Repeats may not be everything in anticipation

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.

Adenosine A2A receptor dysfunction correlates with age at onset anticipation in blood platelets of subjects with Huntington's disease

Huntington's disease (HD) may manifest at an earlier age in affected offspring than in transmitting parents. Earlier onset in successive generations (anticipation) only partially depends on intergenerational parent-child elongation of the CAG expanded mutation. An aberrant amplification of adenosine A(2A) receptor signaling documented in peripheral blood cells of subjects with HD implies that this cellular dysfunction may be related to clinical and genetic features. Prompted by evidence of higher receptor densities in siblings of HD subjects with stronger onset anticipation, in this study we investigated a possible relationship between A(2A) receptor densities and age at onset. We measured adenosine A(2A) receptor densities in blood cell platelets from 32 patients with HD and healthy control siblings, and sought a possible linear correlation between maximum platelet A(2A) receptor binding (B(max)) values for the whole cohort of HD subjects and anticipation in years. The increased B(max) values for the 32 subjects with HD (220 in patients vs. 137 in healthy control subjects, P = 0.0001) correlated significantly with anticipation in years (r2, 0.48, P = 0.0001 by linear correlation analysis). An increased platelet A(2A) receptor B(max) may belong in a cascade of toxic events leading to earlier onset of HD: as such it could be a useful marker of onset anticipation.

New Huntington disease mutation arising from a paternal CAG34 allele showing somatic length variation in serially passaged lymphoblasts

The analysis of somatic CAG triplet variation in lymphoblastoid cell lines from subjects carrying alleles of intermediate length (IA(33CAG) and IA(34CAG)) in Huntington disease (HD) gene disclosed instability in the DNA of the person, from whom a new expansion mutation of 45 triplets originated. The triplet size increased after about 30 passages in cell cultures in lymphoblasts with the IA(34) genotype. Lymphoblasts may provide an appropriate model for studying repeat instability in subjects with poly(CAG) repeat disorders. HD shows somatic, in addition to germ-line instability, highlighting the propensity to somatic CAG variation in human cells even with repeat numbers under the expanded edge. Factors potentially cis acting with the mutation, other than those reported in this study (CCG polymorphic stretch, the deletion of the glutamic acid residue at position 2642 and the 4-codon segment between CAG and CCG polymorphisms), should be searched for and analyzed.

The gender effect in juvenile Huntington disease patients of Italian origin

We analyzed a population of juvenile Huntington disease (HD) subjects of Italian origin (n = 57). The main aim of this study was to analyze the gender effect of the affected parent on age at onset and clinical presentation of offspring with juvenile HD. We also analyzed molecular features of the disease, including CAG mutation length and GluR6 gene polymorphism, according to the affected parent's gender. The mutation length was longer in paternally than in maternally transmitted HD juvenile patients (P = 0.025), nevertheless a similar mean early onset in the two groups (P > 0.05). This data was even enforced by that obtained from the whole cohort of patients included in the databank (n = 600) where, in the presence of increased mean parent-child CAG repeat change in paternal vs. maternal meiotic transmissions (+7.3 vs. +0.7 CAG, P = 0.0002), the mean parent-child year-of-onset change was similar in the two groups (-10.4 and -7.0 years, P > 0.05). A lower TAA-triplet in GluR6 was associated with an earlier age at onset in juvenile patients (P = 0.031, R2 = 0.10). When we added the GluR6 effect on age at onset to the CAG expanded number effect (P = 0.0001, R2 = 0.68) by multiple regression approach, the coefficient of determination R2 increased to 0.81. This effect in addition to the expanded CAG repeat number, found in juvenile and not in adult patients, was slightly enforced by paternal compared to maternal transmissions (R2=0.82). Our findings suggest the occurrence of a weaker effect of the paternal mutation on juvenile age at onset in our population, possibly amplified by other genetic factors, such as the TAA-triplet length in the GluR6 gene.

Onset and pre-onset studies to define the Huntington's disease natural history

Huntington's disease's (HD) clinical history has not been defined yet. However, many aspects of the most confusing clinical stages, i.e., the first and last disease phases, including the symptom progression and the disease duration, have been better approached after discovery of the responsible gene. The existence of accurate genetic tests, available for affected and pre-symptomatic subjects (i.e., mutation carriers) and the possibility to study transgenic in vivo models, are actually helping us to understand some of the aspects of HD clinical presentation. HD may present with motor symptoms other than chorea, the psychiatric manifestations may represent part of the clinical picture and cognitive deterioration may occur very early in the disease and depend on early cortical involvement. Pre-onset studies are of crucial importance in understanding the temporal sequence of the clinical events. This is also very important for future therapeutic strategies in those diseases initiating late in the life, such as HD.

Idiopathic painful sensory neuropathy

Unexplained painful burning feet are common complaints among the elderly. Conventional elettrodiagnostic testing may be normal or only minimally abnormal and the lack of a universally accepted diagnostic test for small fiber dysfunction has posed a significant impediment to evaluation of these patients. The increasing application of newer diagnostic techniques, including quantitative sensory testing, tests of sudomotor function, and skin biopsy for intrnepidermal nerve fiber quantification, has permitted recognition and characterization of the idiopathic painful sensory neuropathies This article reviews the clinical features, evaluation, and management of this syndrome.

Sisters homozygous for the spinocerebellar ataxia type 6 (SCA6)/CACNA1A gene associated with different clinical phenotypes

Spinocerebellar ataxia type 6 (SCA6) is a neurodegenerative disease caused by a CAG repeat expansion in the CACNA1A gene. The neurodegeneration that occurs in CAG repeat diseases is considered to share a common mechanism that may result in the gain of a toxic function related to the expanded polyglutamine tracts. However, the phenotypic expression in homozygotes for CAG repeat diseases has been controversial, and is not clearly related to a gain of functional mechanism. We identified a Japanese family with two sisters who were homozygous for the SCA6 with identical CAG repeat expansion (25/25). They showed an earlier age of onset (27 years in both) than their father (44 years), a heterozygote with an expanded allele showing the same CAG repeat length as the homozygotes (25/14). Interestingly, the two sisters showed differences in disease progression and severity, although the age of onset and CAG repeat length were identical. These findings strongly suggest that the gene dosage influences the age of onset, but other unknown factors are also important in the phenotypic expression of homozygous SCA6.