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

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

Somatic expansion behaviour of the (CTG)n repeat in myotonic dystrophy knock-in mice is differentially affected by Msh3 and Msh6 mismatch-repair proteins

The mechanism of expansion of the (CTG)_n repeat in myotonicdystrophy (DM1) patients and the cause of its pathobiologicaleffects are still largely unknown. Most likely, long repeatsexert toxicity at the level of nuclear RNA transport or splicing.Here, we analyse cis- and trans-acting parameters that determinerepeat behaviour in novel mouse models for DM1. Our mice carry‘humanized’ myotonic dystrophy protein kinase (Dmpk)allele(s) with either a (CTG)₈₄ or a (CTG)₁₁ repeat, insertedat the correct position into the endogenous DM locus. Unlikein the human situation, the (CTG)₈₄ repeat in the syntenic mouseenvironment was relatively stable during intergenerational segregation.However, somatic tissues showed substantial repeat expansionswhich were progressive upon aging and prominent in kidney, andin stomach and small intestine, where it was cell-type restricted.Other tissues examined showed only marginal size changes. The(CTG)₁₁ allele was completely stable, as anticipated. Introducingthe (CTG)₈₄ allele into an Msh3-deficient background completelyblocked the somatic repeat instability. In contrast, Msh6 deficiencyresulted in a significant increase in the frequency of somaticexpansions. Competition of Msh3 and Msh6 for binding to Msh2in functional complexes with different DNA mismatch-recognitionspecificity may explain why the somatic (CTG)_n expansion rateis differentially affected by ablation of Msh3 and Msh6. ⁺ To whom correspondence should be addressed. Tel: +31 24 3614329;Fax: +31 24 3615317; Email: b.wieringa@ncmls.kun.nlPresent address:PatriciaJ. T. A. Groenen, Department of Pathology, UMC Nijmegen, Nijmegen,The Netherlands     

Analysis of the CTG repeat in skeletal muscle of young and adult myotonic dystrophy patients: when does the expansion occur?

The purpose of this investigation was to analyze the CTG expansionin muscle as compared to lymphocytes DNA in a sample of selectedmyotonic dystrophy (DM) patients of different ages and degreesof clinical severity, ranging from severe congenital to minimallyaffected. Results from the present study showed that the sizeof the CTG repeat was markedly larger in skeletal muscle thanin lymphocytes in all DM patients. In contrast to lymphocytes,no significant correlation was found between the size of theCTG expansion in muscle and age at onset. In addition, largeexpansions were observed in muscle from all adult symptomaticpatients independently of the presence of muscle weakness, whichraises the question of the value of analyzing CTG expansionsin muscle for predicting the severity of the phenotype. Differencesbetween the size of the CTG expansions in muscle as comparedto lymphocytes were smaller in affected children suggestingan apparent tendency to increase with aging and reaching a plateauin adulthood.     

Increased (CTG/CAG)(n) lengths in myotonic dystrophy type 1 and Machado-Joseph disease genes in idiopathic azoospermia patients

BACKGROUND: An increase in CAG trinucleotide repeat length in the androgen receptor (AR) gene has been linked to idiopathic azoospermia. METHODS: In order to test whether other (CAG/CTG)(n) loci are also affected, the (CAG/CTG)(n) frequency distribution at myotonic dystrophy type 1 (DM1), Machado-Joseph disease (MJD), dentatorubral-pallidoluysian atrophy (DRPLA) and spinocerebellar ataxia type 8 (SCA8) loci, in addition to the AR gene, was investigated in 48 azoospermia patients and 47 controls. RESULTS: The median CAG repeat length in the AR gene was significantly longer in azoospermia patients than in controls (23 versus 21, P < 0.001). Significant differences were also noted in the upper tails of trinucleotide repeat length distributions at both DM1 and MJD loci between the two populations. At the DM1 locus, alleles of more than 18 repeats were observed only in azoospermia patients, and not in controls (P = 0.014). At the MJD locus, the frequency of normal alleles (ANs) with 29 or more CAG repeats was also much higher in azoospermia patients (29.2 versus 7.4%; P = 0.0001). However, the repeat length distribution at DRPLA and SCA8 loci did not differ in the two groups. CONCLUSIONS: These data indicated that, at least in a subset of azoospermia patients, there was an increase in the number of trinucleotide repeats in some disease loci. Thus, it is noteworthy to evaluate whether offspring of these azoospermia patients, if born by assisted reproductive technologies, have an increased risk of trinucleotide repeat diseases.