataract of adult onset who showed no DM1 symptoms, and were not related to DM1 patients. In four cataract patients (1.46%; 95% CI 0.5-3.7), a protomutation in the myotonin protein kinase gene was found which might lead to a complete mutation after transmission through the next generations. The number of (CTG) repeats in the remaining 270 cataract patients did not differ significantly from the control subjects in terms of the distribution of larger [(CTG)n > or = 19] versus smaller [(CTG)n < 19] alleles. We consider the primary cataract patients to be the pool of DMPK protomutation from which DM1 mutation is maintained in the population.

Simple Procedure for Automatic Detection of Unstable Alleles in the Myotonic Dystrophy and Huntington's Disease Loci

Human neurodegenerative and neuromuscular disorders are associated with a class of gene mutations represented by expansion of trinucleotide repeats. DNA testing is important for the diagnosis of these diseases because clinical discrimination is complicated by their late onset and frequently overlapping symptomatology. However, detection of pathologic alleles expanded up to several thousand trinucleotides poses a challenge for the introduction of rapid, fully automatic, and simple DNA diagnostic procedures. Here we propose a simple two-step polymerase chain reaction (PCR) protocol for rapid molecular diagnostics of myotonic dystrophy, Huntington's disease, and possibly also other triplet expansion diseases. Standard PCR amplification with target repeat flanking primers is used for the detection of alleles of up to 100 repeats; next, triplet-primed PCR is applied for detection of larger expansions. Automated capillary electrophoresis of amplicons allows rapid discrimination between normal, premutated and expanded (CTG/CAG)(n) alleles. Using the suggested protocol, the expanded allele was successfully detected in all test DNA samples with known genotypes. Our experience demonstrates that the suggested two-step PCR protocol provides high sensitivity, specificity, and reproducibility; is significantly less time-consuming; is easier to perform; and provides a better basis for automation than previous methods requiring Southern analysis. Therefore, it can be used for confirmation of uncertain clinical diagnoses, for prenatal testing in at-risk families, and, generally in research on these diseases.

Anatomy of a founder effect: myotonic dystrophy in Northeastern Quebec

Founder effects are largely responsible for changes in frequency profiles of genetic variants in local populations or isolates. They are often recognized by elevated incidence of certain hereditary disorders as observed in regions of Charlevoix and Saguenay-Lac-Saint-Jean (SLSJ) in Northeastern Quebec. Dominantly transmitted myotonic dystrophy (DM1) is highly prevalent in SLSJ where its carrier rate reaches 1/550, compared with 1/5,000 to 1/50,000 elsewhere. To shed light on the origin of DM1 in this region, we have screened 50 nuclear DM1 families from SLSJ and studied the genetic variation in a 2.05 Mb (2.9 cM) segment spanning the site of the expansion mutation. The markers analyzed included 22 biallelic SNPs and two microsatellites. Among 50 independent DM1 chromosomes, we distinguished ten DM1-associated haplotypes and grouped them into three haplotype families, A, B and C, based on the relevant extent of allele sharing between them. To test whether the data were consistent with a single entry of the mutation into SLSJ, we evaluated the age of the founder effect from the proportion of recombinant haplotypes. Taking the prevalent haplotype A1_21 (58%) as ancestral to all the disease-associated haplotypes in this study, the estimated age of the founder effect was 19 generations, long predating the colonization of Nouvelle-France. In contrast, considering A1_21 as ancestral to the haplotype family A only, yielded the estimated founder age of nine generations, consistent with the settlement of Charlevoix at the turn of 17th century and subsequent colonization of SLSJ. We conclude that it was the carrier of haplotype A (present day carrier rate of 1/730) that was a "driver" of the founder effect, while minor haplotypes B and C, with corresponding carrier rates of 1/3,000 and 1/10,000, respectively, contribute DM1 to the incidence level known in other populations. Other studies confirm that this might be a general scenario in which a major "driver" mutation/haplotype issued from a founder effect is found accompanied by distinct minor mutations/haplotypes occurring at background population frequencies.

Relation of CTG expansion and clinical variables to electrocardiogram conduction abnormalities and sudden death in patients with myotonic dystrophy

We prospectively followed 63 patients with myotonic dystrophy (DM) after establishing diagnosis of DM for an average 8 years in an attempt to detect conduction disturbances (by electrocardiography and/or Holter monitoring) and sudden cardiac events (sudden death, cardiac syncope) and correlate them to potential predicting factors (CTG repeat expansion in the myotonin protein kinase gene and several clinical variables: clinical type and duration of DM, age and sex). Twenty-six patients developed conduction disturbances, five patients died suddenly, and two patients experienced cardiac syncope necessitating urgent implantation of pacemaker. Analysis showed no significant correlation between conduction disturbances and/or cardiac events and CTG expansion. Furthermore, no correlation was found with type of DM, whereas conduction disturbances and sudden cardiac events correlated with patients' age, duration of disease and male sex. Results on our cohort of DM patients show that CTG expansion has no role in predicting neither conduction abnormalities nor sudden death. It seems that risk of sudden death increases with duration of disease and age, and that risk is higher in male patients.

Paternal transmission of the congenital form of myotonic dystrophy type 1: a new case and review of the literature

Myotonic dystrophy type 1 (DM1) is an autosomal dominant trinucleotide repeat disorder that shows anticipation. The mildest manifestations of the DM gene are usually noted in individuals with the smallest repeat sizes, while congenital myotonic dystrophy (CDM) is the most common clinical outcome of the larger expansions. For many years, it was thought that CDM could only be maternally transmitted. However, in the last few years, cases of paternal transmission of CDM have been described. We report a child with the CDM phenotype and 1, 800 CTG repeats born to an asymptomatic father with 65 repeats and compare this case to the four currently in the literature. We note that polyhydramnios was present in the majority of cases and that all fathers whose status was known had small repeat sizes and/or were asymptomatic at the time of their child's birth. Although it may be unusual, the possibility of the paternal transmission of CDM should be mentioned when counseling families with DM. The men who are at highest risk may be those who have small repeats sizes and are asymptomatic.

"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.

Diagnosis of pediatric neuromuscular disorders in the era of DNA analysis

Extraordinary breakthroughs in the molecular pathogenesis of muscle and nerve disease have resulted in an evolving genetic classification of neuromuscular disorders and the development of new diagnostic methods. This remarkable progress has introduced new genetic tests and has changed the indications for use of certain invasive diagnostic procedures in the evaluation of children with presumed disorders of the motor unit. In this review, we present the current diagnostic approach to the more common neuromuscular diseases of infancy and childhood and define the diagnostic role of muscle biopsy and pediatric electromyography/nerve conduction studies in the era of genetic analysis.

Independent regulation of the myotonic dystrophy 1 locus genes postnatally and during adult skeletal muscle regeneration

Myotonic dystrophy is caused by a CTG(n) expansion in the 3'-untranslated region of a serine/threonine protein kinase gene (DMPK), which is flanked by two other genes, DMWD and SIX5. One hypothesis to explain the wide-ranging effects of this expansion is that, as the mutation expands, it alters the expression of one or more of these genes. The effects may vary in different tissues and developmental stages, but it has been difficult to develop these hypotheses as the normal postnatal developmental expression patterns of these genes have not been adequately investigated. We have developed accurate transcript quantification based on fluorescent real-time reverse transcription-polymerase chain reaction (TaqMan) to develop gene expression profiles during postnatal development in C57Bl/10 mice. Our results show extensive independent postnatal regulation of the myotonic dystrophy-locus genes in selected tissues and demonstrate which are the most highly expressed of the genes in each tissue. All three genes at the locus are expressed in the adult lens, questioning a previous model of cataractogenesis mediated solely by effects on Six5 expression. Additionally, using an in vivo model, we have shown that Dmpk levels decrease during the early stages of muscle regeneration. Our data provide a framework for investigation of tissue-specific pathological mechanisms in this disorder.

Over expression of the murine myotonic dystrophy protein kinase in the mouse myogenic C2C12 cell line leads to inhibition of terminal differentiation

Myotonic dystrophy (DM) is an autosomal dominant human disorder, caused by the abnormal expansion of a CTG trinucleotide repeat in the 3' untranslated region of a protein kinase gene (DMPK). Muscle symptoms are a common feature of the disorder and in the adult onset cases there are increased patterns of muscle fibre degeneration and regeneration. In the congenitally affected infants there is a failure of muscle maturation, with the histological presence of numerous immature fibres. However, the pathological mechanism in both forms of the disease is unclear. We report that over-expression of the murine dmpk gene, in a murine myogenic cell line, leads to markedly reduced levels of fusion to the terminally differentiated state. These findings complement recently published data using a heterologous expression/cell system and may have implications for the understanding of the disease process in this disorder.

Cost effectiveness of DNA diagnosis for four monogenic diseases

In this paper the costs and benefits associated with DNA diagnosis of subjects who are at risk of having a child with a monogenic disease and who seek genetic counselling because of their reproductive plans are predicted under various assumptions using a mathematical model. Four monogenic diseases have been considered: cystic fibrosis, Duchenne muscular dystrophy, myotonic dystrophy, and fragile X syndrome. Counselling (triggered by previous information) on the basis of DNA diagnosis is compared to the situation that only risk evaluation based on pedigree analysis is possible. The results show for each disease that with DNA diagnosis, couples can be more confident in choosing (further) offspring leading to the birth of more healthy children while the number of affected children is reduced. The costs minus savings within the health care sector depend on the prior risks and on the future burden of the monogenic illness under consideration. DNA diagnosis of relative "low" prior risks of a child with CF (for example, 1:180, 1:240 and 1:480) leads to costs instead of savings. For higher prior risks of CF and for the three other diseases, DNA diagnosis produces considerable savings. This result remains valid when assumptions regarding behaviour, reproduction, and receiving DNA diagnosis under different circumstances are varied.

Characterisation of expression of mDMAHP, a homeodomain-encoding gene at the murine DM locus

We examined the expression of the murine homologue of myotonic dystrophy associated homeodomain protein (mDMAHP) using two different strategies. The first approach, RT-PCR, detected spliced transcripts in a wide range of embryonic and adult tissues, in a pattern overlapping substantially with the expression of mDMPK. A second approach, the generation of transgenic mice expressing the lacZ reporter gene from a 4.3 kb promoter fragment, also demonstrated expression in a range of tissues with potential links to the phenotype in myotonic dystrophy. We conclude that murine DMAHP has a similar pattern of expression to human DMAHP and will serve as a useful model for functional studies of this gene, although species differences, such as the reduced CpG island (1.8 kb compared with 3.5 kb) must be carefully considered.

Reduction of serum IgG level and peripheral T-cell counts are correlated with CTG repeat lengths in myotonic dystrophy patients

Myotonic dystrophy (DM) is an autosomal dominant multisystem disorder associated with expansion of the CTG repeat within the 3' non-coding region of the myotonin protein kinase (MT-PK) gene. CTG repeat length has been shown to correlate with the clinical category and age at onset of the disease. The relationship between CTG repeat length and immunological parameters were analyzed in this study. We determined CTG repeat length in 14 DM patients and 15 normal controls using Southern and PCR analyses, and then correlated their CTG repeat lengths with their serum immunoglobulin (IgG, IgA, IgM) levels and the number of peripheral white blood cells, including lymphocyte subsets. In DM patients, increasing CTG repeat lengths correlated significantly with decreasing serum IgG levels, decreasing total lymphocyte counts, and decreasing CD2+, CD3+, and CD4+ cell counts. Immunological parameters were also influenced by the expansion of CTG repeat in DM patients.

Single-cell analysis of unstable genes

PURPOSE

We have developed sensitive diagnostic procedures for studies on the normal and mutant alleles of the triplet repeat genes associated with myotonic dystrophy and fragile X in single human somatic cells, gametes and embryos.

METHODS

Polymerase chain reaction (PCR) assays for the normal alleles of the myotonic dystrophy and fragile X loci have been refined to the sensitivity of the single cell. In addition, we have developed a simple PCR-based technique, termed ¿Repeat Primer PCR', which can detect the full fragile X expansion in small samples of buccal cells.

CONCLUSIONS

The assay for the triplet repeat sequence in the myotonic dystrophy locus could not be used to study stability since we observed additional PCR products derived from in vitro expansion of the triplet repeat sequence during the PCR reaction itself. The implications of in vitro expansion and allele drop-out for studies on the timing of the expansion in development and preimplantation diagnosis of triplet repeat diseases are discussed. The development of a new PCR procedure to identify the expanded alleles of the fragile X locus could prove invaluable for monitoring the timing of repeat expansion in early embryonic development. Triplet repeat polymorphisms provide a means of identifying the maternally and paternally-derived alleles of the myotonic dystrophy gene. Using single cell reverse transcriptase PCR analysis, we have monitored the onset of the myotonic dystrophy gene transcription in early preimplantation embryos. Transcripts from the paternally-inherited allele of the myotonic dystrophy gene are already detectable in the 1-cell stage human embryo.

Somatic stability of the expanded CAG trinucleotide repeat in X-linked spinal and bulbar muscular atrophy

Expansion of trinucleotide repeats has now been associated with eight inherited diseases: X-linked spinal and bulbar muscular atrophy, two fragile X syndromes, myotonic dystrophy, Huntington's disease, spinocerebellar ataxia type I, dentatorubral pallidoluysian atrophy and Machado-Joseph disease. It has been shown that these expanded DNA repeats are unstable in number when transmitted from parents to offspring ("meiotic instability"), while somatic variation in repeat number has also been found in the fragile X syndrome and myotonic dystrophy. Moderate meiotic instability has been demonstrated in X-linked spinal and bulbar muscular atrophy (SBMA, Kennedy's disease). In order to determine if the expanded CAG repeat in SBMA also shows somatic instability, we compared different tissues from two patients with SBMA. We then examined the in vitro stability of the CAG repeat expansion by analyzing fibroblast cell cultures. Length comparison of expanded CAG repeats from all these materials clearly demonstrates that the CAG trinucleotide repeat in SBMA does not exhibit somatic variation.

Ascertainment of myotonic dystrophy through cataract by selective screening

Myotonic dystrophy (DM) almost always results from the expansion of an unstable (CTG)n repeat. The mutation can be detected directly. Affected patients with cataracts may have minimal additional signs of the disorder, but all are at risk of life threatening complications. We have studied the efficacy of detecting new families with myotonic dystrophy by selectively screening cataract patients. Selection criteria were: age under 60 with no obvious precipitating factor (except non-insulin dependent diabetes mellitus (NIDDM)); patients of any age with other signs suggestive of myotonic dystrophy detected by the ophthalmologist. Ninety-six patients were tested prospectively; 17 others under 55 were screened retrospectively. All patients were counselled by a clinical geneticist before testing. The patients' DNA was analysed using the DNA probe/restriction enzyme combinations GB2.6/EcoRI, KB1.4/BglI and polymerase chain reaction (PCR). Six patients have been found to have a mutation, three (3.1%) in the prospective group and three (17.6%) in the retrospective group. Three of these patients had minimal myotonic dystrophy and three had classical DM.

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.

Myotonic dystrophy: genetic, clinical, and molecular analysis of patients from 41 Brazilian families

Results of genealogical, DNA, and clinical findings in 41 families with 235 patients affected with myotonic dystrophy (DM) led to the following observations. (1) The relative proportion of affected patients among blacks is apparently lower than among whites or orientals. (2) A significant excess of males was observed. (3) The frequency of DM patients who did not reproduce was similar for males and females; however, female patients had on average 25% fewer children than male patients. (4) There was a significant intergenerational increase in the mean length of the CTG repeat which was also correlated with the severity of the phenotype. (5) No significant difference was observed in the mean size of the CTG repeat in offspring of male as compared to female transmitters. (6) With the exception of the congenital cases of maternal origin, the largest expansions were paternally inherited, but did not lead to congenital DM.

[Myotonic dystrophy: study of clinico-genetic correlation in a pair of relatives (father-son)]

We report the case of a child with myotonic dystrophy (DM) with symptoms beginning at the age of seven, whose genetic study showed an additional DNA fragment, greater than of his father, an asymptomatic carrier. The clinical and molecular analysis of this parent-child pair are probably the first described in Brazil, since the recent discovery of genetic abnormality in DM by American and European researchers, that explained the long-debated phenomenon of "anticipation" in this disease. The main advances in molecular genetics in DM and its correlation with increasing severity and earlier onset of the symptoms in successive generations of a family are commented briefly.

Anticipation resulting in elimination of the myotonic dystrophy gene: a follow up study of one extended family

We have re-examined an extended myotonic dystrophy (DM) family, previously described in 1955, in order to study the long term effects of anticipation in DM and in particular the implications for families affected by this disease. This follow up study provides data on 35 gene carriers and 46 asymptomatic at risk family members in five generations. Clinical anticipation, defined as the cascade of mild, adult, childhood, or congenital disease in subsequent generations, appeared to be a relentless process, occurring in all affected branches of the family. The cascade was found to proceed asynchronously in the different branches, mainly because of an unequal number of generations with mild disease. The transition from the mild to the adult type was associated with transmission through a male parent. Stable transmission of the asymptomatic/mild phenotype showed a female transmission bias. We further examined the extent and causes of gene loss in this pedigree. Gene loss in the patient group was complete, owing to infertility of the male patients with adult onset disease and the fact that mentally retarded patients did not procreate. Out of the 46 at risk subjects in the two youngest generations, only one was found to have a full mutation. This is the only subject who may transmit the gene to the sixth generation. No protomutation carriers were found in the fourth and fifth generations. Therefore it is highly probable that the DM gene will be eliminated from this pedigree within one generation. The high population frequency of DM can at present not be explained by the contribution of asymptomatic cases in the younger generations of known families, but is probably caused by the events in the ancestral generations.

Molecular basis and diagnosis of neurogenetic disorders

Over the past few years, molecular neurogenetics has developed into one of the most promising and active research fields. The new discipline applies modern molecular genetic techniques to the investigation of classical neurological disorders. In the following article, a definition of neurogenetic disease is introduced, the molecular basis of four groups of neurogenetic disorders is described and recent diagnostic developments are presented. The first group of diseases is caused by trinucleotide expansions. "Expanding" trinucleotide repeats were not known to occur in any species until about three years ago. Today, disorders such as Huntington's disease, spinocerebellar ataxia type 1, fragile X mental retardation, spinobulbar muscular atrophy and myotonic dystrophy are all known to be caused by the expansion of trinucleotides. The second group is characterized by chromosomal deletions or uniparental disomies. Lissencephaly and the Miller-Dieker syndrome, Prader-Willi and Angelman syndromes and Duchenne and Becker muscular dystrophies belong to this category. The third group includes those neurogenetic disorders that are mainly caused by point mutations such as the X-linked leukodystrophies, including Pelizaeus-Merzbacher disease and adrenoleukodystrophy, Charcot-Marie-Tooth syndrome type 1, familial forms of amyotrophic lateral sclerosis, several types of craniosynostoses and some CNS tumor syndromes. Finally, Alzheimer's and Parkinson's disease are discussed as representatives of group four, i.e. genetically heterogeneous neurological disorders.

Instability of CAG repeats in Huntington's disease: relation to parental transmission and age of onset

Huntington's disease (HD) has recently been found to be caused by expansion of a trinucleotide (CAG) repeat within the putative coding region of a gene with an unknown function. We report here an analysis of HD mutation and the characteristics of its transmission in 36 HD families. CAG repeats on HD chromosomes were unstable when transmitted from parent to offspring. Instability appeared more frequent and stronger upon transmission from a male than from a female, with a clear tendency towards increased size. We have also found a significant inverse correlation (p = 0.0001) between the age of onset and the CAG repeat length. The observed scatter would, however, not allow an accurate individual prediction of age of onset. Three juvenile onset cases analysed had an HD mutation of paternal origin. In at least two of these cases a large expansion of the HD allele upon paternal transmission may explain the major anticipation observed. Our results suggest that several features of the expansion mutation in HD are similar to those previously observed for mutations of similar size in spinobulbar muscular atrophy and in myotonic dystrophy, and to those observed more recently in spinocerebellar ataxia type 1 and in dentatorubropallidoluysian atrophy, four diseases also caused by expansion of CAG repeats.

Gonosomal mosaicism in myotonic dystrophy patients: involvement of mitotic events in (CTG)n repeat variation and selection against extreme expansion in sperm

Myotonic dystrophy (DM) is caused by abnormal expansion of a polymorphic (CTG)n repeat, located in the DM protein kinase gene. We determined the (CTG)n repeat lengths in a broad range of tissue DNAs from patients with mild, classical, or congenital manifestation of DM. Differences in the repeat length were seen in somatic tissues from single DM individuals and twins. Repeats appeared to expand to a similar extent in tissues originating from the same embryonal origin. In most male patients carrying intermediate- or small-sized expansions in blood, the repeat lengths covered a markedly wider range in sperm. In contrast, male patients with large allele expansions in blood (> 700 CTGs) had similar or smaller repeats in sperm, when detectable. Sperm alleles with > 1,000 CTGs were not seen. We conclude that DM patients can be considered gonosomal mosaics, i.e., combined somatic and germ-line tissue mosaics. Most remarkably, we observed multiple cases where the length distributions of intermediate- or small-sized alleles in fathers' sperm were significantly different from that in their offspring's blood. Our combined findings indicate that intergenerational length changes in the unstable CTG repeat are most likely to occur during early embryonic mitotic divisions in both somatic and germ-line tissue formation. Both the initial CTG length, the overall number of cell divisions involved in tissue formation, and perhaps a specific selection process in spermatogenesis may influence the dynamics of this process. A model explaining mitotic instability and sex-dependent segregation phenomena in DM manifestation is discussed.

Characteristics of intergenerational contractions of the CTG repeat in myotonic dystrophy

In myotonic dystrophy (DM), the size of a CTG repeat in the DM kinase gene generally increases in successive generations with clinical evidence of anticipation. However, there have also been cases with an intergenerational contraction of the repeat. We examined 1,489 DM parent-offspring pairs, of which 95 (6.4%) showed such contractions in peripheral blood leukocytes (PBL). In 56 of the 95 pairs, clinical data allowed an analysis of their anticipation status. It is surprising that anticipation occurred in 27 (48%) of these 56 pairs, while none clearly showed a later onset of DM in the symptomatic offspring. The contraction occurred in 76 (10%) of 753 paternal transmissions and in 19 (3%) of 736 maternal transmissions. Anticipation was observed more frequently in maternal (85%) than in paternal (37%) transmissions (P < .001). The parental repeat size correlated with the size of intergenerational contraction (r2 = .50, P << .001), and the slope of linear regression was steeper in paternal (-.62) than in maternal (-.30) transmissions (P << .001). Sixteen DM parents had multiple DM offspring with the CTG repeat contractions. This frequency was higher than the frequency expected from the probability of the repeat contractions (6.4%) and the size of DM sib population (1.54 DM offspring per DM parent, in 968 DM parents). We conclude that (1) intergenerational contractions of the CTG repeat in leukocyte DNA frequently accompanies apparent anticipation, especially when DM is maternally transmitted, and (2) the paternal origin of the repeat and the presence of the repeat contraction in a sibling increase the probability of the CTG repeat contraction.

Myotonic dystrophy with no trinucleotide repeat expansion

We report 3 patients from 2 families with myotonic dystrophy who do not show an abnormal expansion of CTG trinucleotide repeats within the myotonic dystrophy gene. Characteristic features of myotonic dystrophy in these patients were frontal balding, cataracts, cardiac conduction abnormalities, and testicular atrophy with myotonia and muscle weakness. Results of muscle histopathology were consistent with myotonic dystrophy. Genetic analysis of leukocyte and muscle DNA showed a normal number of CTG repeats. The demonstration of normal CTG repeat number for the myotonic dystrophy gene does not exclude the diagnosis of myotonic dystrophy.

Influence of sex of the transmitting parent as well as of parental allele size on the CTG expansion in myotonic dystrophy (DM)

In patients with myotonic dystrophy (DM), the severity of clinical signs is correlated with the length of a (CTG)n trinucleotide repeat sequence. This sequence tends to expand in subsequent generations. In order to examine the kinetics of this process and, in particular, the influence of the mutant-allele size and the sex of the transmitting parent, we have studied (CTG)n repeat lengths in the offspring of 38 healthy carriers with small mutations (less than 100 CTG trinucleotides, mean length [CTG]67). In these studies, we found a weakly positive correlation between the size of the mutation in the carrier parents and that in their offspring. Furthermore, we observed that, in the offspring of male transmitters, repeat lengths exceeding 100 CTG trinucleotides were much more frequent than in the offspring of carrier females (48 [92%] of 52 vs. 7 [44%] of 16, P = .0002). Similarly, in genealogical studies performed in 38 Dutch DM kindreds, an excess of nonmanifesting male transmitters was noted, which was most conspicuous in the generation immediately preceding that with phenotypic expression of DM. Thus, two separate lines of evidence suggest that the sex of the transmitting parent is an important factor that determines DM allele size in the offspring. On the basis of our data, we estimate that when both parents are asymptomatic, the odds are approximately 2:1 that the father carries the DM mutation. Because expansion of the CTG repeat is more rapid with male transmission, negative selection during spermatogenesis may be required to explain the exclusive maternal inheritance of severe congenital onset DM.

Direct molecular diagnosis of myotonic dystrophy

Myotonic dystrophy (DM) arises from an unstable trinucleotide (CTGn) repeat sequence within the DM locus at 19q13.3. Twenty-three myotonic dystrophy families containing 205 persons with no symptoms, minimal manifestations, classic DM or congenital DM were investigated to validate the application of the pM10M6 probe to direct molecular diagnosis. Affected family members had been diagnosed clinically and the unaffected family members had been assigned carrier probabilities close to either zero or 100%, using closely linked flanking markers. Southern analysis identified all 89 DM gene carriers as having expansions of the unstable element. PstI detected all small expansions of the repeat sequence as easily seen discrete bands; but large expansions were usually seen as diffuse smears, sometimes difficult to distinguish from lane background. EcoRI concentrated these diffuse smears, associated with somatic instability, into discrete bands which were easy to detect; but it did not resolve the smaller expansions present in 9 (10%) of the DM carriers. It is essential that PstI and EcoRI gels are run in parallel to detect all DM gene carriers. The extent of expansion of CTG correlated with age of onset and disease severity. Biopsies of various fetal tissues from two terminated pregnancies confirmed the diagnosis obtained by CVS and revealed no heterogeneity between tissues at this developmental stage. Further expansion occurred during the culture of CVS cells, indicating that direct prenatal diagnosis needs to be carried out on CVS tissue rather than on cultured cells. The intergenerational change of the repeat sequence from DM parent to DM offspring showed a significant parental sex difference for those parents with large expansions. Contraction of the unstable element was observed in the three males carrying the largest expansions and could explain why congenital DM is exclusively of maternal origin.

Origin of the expansion mutation in myotonic dystrophy

Myotonic dystrophy (DM) is caused by the expansion of a CTG trinucleotide repeat. The mutation is in complete linkage disequilibrium with a nearly two-allele insertion/deletion polymorphism, suggesting a single origin for the mutation or predisposing mutation. To trace this-ancestral event, we have studied the association of CTG repeat alleles in a normal population to alleles of the insertion/deletion polymorphism and of a (CA)n repeat marker 90 kilobases from the DM mutation. The results strongly suggest that the initial predisposing event(s) consisted of a transition from a (CTG)5 allele to an allele with 19 to 30 repeats. The heterogeneous class of (CTG)19-30 alleles which has an overall frequency of about 10%, may constitute a reservoir for recurrent DM mutations.