Brief report: reverse mutation in myotonic dystrophy

Revertant Mosaicism in Genodermatoses: Natural Gene Therapy Right before Your Eyes

Revertant mosaicism (RM) is the intriguing phenomenon in which nature itself has successfully done what medical science is so eagerly trying to achieve: correcting the effect of disease-causing germline variants and thereby reversing the disease phenotype back to normal. RM was molecularly confirmed for the first time in a genodermatosis in 1997, the genetic skin condition junctional epidermolysis bullosa (EB). At that time, RM was considered an extraordinary phenomenon. However, several important discoveries have changed this conception in the past few decades. First, RM has now been identified in all major subtypes of EB. Second, RM has also been identified in many other genodermatoses. Third, a theoretical mathematical exercise concluded that reverse mutations should be expected in all patients with a recessive subtype of EB or any other genodermatosis. This has shifted the paradigm from RM being an extraordinary phenomenon to it being something that every physician working in the field of genodermatoses should be looking for in every patient. It has also raised hope for new treatment options in patients with genodermatoses. In this review, we summarize the current knowledge on RM and discuss the perspectives of RM for the future treatment of patients with genodermatoses.

A "late-but-fitter revertant cell" explains the high frequency of revertant mosaicism in epidermolysis bullosa

Revertant mosaicism, or "natural gene therapy", is the phenomenon in which germline mutations are corrected by somatic events. In recent years, revertant mosaicism has been identified in all major types of epidermolysis bullosa, the group of heritable blistering disorders caused by mutations in the genes encoding epidermal adhesion proteins. Moreover, revertant mosaicism appears to be present in all patients with a specific subtype of recessive epidermolysis bullosa. We therefore hypothesized that revertant mosaicism should be expected at least in all patients with recessive forms of epidermolysis bullosa. Naturally corrected, patient-own cells are of extreme interest for their promising therapeutic potential, and their presence in all patients would open exciting, new treatment perspectives to those patients. To test our hypothesis, we determined the probability that single nucleotide reversions occur in patients' skin using a mathematical developmental model. According to our model, reverse mutations are expected to occur frequently (estimated 216x) in each patient's skin. Reverse mutations should, however, occur early in embryogenesis to be able to drive the emergence of recognizable revertant patches, which is expected to occur in only one per ~10,000 patients. This underestimate, compared to our clinical observations, can be explained by the "late-but-fitter revertant cell" hypothesis: reverse mutations arise at later stages of development, but provide revertant cells with a selective growth advantage in vivo that drives the development of recognizable healthy skin patches. Our results can be extrapolated to any other organ with stem cell division numbers comparable to skin, which may offer novel future therapeutic options for other genetic conditions if these revertant cells can be identified and isolated.

Absence of MutSβ leads to the formation of slipped-DNA for CTG/CAG contractions at primate replication forks

Typically disease-causing CAG/CTG repeats expand, but rare affected families can display high levels of contraction of the expanded repeat amongst offspring. Understanding instability is important since arresting expansions or enhancing contractions could be clinically beneficial. The MutSβ mismatch repair complex is required for CAG/CTG expansions in mice and patients. Oddly, by unknown mechanisms MutSβ-deficient mice incur contractions instead of expansions. Replication using CTG or CAG as the lagging strand template is known to cause contractions or expansions respectively; however, the interplay between replication and repair leading to this instability remains unclear. Towards understanding how repeat contractions may arise, we performed in vitro SV40-mediated replication of repeat-containing plasmids in the presence or absence of mismatch repair. Specifically, we separated repair from replication: Replication mediated by MutSβ- and MutSα-deficient human cells or cell extracts produced slipped-DNA heteroduplexes in the contraction- but not expansion-biased replication direction. Replication in the presence of MutSβ disfavoured the retention of replication products harbouring slipped-DNA heteroduplexes. Post-replication repair of slipped-DNAs by MutSβ-proficient extracts eliminated slipped-DNAs. Thus, a MutSβ-deficiency likely enhances repeat contractions because MutSβ protects against contractions by repairing template strand slip-outs. Replication deficient in LigaseI or PCNA-interaction mutant LigaseI revealed slipped-DNA formation at lagging strands. Our results reveal that distinct mechanisms lead to expansions or contractions and support inhibition of MutSβ as a therapeutic strategy to enhance the contraction of expanded repeats.

Molecular genetics and genetic testing in myotonic dystrophy type 1

Myotonic dystrophy type 1 (DM1) is the most common adult onset muscular dystrophy, presenting as a multisystemic disorder with extremely variable clinical manifestation, from asymptomatic adults to severely affected neonates. A striking anticipation and parental-gender effect upon transmission are distinguishing genetic features in DM1 pedigrees. It is an autosomal dominant hereditary disease associated with an unstable expansion of CTG repeats in the 3'-UTR of the DMPK gene, with the number of repeats ranging from 50 to several thousand. The number of CTG repeats broadly correlates with both the age-at-onset and overall severity of the disease. Expanded DM1 alleles are characterized by a remarkable expansion-biased and gender-specific germline instability, and tissue-specific, expansion-biased, age-dependent, and individual-specific somatic instability. Mutational dynamics in male and female germline account for observed anticipation and parental-gender effect in DM1 pedigrees, while mutational dynamics in somatic tissues contribute toward the tissue-specificity and progressive nature of the disease. Genetic test is routinely used in diagnostic procedure for DM1 for symptomatic, asymptomatic, and prenatal testing, accompanied with appropriate genetic counseling and, as recommended, without predictive information about the disease course. We review molecular genetics of DM1 with focus on those issues important for genetic testing and counseling.

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.

Revertant mosaicism in skin: natural gene therapy

Revertant mosaicism is a naturally occurring phenomenon involving spontaneous correction of a pathogenic mutation in a somatic cell. Recent studies suggest that it is not a rare event and that it could be clinically relevant to phenotypic expression and patient treatment. Indeed, revertant cell therapy represents a potential 'natural gene therapy' because in vivo reversion obviates the need for further genetic correction. Revertant mosaicism has been observed in several inherited conditions, including epidermolysis bullosa, a heterogeneous group of blistering skin disorders. These diseases provide a useful model for studying revertant mosaicism because of the visual and accessible nature of skin. This overview highlights the latest developments in revertant mosaicism and the translational implications germane to heritable skin disorders.

CLINICAL CASE REPORT ATYPICAL MYOPATHY IN A YOUNG GIRL WITH 91 CTG REPEATS IN DM1 LOCUS AND A POSITIVE DM1 FAMILY HISTORY

Myotonic dystrophy type 1 (DM1) is an autosomal dominant inheritable disease associated with an expansion of CTG repeats in the 3' UTR of the DMPK gene. The subject is an 11-year-old girl with atypical myopathy. Because the proband's family has a positive DM1 history, a molecular-genetic analysis for DM1 was performed. This study showed that proband had a small DMPK expansion (91 CTG repeats) although the observed myopathy would not normally be associated with DM1. These results show how the phenotypic manifestation of DM1 can have unusual symptoms with a completely unexpected relationship to genotype.

Analysis of trinucleotide repeats in myotonic dystrophy

Myotonic dystrophy is a genetic disorder characterized in 99% of clinically diagnosed families by an unstable CTG repeat in the 3-untranslated region of a gene encoding a serine-threonine protein kinase. There is no one method to detect the entire range of expansion sizes possible in affected patients, so current diagnostic approaches rely on analyzing samples by hybridization of both polymerase chain reaction (PCR)-amplified CTG repeats (CTG-PCR) and genomic DNA. In this unit, the the Basic Protocol 1 describes the analysis of PCR-amplified repeats transferred to a nylon membrane by Southern blotting and hybridized to an alkaline phosphatase-labeled probe. The first support protocol describes a vacuum blotting technique for rapid transfer of the PCR product to the nylon membrane and the second support protocol describes the use of a radiolabeled oligonucleotide probe for hybridization. Analysis of genomic DNA by similar hybridization techniques is outlined in the second basic protocol. Myotonic dystrophy is a genetic disorder characterized in 99% of clinically diagnosed families by an unstable CTG repeat Myotonic dystrophy is a genetic disorder characterized in 99% of clinically diagnosed families by an unstable CTG repeat.

Double-strand breaks in the myotonic dystrophy type 1 and the fragile X syndrome triplet repeat sequences induce different types of mutations in DNA flanking sequences in Escherichia coli

The putative role of double-strand breaks (DSBs) created in vitro by restriction enzyme cleavage in or near CGG*CCG or CTG*CAG repeat tracts on their genetic instabilities, both within the repeats and in their flanking sequences, was investigated in an Escherichia coli plasmid system. DSBs at TRS junctions with the vector generated a large number of mutagenic events in flanking sequences whereas DSBs within the repeats elicited no similar products. A substantial enhancement in the number of mutants was caused by transcription of the repeats and by the absence of recombination functions (recA-, recBC-). Surprisingly, DNA sequence analyses on mutant clones revealed the presence of only single deletions of 0.4-1.6 kb including the TRS and the flanking sequence from plasmids originally containing (CGG*CCG)43 but single, double and multiple deletions as well as insertions were found for plasmids originally containing (CTG*CAG)n (where n = 43 or 70). Non-B DNA structures (slipped structures with loops, cruciforms, triplexes and tetraplexes) as well as microhomologies are postulated to participate in the recombination and/or repair processes.

A multistep mutation mechanism drives the evolution of the CAG repeat at MJD/SCA3 locus

Despite the intense debate around the repeat instability reported on the large group of neurological disorders caused by trinucleotide repeat expansions, little is known about the mutation process underlying alleles in the normal range that, ultimately, expand to pathological size. In this study, we assessed the mutation mechanisms by which wild-type Machado-Joseph disease (MJD) alleles have been generated throughout human evolution. Haplotypes including the CAG repeat, six intragenic SNPs and four flanking microsatellites were analysed in 431 normal chromosomes of European, Asian and African origin. A bimodal CAG repeat length frequency distribution was found in the four most frequent wild-type lineages (H1-GCGGCA; H2-GTGGCA; H3-TTAGAC and H4-TTACAC). Based on flanking microsatellite haplotypes, the variance calculated by analysis of molecular variance between modal (CAG)n alleles was little or null in lineages H1, H2 and H4, as were the pairwise differences. Moreover, genetic distances among all the alleles from each lineage did not reflect the allele sizes differences, as expected if a stepwise mutation model was the main process of evolution. On the contrary, when exposed in maximum parsimonious phylogenetic trees, a large number of mutation steps separated same-size alleles, whereas several microsatellite haplotypes were shared by modal CAGs. In conclusion, our results suggest that the main mutation mechanism occurring in the evolution of the polymorphic CAG region at MJD/SCA3 locus is a multistep one, either by gene conversion or DNA slippage; repeats with 14, 21, 23 and 27 CAGs are the main alleles involved in this process.

The contribution of cis-elements to disease-associated repeat instability: clinical and experimental evidence

Alterations in the length (instability) of gene-specific microsatellites and minisatellites are associated with at least 35 human diseases. This review will discuss the various cis-elements that contribute to repeat instability, primarily through examination of the most abundant disease-associated repetitive element, trinucleotide repeats. For the purpose of this review, we define cis-elements to include the sequence of the repeat units, the length and purity of the repeat tracts, the sequences flanking the repeat, as well as the surrounding epigenetic environment, including DNA methylation and chromatin structure. Gender-, tissue-, developmental- and locus-specific cis-elements in conjunction with trans-factors may facilitate instability through the processes of DNA replication, repair and/or recombination. Here we review the available human data that supports the involvement of cis-elements in repeat instability with limited reference to model systems. In diverse tissues at different developmental times and at specific loci, repetitive elements display variable levels of instability, suggesting vastly different mechanisms may be responsible for repeat instability amongst the disease loci and between various tissues.

Fidelity of primate cell repair of a double-strand break within a (CTG).(CAG) tract. Effect of slipped DNA structures

At least 15 human diseases are caused by the instability of gene-specific (CTG).(CAG) repeats. The precise mechanism of instability remains unknown, though bacterial and yeast models have suggested a role for aberrant repair of double-strand breaks (DSBs). Using an established primate DSB repair system, we have investigated the fidelity of repair of a DSB within a (CTG).(CAG) repeat tract. DSB repair substrates were generated from plasmids that are stably replicated in their circular form, permitting us to highlight the effects of DSB repair on repeat stability and minimize the contribution of replication. DSBs were introduced into repeat-containing plasmids using a unique BsmI site, such that the entire repeat tract comprised one free end of the linearized plasmid. Substrates containing 17, 47, and 79 repeats, in either their linear duplex form or containing slipped structures (out-of-register interstrand mispairings at repeat sequences), were transiently transfected into primate cells. Linearized plasmids with repeats were repaired with mildly reduced efficiency, while the presence of slipped structures considerably reduced repair efficiency. The repaired products were characterized for alterations within the repeat tract and flanking sequence. DSB repair induced predominantly repeat deletions. Notably, a polarized/directional deletion effect was observed, in that the repetitive end of the DSB was preferentially removed. This phenomenon was dramatically enhanced when slipped structures were present within the repeat tract, providing the first evidence for error-prone processing of slipped-strand structures. These results suggest the existence of primate nuclease activities that are specific for (CTG).(CAG) repeats and the structures they form.

The extinction program for Homo sapiens and cloning humans: trinucleotide expansion as a one-way track to extinction

It has been uncovered that expansion of trinucleotide repeats in the human genome are causing a variety of genetic diseases. They are also identified in other loci that are not clearly related to particular diseases, which indicates such repeat expansion is one of the general forms of evolution occurring throughout the human genome. Trinucleotide repeat expansion has been shown to occur during meiosis, and is generally irreversible. In consequence, every human chromosome suffers from the burden of accumulating trinucleotide repeats along with the passage of generation, which eventually ends up in a deficiency of replication. Thus, all the human chromosomes are predicted to be 'mortal', so far as they replicate through meiosis. Naturally, the mortality of human chromosomes leads to the mortality of Homo sapiens as a species on earth. Although it is quite controversial, a radical breakthrough for humans to survive might be found through cloning reproduction without meiosis.

Myotonic dystrophy protein kinase (DMPK) gene expression in lymphocytes of patients with myotonic dystrophy

BACKGROUND

Myotonic dystrophy (DM) is an autosomal dominant neuromuscular disorder with defects in many tissues, including skeletal muscle myotonia, progressive myopathy, and abnormalities in heart, brain, and endocrine systems. It is associated with a trinucleotide repeat occurring in the 3' (UTR) untranslated region of the myotonic dystrophy protein kinase (DMPK) gene. Several studies have been carried out to determine DMPK gene expression in muscle and non-muscle tissues.

METHODS

DMPK gene expression was determined in lymphocytes of adult-onset patients with DM and normal controls. To quantitate total locus expression as well as allele-specific mRNA levels, semiquantitative RT-PCR assay was used. Allele-specific expression was analyzed using a Bpm1 polymorphism located at exon 10 of the DMPK gene.

RESULTS

In heterozygous patients with DM, we observed a fourfold difference between mRNA levels produced by the Bpm1-undigested allele (187 bp) compared to the Bpm1-digested allele (136 bp). By using (CTG) trinucleotide (with cytosine, thymine, and guanine) expansion polymorphism, it was shown that the down-regulated allele corresponds to the mutant allele. Interestingly, the reduction in the mutant allele-transcript levels is compensated by an increase of the wild-type allele, yielding no significant differences in total locus mRNA amount between patients and normal individuals.

CONCLUSIONS

These results suggest that the expression of the two alleles at the DMPK locus in lymphocytes is coordinated. The reduction in mutant-allele transcript levels is compensated by an increase in wild-type allele mRNA levels.

Revertant mosaicism in human genetic disorders

Somatic reversion of inherited mutations is known for many years in plant breeding, however it was recognized only recently in humans. The concept of revertant mosaicism is important in medical genetics.

Parental gender, age at birth and expansion length influence GAA repeat intergenerational instability in the X25 gene: pedigree studies and analysis of sperm from patients with Friedreich's ataxia

Friedreich's ataxia is the first known autosomal recessive disease caused by an unstable trinucleotide expansion mutation. The most frequent mutation is expansion of a GAA repeat in the first intron of gene X25. We studied transmission of the expanded GAA repeat in 37 Friedreich's ataxia pedigrees and analysed blood and sperm alleles in eight patients. We showed intergenerational instability in 84% of the alleles with an overall excess of contractions. Both contractions and expansions of the GAA repeat occurred in maternal transmission with a stronger tendency to expand for smaller repeats and to contract for longer repeats. Paternally transmitted alleles contracted only. Parental age and the intergenerational change in expansion size were directly correlated in maternal transmission and inversely in paternal transmission. The size of the GAA expansion was slightly lower in patients than heterozygous carriers. Sperm analysis confirmed the tendency to contract of paternal alleles, which was more marked with ageing. The degree of contraction of the GAA repeat in sperm was much higher than that found in intergenerational transmission and was directly related to the repeat size. A blood expanded allele reverted to normal size in the sperm of one patient. This study suggests the existence of different mutational mechanisms in Friedreich's ataxia alleles, which occur both pre- and post-zygotically.

Paternal transmission of congenital myotonic dystrophy

We report a rare case of paternally transmitted congenital myotonic dystrophy (DM). The proband is a 23 year old, mentally retarded male who suffers severe muscular weakness. He presented with respiratory and feeding difficulties at birth. His two sibs suffer from childhood onset DM. Their late father had the adult type of DM, with onset around 30 years. Only six other cases of paternal transmission of congenital DM have been reported recently. We review the sex related effects on transmission of congenital DM. Decreased fertility of males with adult onset DM and contraction of the repeat upon male transmission contribute to the almost absent occurrence of paternal transmission of congenital DM. Also the fathers of the reported congenitally affected children showed, on average, shorter CTG repeat lengths and hence less severe clinical symptoms than the mothers of children with congenital DM. We conclude that paternal transmission of congenital DM is rare and preferentially occurs with onset of DM past 30 years in the father.

Asperger syndrome associated with Steinert's myotonic dystrophy

Steinert's myotonic dystrophy was diagnosed in a 10-year-old girl with Asperger syndrome. This form of myotonic dystrophy is one of a growing list of genetic diseases that can be diagnosed by analysis of a gene with trinucleotide repeats that may expand. The patient has the largest such expansion known within her pedigree. She has many of the psychological deficits that have been reported in adult-onset myotonic dystrophy. This is the first known report of an association of this disorder with Asperger syndrome. Recently Moebius syndrome has been found in association with disorders of the autistic spectrum; physicians practicing developmental medicine should now also consider Steinert's myotonic dystrophy when evaluating children with facial diplegia.

Reverse mutations in the fragile X syndrome

Three females were identified who have apparent reversal of fragile X premutations. Based on haplotype analysis of nearby markers, they were found to have inherited a fragile X chromosome from their premutation carrier mothers, and yet had normal size FMR1 repeat alleles. The changes in repeat sizes from mother to daughter was 95 to 35 in the first, 145 to 43 in the second, and 82 to 33 in the third. In the first family, mutations of the nearby microsatellites FRAXAC2 and DXS548 were also observed. In the other two, only mutations involving the FMR1 repeats were found. We suggest differing mutational mechanisms such as gene conversion versus DNA replication slippage may underlie such reversions. We estimate that such revertants may occur among 1% or less of premutation carrier offspring. Our results indicate that women identified to be carriers by linkage should be retested by direct DNA analysis.

Restrictive and hypertrophic cardiomyopathies in Noonan syndrome: the overlap syndromes

A woman with Noonan syndrome had clinical and haemodynamic features of restrictive cardiomyopathy. There was no ventricular hypertrophy on echocardiography but myocardial biopsies showed myocyte hypertrophy without pathological disarray. This case illustrates the overlap of the cardiac phenotypes of Noonan syndrome, restrictive cardiomyopathy, and hypertrophic cardiomyopathy.

Parents do matter: genomic imprinting and parental sex effects in neurological disorders

Genomic imprinting is a recently recognized phenomenon of differential expression of genetic material depending upon whether the genetic material has come from the male or female parent. This process of differential phenotypic expression involves mammalian development both in the normal and abnormal situations, resulting in parental sex effects. However, some parental sex effects may be due to other mechanisms such as mitochondrial inheritance. In the following article, evidence for genomic imprinting in experimental animals and in diseases are summarized. Relevant human neurological disorders manifesting parental sex effects discussed here include myotonic dystrophy, Huntington's disease, fragile X syndrome, spinocerebellar ataxia type 1, and neurofibromatosis type 1 and 2. A possible mechanism of imprinting involves the processes of methylation imprint and replication imprint. The knowledge of imprinting is helpful in clinical practice particularly in the areas of genetic counseling, prenatal diagnosis, and possible future gene therapy.

Unstable triplet repeat sequences: a source of cancer mutations?

Numerous mutations have been related to various types of cancer. Short tandem repeats (STRs) are repetitive DNA elements that are often polymorphic in normal populations. Triplet repeat expansion has been related pathogenetically to six diseases: fragile X syndrome, fragile X E syndrome, spinobulbar muscular atrophy, myotonic dystrophy, Huntington's disease, and spinocerebellar ataxia type 1. The characteristics of the GC-rich repeat expansion are diverse and result in profound changes in phenotype, sometimes within a single generation in affected families. We expect that simple repeat expansion will cause some cancers based on our knowledge of these unstable DNA sequences in the previously mentioned genes. This may occur by alteration of tumor suppressor gene expression, alteration in coding features of proteins, or change in bystander oncogene expression such as that which occurs with DNA methylation. The demonstrated meiotic instability could link this mechanism of mutation of familial cancer syndromes. The recent discovery of STR instability at multiple sites in hereditary nonpolyposis colon cancer suggests sequence instability may be a factor in cancer progression. Continued identification of candidate genes containing triplet repeats should allow a ready testing of the hypothesis that unstable simple repeat sequences can cause cancer.

Myotonic dystrophy: an unstable CTG repeat in a protein kinase gene

Myotonic dystrophy (DM) is caused by the amplification of CTG repeats in the 3' untranslated region of a gene encoding a protein homologous to serine/threonine protein kinases. In DM patients the CTG repeats are extremely unstable, varying in length from patient to patient and generally increasing in length in successive generations. There is a strong correlation between the size of the repeats and the age of onset and severity of the disease. The molecular basis of the effect of the CTG expansion on the development of the DM phenotype continues to be investigated. The first working hypothesis of the molecular mechanism of DM was a reduction in steady-state myotonin-protein kinase (Mt-PK) mRNA and protein levels. However, although the consensus finding is that the Mt PK mRNA and protein levels are decreased in DM patients, it is still not clear if this reduction leads directly to the DM phenotype. In this short review we discuss the molecular aspects of CTG instability and the expression of the myotonin-protein kinase gene in normal and DM populations.

Unstable triplet repeat diseases

Seven inherited human disorders are now associated with the intragenic expansion of triplet repeat DNA sequences. These repeats demonstrate extreme instability in both germline and somatic tissue, accounting for the unusual genetic inheritance patterns and symptom variability associated with these diseases.

Androgen receptor gene (CAG)n repeat analysis in the differential diagnosis between Kennedy disease and other motoneuron disorders

An increase in the number of (CAG)n repeats in the first coding exon of the androgen receptor (AR) gene has been strongly associated with Kennedy disease (KD) (spinal and bulbar muscular atrophy). This is an X-linked hereditary disorder characterized by motoneuron degeneration occurring in adults together with gynecomastia and hyperestrogenemia. We have performed AR gene molecular analysis in several members of a large family with KD as well as in 25 sporadic patients suffering from heterogeneous motoneuron disease (MND). An increase in the length of the (CAG)n repeats was detected, as expected, in all the affected males and in obligatory carrier females, some of which had minor signs of lower motoneuron involvement. There was only one possible exception, one young male with initial signs of the disease, who had an apparent normal length allele. An increased pathological allele was also found in 3 patients with MND. This indicates that the analysis of (CAG)n repeats of the AR gene plays a role in the differential diagnosis of this heterogeneous group of neurological diseases.

Correlation between cardiac involvement and CTG trinucleotide repeat length in myotonic dystrophy

OBJECTIVES

Because sudden death due to complete atrioventricular (AV) block or ventricular arrhythmias is the most dramatic event in myotonic dystrophy, we assessed the relation of cardiac disease to cytosine-thymine-guanine (CTG) triplet mutation in adults affected with myotonic dystrophy.

BACKGROUND

The myotonic dystrophy mutation, identified as an unstable deoxyribonucleic acid (DNA) sequence (CTG) prone to increase the number of trinucleotide repeats, produces clinical manifestations of the disease in skeletal muscle, the heart and many organ systems.

METHODS

Forty-two adult patients underwent electrocardiography and echocardiography; in addition, signal-averaging electrocardiography was performed in 22, and 24-h Holter monitoring was recorded in 32. The diagnosis was established by neurologic examination, electromyography, muscle biopsy and DNA analysis. The patients were then classified into three subgroups on the basis of the number of CTG trinucleotide repeat expansions: E1 = 18 patients with 0 to 500 CTG repeats; E2 = 12 patients with up to 1,000 repeats; E3 + E4 = 10 patients with up to 1,500 repeats and 2 patients with > 1,500 repeats.

RESULTS

The incidence of normal electrocardiographic (ECG) results was found to be significantly different in the three subgroups (55%, 50%, 17% in E1, E2, E3, + E4, respectively, p = 0.04), with the highest values in the group with fewer repeat expansions. The incidence of complete left bundle branch block was also significantly different among the groups (5% in E1, 0% in E2, 42% in E3 + E4 p = 0.01) and was directly correlated with the size of the expansion. A time-domain analysis of the signal-averaged ECG obtained in 12 patients in E1, 4 in E2, 5 in E3 and 1 in E4 showed that abnormal ventricular late potentials were directly correlated with CTG expansion (33% in E1, 75% in E2, 83% in E3 + E4, p = 0.05). Moreover, the incidence of ventricular couplets or triplets showed a positive correlation with size of CTG expansion (0 in E1, 0 in E2, 29% in E3 + E4, chi square 0.02).

CONCLUSIONS

Our findings suggest that the involvement of specialized cardiac tissue, accounting for severe AV and intraventricular conduction defects, is related to CTG repeat length. In addition, the presence of abnormal late potentials directly correlates to CTG expansion. Abnormal late potentials, caused by slowed and fragmented conduction through damaged areas of myocardium, represent a substrate for malignant reentrant ventricular arrhythmias. In the future, therefore, molecular analysis of DNA should identify patients with cardiac disease at high risk for development of AV block or lethal ventricular arrhythmias.

Trinucleotide repeat expansion in neurological disease

Expansion of trinucleotide repeats is now recognized as a major cause of neurological disease. At least seven disorders result from trinucleotide repeat expansion: X-linked spinal and bulbar muscular atrophy (SBMA), two fragile X syndromes of mental retardation (FRAXA and FRAXE), myotonic dystrophy, Huntington's disease, spinocerebellar ataxia type 1 (SCA1), and dentatorubral-pallidoluysian atrophy (DRPLA). The expanded trinucleotide repeats are unstable, and the phenomenon of anticipation, i.e., worsening of disease phenotype over successive generations, correlates with increasing expansion size. In this review, we compare the clinical and molecular features of the trinucleotide repeat diseases, which may be classified into two types. Fragile X and myotonic dystrophy are multisystem disorders usually associated with large expansions of untranslated repeats, while the four neurodegenerative disorders, SBMA, Huntington's disease, SCA1, and DRPLA, are caused by smaller expansions of CAG repeats within the protein coding portion of the gene. CAG repeats encode polyglutamine tracts. Polyglutamine tract expansion thus appears to be a common mechanism of inherited neurodegenerative disease. Although polyglutamine tract lengthening presumably has a toxic gain of function effect in the CAG trinucleotide repeat disorders, the basis of this neuronal toxicity remains unknown.

Heritable trinucleotide repeats and neurological disorders

In the past 3 years, seven human neurological disorders have been found to be associated with an abnormal number of unstable trinucleotide repeats within exons or non-expressed regions of a gene. These forms of mutations are called dynamic mutations. The expansion in copy number of trinucleotide repeats may represent a large number of hereditary disorders. The correlation between the length of the repeated size and the disease severity and variable onset has provided some genetic explanation for a phenomenon called anticipation. However, there are numerous questions which cannot be explained by anticipation. Many other factors such as genomic imprinting and variable DNA methylation may also contribute to the puzzling features of these phenotypes.

Apparent regression of the CGG repeat in FMR1 to an allele of normal size

The fragile X syndrome is the result of amplification of a CGG trinucleotide repeat in the FMR1 gene and anticipation in this disease is caused by an intergenerational expansion of this repeat. Although regression of a CGG repeat in the premutation range is not uncommon, regression from a full premutation (> 200 repeats) or premutation range (50-200 repeats) to a repeat of normal size (< 50 repeats) has not yet been documented. We present here a family in which the number of repeats apparently regressed from approximately 110 in the mother to 44 in her daughter. Although the CGG repeat of the daughter is in the normal range, she is a carrier of the fragile X mutation based upon the segregation pattern of Xq27 markers flanking FMR1. It is unclear, however, whether this allele of 44 repeats will be stably transmitted, as the daughter has as yet no progeny. Nevertheless, the size range between normal alleles and premutation alleles overlap, a factor that complicates genetic counseling.

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.

DNA diagnosis of the fragile X syndrome in a series of 236 mentally retarded subjects and evidence for a reversal of mutation in the FMR-1 gene

The cloning of the FMR-1 gene and the identification of an expanded CGG repeat in DNA of fragile X patients has made reliable DNA diagnosis feasible. Southern blotting and PCR assays of the CGG repeat in an unselected series of 236 mentally retarded subjects resulted in the identification of 10 new fragile X families. Reevaluation of previously assessed fragile X families resulted in the first observation of the presence of a reversal of mutation in the FMR-1 gene.

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.

Is there evidence for anticipation in autosomal-dominant polycystic kidney disease?

The heritability of autosomal-dominant polycystic kidney disease (ADPKD) is marked by an apparent high mutation rate, neonatal onset of disease in some patients and intrafamily variability. These findings raise the possibility of genetic anticipation in ADPKD as has been observed in fragile-X syndrome, myotonic dystrophy and Huntington's disease. We reviewed 242 pedigrees obtained during our prospective studies on the natural history of ADPKD. Anticipation was defined as a 10 year earlier onset of ESRD in offspring as compared to their affected parent or a child diagnosed in the first year of life. Due to the slowly progressive nature of ADPKD, 148 pedigrees were uninformative. Anticipation of ESRD was found in 49% of informative families in at least one parent-offspring pair, and when early onset children were included, 53% of informative families had at least one parent-offspring pair with anticipation. Moreover, the transmitting parent in the pairs with anticipation was more often the mother than the father, similar to myotonic dystrophy, where the most dramatic form of anticipation, congenital disease, occurs almost exclusively with maternal transmission. These observations suggest that ADPKD may be another genetic disorder characterized by heritable unstable DNA.

Trinucleotide repeat expansions and human genetic disease

Trinucleotide repeat expansions are now a well-established mutational mechanism in human genetic disease. An unstable CAG repeat is known to be responsible for three neurodegenerative disorders: Huntington's disease, spinal and bulbar muscular atrophy and spinocerebellar ataxia type 1. Similarities in the genetics of these diseases, the size of the repeat expansions and the position of the unstable repeat within the gene (when known) suggest a common basis to the observed phenotypes. The cloning of two regions at which chromosome breakage can be induced (FRAXA and FRAXE) has in each case uncovered an unstable CG-rich triplet repeat which becomes methylated when fully expanded. In addition to these two classes of mutation, the presence of an expanded CTG repeat in the 3' untranslated region of a protein kinase causes myotonic dystrophy. The size of the respective expansions, repeat stability, mutational origins and possible mechanisms of action are discussed.

Origin of a regressed myotonic dystrophy allele

A new case of regression of the CTG copy number in the myotonic dystrophy allele was observed in a 7 year old boy. His affected father had an expanded allele of about 100 repeats in his lymphocyte DNA while the child showed a 60 repeat allele, of the same size as the present in the grandfather. Analysis of the father's sperm DNA allowed us to detect an expanded fragment of approximately the same size (62 repeats) as that present in the child's and grandfather's lymphocytes. This fragment was not detectable in the father's lymphocytes. Thus the regression is constitutive in the child, being already present in his father's germline. It is therefore likely that the regressed allele is present in all the tissues of the child, allowing a favourable prognosis.

Trinucleotide repeat length instability and age of onset in Huntington's disease

The initial observation of an expanded and unstable trinucleotide repeat in the Huntington's disease gene has now been confirmed and extended in 150 independent Huntington's disease families. HD chromosomes contained 37-86 repeat units, whereas normal chromosomes displayed 11-34 repeats. The HD repeat length was inversely correlated with the age of onset of the disorder. The HD repeat was unstable in more than 80% of meiotic transmissions showing both increases and decreases in size with the largest increases occurring in paternal transmissions. The targeting of spermatogenesis as a particular source of repeat instability is reflected in the repeat distribution of HD sperm DNA. The analysis of the length and instability of individual repeats in members of these families has profound implications for presymptomatic diagnosis.

Expansion of an unstable trinucleotide CAG repeat in spinocerebellar ataxia type 1

Spinocerebellar ataxia type 1 (SCA1) is an autosomal dominant disorder characterized by neurodegeneration of the cerebellum, spinal cord and brainstem. A 1.2-Megabase stretch of DNA from the short arm of chromosome 6 containing the SCA1 locus was isolated in a yeast artificial chromosome contig and subcloned into cosmids. A highly polymorphic CAG repeat was identified in this region and was found to be unstable and expanded in individuals with SCA1. There is a direct correlation between the size of the (CAG)n repeat expansion and the age-of-onset of SCA1, with larger alleles occurring in juvenile cases. We also show that the repeat is present in a 10 kilobase mRNA transcript. SCA1 is therefore the fifth genetic disorder to display a mutational mechanism involving an unstable trinucleotide repeat.

Genes with triplet repeats: candidate mediators of neuropsychiatric disorders

Recently a new form of human mutation-expansion of trinucleotide repeats-has been found to cause the diseases of fragile X syndrome, spinal and bulbar muscular atrophy, myotonic dystrophy and, most recently, Huntington's disease. We review the emerging data on the genetics and neurobiology of these disorders. Three are characterized by unusual patterns of inheritance, in particular, genetic 'anticipation', in which the severity of the disorder increases and the age of onset decreases in successive generations of a pedigree. Several idiopathic neuropsychiatric disorders have features of inheritance consistent with anticipation. In bipolar affective disorder, there is evidence for both earlier age of onset and more severe illness in the second generation of a subset of unilineal pedigrees. There is also the suggestion of anticipation in some forms of schizophrenia, spinocerebellar atrophy and autism. Triplet repeats are present in additional known genes, both in coding regions and untranslated regions. Furthermore, many novel genes with triplet repeats are expressed in the human brain, and these are candidates to cause some forms of these neuropsychiatric disorders.

The full mutation in the FMR-1 gene of male fragile X patients is absent in their sperm

Fragile X syndrome is characterized at the molecular level by amplification of a (CGG)n repeat and hypermethylation of a CpG island preceeding the open reading frame of the fragile X gene (FMR-1) located in Xq27.3. Anticipation in this syndrome is associated with progressive amplification of the (CGG)n repeat from a premutation to a full mutation through consecutive generations. Remarkably, expansion of the premutation to the full mutation is strictly maternal. To clarify this parental influence we studied FMR-1 in sperm of four male fragile X patients. This showed that only the premutation was present in their sperm, although they had a full mutation in peripheral lymphocytes. This might suggest that expansion of the premutation to the full mutation in FMR-1 does not occur in meiosis but in a postzygotic stage.