DM1 Phenotype Variability and Triplet Repeat Instability: Challenges in the Development of New Therapies

Myotonic dystrophy type 1 (DM1) is a complex neuromuscular disease caused by an unstable cytosine thymine guanine (CTG) repeat expansion in the DMPK gene. This disease is characterized by high clinical and genetic variability, leading to some difficulties in the diagnosis and prognosis of DM1. Better understanding the origin of this variability is important for developing new challenging therapies and, in particular, for progressing on the path of personalized treatments. Here, we reviewed CTG triplet repeat instability and its modifiers as an important source of phenotypic variability in patients with DM1.

Genetic and chemical modifiers of a CUG toxicity model in Drosophila

Non-coding CUG repeat expansions interfere with the activity of human Muscleblind-like (MBNL) proteins contributing to myotonic dystrophy 1 (DM1). To understand this toxic RNA gain-of-function mechanism we developed a Drosophila model expressing 60 pure and 480 interrupted CUG repeats in the context of a non-translatable RNA. These flies reproduced aspects of the DM1 pathology, most notably nuclear accumulation of CUG transcripts, muscle degeneration, splicing misregulation, and diminished Muscleblind function in vivo. Reduced Muscleblind activity was evident from the sensitivity of CUG-induced phenotypes to a decrease in muscleblind genetic dosage and rescue by MBNL1 expression, and further supported by the co-localization of Muscleblind and CUG repeat RNA in ribonuclear foci. Targeted expression of CUG repeats to the developing eye and brain mushroom bodies was toxic leading to rough eyes and semilethality, respectively. These phenotypes were utilized to identify genetic and chemical modifiers of the CUG-induced toxicity. 15 genetic modifiers of the rough eye phenotype were isolated. These genes identify putative cellular processes unknown to be altered by CUG repeat RNA, and they include mRNA export factor Aly, apoptosis inhibitor Thread, chromatin remodelling factor Nurf-38, and extracellular matrix structural component Viking. Ten chemical compounds suppressed the semilethal phenotype. These compounds significantly improved viability of CUG expressing flies and included non-steroidal anti-inflammatory agents (ketoprofen), muscarinic, cholinergic and histamine receptor inhibitors (orphenadrine), and drugs that can affect sodium and calcium metabolism such as clenbuterol and spironolactone. These findings provide new insights into the DM1 phenotype, and suggest novel candidates for DM1 treatments.

Male infertility and variation in CAG repeat length in the androgen receptor gene: a meta-analysis

CONTEXT

Many studies have investigated the association between male infertility and trinucleotide repeat polymorphisms in the androgen receptor (AR) gene, but no comprehensive meta-analysis of all published studies has been conducted.

OBJECTIVE

Our goals were to summarize published data on associations between AR CAG and GGC repeat lengths and male infertility and investigate sources of variation between study results.

DATA SOURCES

We searched for reports published before October 2006 using Medline, PubMed, and Web of Science.

STUDY SELECTION

All selected studies included the following: a case group with infertility as measured by semen parameters, a control group of known or presumed fertile men, and measurement of CAG and/or GGC repeat lengths among cases and controls. Thirty-nine reports were selected based on these criteria, and 33 were ultimately included in the meta-analysis.

DATA EXTRACTION

One investigator extracted data on sample size, mean and sd of trinucleotide repeat length, and study characteristics.

DATA SYNTHESIS

Estimates of the standardized mean difference (95% confidence interval) were 0.19 (0.09-0.29) for the 33 studies and 0.31 (0.14-0.47) for a subset of 13 studies that used more stringent case and control selection criteria. Thus, in both groups, cases had statistically significantly longer CAG repeat length than controls. Publication date appeared to be a significant source of variation between studies.

CONCLUSIONS

This meta-analysis provides support for an association between increased androgen receptor CAG length and idiopathic male infertility, suggesting that even subtle disruptions in the androgen axis may compromise male fertility.

Nuclear targeting of the CaMKII anchoring protein αKAP is regulated by alternative splicing and protein kinases

alphaKAP is an anchoring protein for the Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and is encoded within the same gene as the CaMKIIalpha isoform. alphaKAP co-assembles with CaMKII and targets such heteromers to the membrane of the sarcoplasmic reticulum, where CaMKII can regulate Ca(2+) homeostasis. CaMKII has also nuclear functions in skeletal muscle, however, the nuclear targeting mechanism has been elusive. We show here that developmentally regulated splicing of exon Ealpha(B) generates a functional nuclear localization signal (NLS) in alphaKAP(B), the dominant alphaKAP variant in mature muscle. The alphaKAP(A) variant lacks the NLS and dominates in developing muscle before and around birth. Both alphaKAP variants localize to membranes, but a small fraction of alphaKAP(B) is additionally found in the nucleus. Indeed, alpha-karyopherins that mediate nuclear import bound to alphaKAP(B) but not alphaKAP(A) in vitro. When the N-terminal membrane anchor of alphaKAP was deleted, localization of alphaKAP(B) but not alphaKAP(A) became predominantly nuclear. Co-expression of constitutively active CaMKI and IV, which do not bind to alphaKAP, interfered with nuclear localization of alphaKAP(B). CaMKIIalpha was found essentially exclusively in the cytoplasm when expressed in cell lines but was targeted to the nucleus when co-expressed with the nuclear form of alphaKAP(B). Thus, nuclear targeting of cytoplasmic CaMKII isoforms by alphaKAP may be regulated by developmentally controlled alternative splicing and by protein kinases.

[Features of interaction of complexes cortisol-apolipoprotein A-I and tetrahydrocortisol-apolipoprotein A-I with eukariotic DNA]

On primary culture of hepatocytes it is shown, that a complex cortisol-apolipoprotein A-I did not change rate of biosynthesis DNA and protein, whereas the complex tetrahydrocortisol-apolipoprotein A-I (THC-apoA-I) essentially raised rate of incorporation 3H-thymidine in DNA and 14C-leucine into protein. By a method of small-angle X-ray scattering it is shown, that appreciable interaction with eukariotic DNA is marked only in case of use of a complex THC-apoA-I, thus there is local fusion of DNA. The most probable region of interaction of the given complex with DNA is repetition (GCC)n the type, included in structure of many genes eukariot, including the human. It is synthesized oligonucleotid (duplex) of this type. It is shown, that at his interaction with complex THC-apoA-I there is a formation of more difficult complex, which breaks up with formation of complementary chains of oligonucleotides. The last also enter interaction with complex THC-apoA-I. It is given of kinetic this multiphasic process. Interaction of a complex cortisol-anoA-I with a duplex is less specific and does not result reduce in decay of the duplex and in formation of complementary oligonucleotides.

Androgen receptor gene CAG length polymorphism in women with polycystic ovary syndrome

OBJECTIVE

To determine whether the CAG repeat length of the androgen receptor (AR) gene contributes to individual differences in the susceptibility to the development of polycystic ovary syndrome (PCOS).

DESIGN

Retrospective case-control study.

SETTING

University-based clinic.

PATIENT(S)

One hundred six nondiabetic women with PCOS and 112 nonhirsute fertile controls.

INTERVENTION(S)

Androgen receptor gene CAG repeat length was analyzed in women with PCOS and their controls.

MAIN OUTCOME MEASURE(S)

Androgen receptor gene CAG repeat length in both groups.

RESULT(S)

The mean CAGn was 21.5 repeats in both groups (NS; t-test). Furthermore, when the CAGn lengths were divided into three categories (CAGn < or =18, 19-24, > or =25), the distribution was similar in both groups (NS; chi2 test). However, all five women carrying < or =15 CAG repeats belonged to the PCOS group. In the PCOS group, CAGn did not correlate with body mass index or serum testosterone concentration.

CONCLUSION(S)

Androgen receptor CAGn is not a major determinant of PCOS. However, it may be a significant modulator of androgen-related diseases in some individuals.

Drosophilaas a Model for Human Neurodegenerative Disease

Among many achievements in the neurodegeneration field in the past decade, two require special attention due to the huge impact on our understanding of molecular and cellular pathogenesis of human neurodegenerative diseases. First is defining specific mutations in familial neurodegenerative diseases and second is modeling these diseases in easily manipulable model organisms including the fruit fly, nematode, and yeast. The power of these genetic systems has revealed many genetic factors involved in the various pathways affected, as well as provided potential drug targets for therapeutics. This review focuses on fruit fly models of human neurodegenerative diseases, with emphasis on how fly models have provided new insights into various aspects of human diseases.

Orexin loss in Huntington's disease

Huntington's disease (HD) is a devastating neurodegenerative disorder caused by an expanded CAG repeat in the gene encoding huntingtin, a protein of unknown function. Mutant huntingtin forms intracellular aggregates and is associated with neuronal death in select brain regions. The most studied mouse model (R6/2) of HD replicates many features of the disease, but has been reported to exhibit only very little neuronal death. We describe for the first time a dramatic atrophy and loss of orexin neurons in the lateral hypothalamus of R6/2 mice. Importantly, we also found a significant atrophy and loss of orexin neurons in Huntington patients. Like animal models and patients with impaired orexin function, the R6/2 mice were narcoleptic. Both the number of orexin neurons in the lateral hypothalamus and the levels of orexin in the cerebrospinal fluid were reduced by 72% in end-stage R6/2 mice compared with wild-type littermates, suggesting that orexin could be used as a biomarker reflecting neurodegeneration. Our results show that the loss of orexin is a novel and potentially very important pathology in HD.

FRAXE intermediate alleles are associated with Parkinson’s disease

There is evidence that male subjects with a clinical picture of action tremor, Parkinsonism, and cerebellar ataxia may have Fragile X premutations (FRAXA). We analyzed FRAXA and FRAXE triplet repeats in 203 male subjects with Parkinson's disease (PD) and 370 healthy controls. No full mutations or premutations at the FRAXA and FRAXE loci were found in the subjects with PD or in the controls. FRAXA allele distribution was similar in patients and controls. FRAXE intermediate alleles (31-60 repeats CCG) were found in 13 of 203 (6.4%) subjects with PD and in only one of the 370 (0.27%) healthy controls (P < 0.001), thus indicating that these relatively large alleles may be associated with PD.

Role of proteolysis in polyglutamine disorders

To date, nine polyglutamine disorders have been characterised, including Huntington's disease (HD), spinobulbar muscular atrophy (SBMA), dentatorubral-pallidoluysian atrophy (DRPLA), and spinocerebellar ataxias 1, 2, 3, 6, 7 and 17 (SCAs). Although knockout and transgenic mouse experiments suggest that a toxic gain of function is central to neuronal death in these diseases (with the probable exception of SCA6), the exact mechanisms of neurotoxicity remain contentious. A further conundrum is the characteristic distribution of neuronal damage in each disease, despite ubiquitous expression of the abnormal proteins. One mechanism that could possibly underlie the specific distribution of neuronal toxicity is proteolytic cleavage of the full-length expanded polyglutamine tract-containing proteins. There is evidence found in vitro or in vivo (or both) of proteolytic cleavage in HD, SBMA, DRPLA, and SCAs 2, 3, and 7. In HD, cleavage has been demonstrated to be regionally specific, occurring as a result of caspase activation. These diseases are also characterised by development of intraneuronal aggregates of the abnormal protein that co-localise with components of the ubiquitin-proteasome pathway. It remains unclear whether these aggregates are pathogenic or merely disease markers; however, at least in the case of ataxin-3, cleavage promotes aggregation. Inhibition of specific proteases constitutes a potential therapeutic approach in these diseases.

DNA triplet repeats mediate heterochromatin-protein-1-sensitive variegated gene silencing

Gene repression is crucial to the maintenance of differentiated cell types in multicellular organisms, whereas aberrant silencing can lead to disease. The organization of DNA into chromatin and heterochromatin is implicated in gene silencing. In chromatin, DNA wraps around histones, creating nucleosomes. Further condensation of chromatin, associated with large blocks of repetitive DNA sequences, is known as heterochromatin. Position effect variegation (PEV) occurs when a gene is located abnormally close to heterochromatin, silencing the affected gene in a proportion of cells. Here we show that the relatively short triplet-repeat expansions found in myotonic dystrophy and Friedreich's ataxia confer variegation of expression on a linked transgene in mice. Silencing was correlated with a decrease in promoter accessibility and was enhanced by the classical PEV modifier heterochromatin protein 1 (HP1). Notably, triplet-repeat-associated variegation was not restricted to classical heterochromatic regions but occurred irrespective of chromosomal location. Because the phenomenon described here shares important features with PEV, the mechanisms underlying heterochromatin-mediated silencing might have a role in gene regulation at many sites throughout the mammalian genome and modulate the extent of gene silencing and hence severity in several triplet-repeat diseases.

Molecular mechanisms of TRS instability

To date several neurodegenerative disorders including myotonic dystrophy, Huntington's disease, Kennedy's disease, fragile X syndrome, spinocerebellar ataxias or Friedreich's ataxia have been linked to the expanding trinucleotide sequences. Although phenotypic features vary among these debilitating diseases, the structural abnormalities of the triplet repeat containing DNA sequences is the primary cause for all of these disorders. Expansions of the CAG repeat within coding regions of miscellaneous genes result in the synthesis of aberrant proteins containing enormously long polyglutamine stretches. Such proteins acquire toxic functions and/or may direct cells into the apoptotic cycle. On the other hand, massive expansions of various triplet repeats (i.e., CTG/CAG, CGG/CCG/, GAA/TTC) inside the noncoding regions lead to the silencing of transcription and therefore affect expression of the adjacent genes. The repetitive character of TRS allows stretches of such tracts to form slipped-stranded structures, self-complementary hairpins, triplexes or more complex configurations called "sticky DNA", which are not equally processed by some cellular mechanisms, as compared to random DNA. It is likely that the instability of the short TRS (below the threshold level) occurs due to the SILC pathway, which is driven by the DNA slippage. Accumulation of the short expansions leads to the disease premutation state where the MLC pathway becomes predominant. Independent of which mechanism is involved in the MLC pathway (replication, transcription, repair or recombination) the process of complementary strand synthesis is crucial for the TRS instability. Generally, dependent on the location of the tract which has higher potential to form secondary DNA structure, further processing of such tract may result in expansions (secondary structure formed at the newly synthesized strand) or deletions (structure present on the template strand). Analyses of molecular mechanisms of the TRS genetic instability using bacteria, yeast, cell lines and transgenic animals as models allowed the scientists to better understand the role of some major cellular processes in the development of neurodegenerative disorders in humans. However, it is necessary to remember that most of these investigations were focused on the involvement of each particular process separately. Much less of this work though was dedicated to the search for the interactions between such cellular systems that in effect could result in different rate of TRS expansions. Thus, more intensive studies are necessary in order to fully understand the phenomenon ofthe dynamic mutations leading to the human hereditary neurodegenerative diseases.

Androgen receptor gene and male infertility

Androgens are critical steroid hormones that determine the expression of the male phenotype. Their actions are mediated by a single androgen receptor (AR) which, upon ligand binding, translocates to the nucleus to regulate the expression of androgen-responsive genes. Mutations that disrupt AR function totally result in the complete feminization of 46 XY individuals and the complete androgen insensitivity syndrome. Studies have revealed that AR mutations that do not lead to complete abrogation of its activity can cause a wide spectrum of milder androgen insensitivity syndromes, from ambiguous genitalia in newborn infants to 'idiopathic' male infertility. Recent studies indicate that missense amino-acid substitutions in the ligand-binding domain of the AR result in infertility through a novel mechanism that involves defective protein-protein interactions between receptor domains and coactivator proteins. Independent of missense mutations, studies involving Singaporean, Australian, North American and Japanese subjects indicate that increases in length of a trinucleotide repeat (CAG) tract, encoding a polyglutamine stretch in the transactivation domain of the AR, are associated with increased risk of defective spermatogenesis and undermasculinization. This association was however not observed in European populations, suggesting that the genetic background may play a significant role in the expression of the AR defects.

Fragile X premutation carriers: characteristic MR imaging findings of adult male patients with progressive cerebellar and cognitive dysfunction

BACKGROUND AND PURPOSE

Our purpose was to characterize the findings of MR imaging of the brain of adult male fragile X premutation carriers with a recently identified disorder characterized by ataxia, tremor, rigidity, and cognitive dysfunction.

METHODS

MR imaging studies of the brain of 17 male patients were characterized for signal intensity and for size of ventricles, cerebral and cerebellar sulci, and brain stem. Comparison was made with age- and sex-matched control participants. Southern blot and/or polymerase chain reaction methods were used to analyze CGG trinucleotide repeats in the fragile X mental retardation 1 gene.

RESULTS

Fifteen of 17 patients showed symmetrically decreased T1 and increased T2 signal intensity in cerebellar white matter lateral, superior, and inferior to the dentate nuclei. Fourteen of 17 had similar signal intensity alterations in the middle cerebellar peduncles. Cerebellar cortical atrophy was present in 16 of 17 and cerebral atrophy in 17 of 17. Evan's Index as a measure of ventricular size averaged 0.35 (range, 0.25-0.46), with that for age-matched control participants averaging 0.28 (range, 0.24-0.31) (P <.005). The mean third ventricle width was 11 mm (for control participants, 6 mm; P <.01). Corpus callosum was thinned in 14 of 16 participants. Middle cerebellar peduncles were atrophic when compared with those of control participants (P <.005). Pontine transverse dimension was 25 mm (for control participants, 31 mm; P <.005), and rostral-caudal length averaged 26 mm (for control participants, 29 mm; P <.005). CGG repeats clustered in the low to mid premutation range (86 +/- 10 CGG repeats) in the 17 patients.

CONCLUSION

MR imaging findings in symptomatic male fragile X premutation carriers are characteristic of this disorder. Recognition of these alterations may support a specific diagnosis and may have implications for the potential occurrence of fragile X syndrome in the children of reproductive age female relatives.

Identification of RTG2 as a modifier gene for CTG*CAG repeat instability in Saccharomyces cerevisiae

Trinucleotide repeats (TNRs) undergo frequent mutations in families affected by TNR diseases and in model organisms. Much of the instability is conferred in cis by the sequence and length of the triplet tract. Trans-acting factors also modulate TNR instability risk, on the basis of such evidence as parent-of-origin effects. To help identify trans-acting modifiers, a screen was performed to find yeast mutants with altered CTG.CAG repeat mutation frequencies. The RTG2 gene was identified as one such modifier. In rtg2 mutants, expansions of CTG.CAG repeats show a modest increase in rate, depending on the starting tract length. Surprisingly, contractions were suppressed in an rtg2 background. This creates a situation in a model system where expansions outnumber contractions, as in humans. The rtg2 phenotype was apparently specific for CTG.CAG repeat instability, since no changes in mutation rate were observed for dinucleotide repeats or at the CAN1 reporter gene. This feature sets rtg2 mutants apart from most other mutants that affect genetic stability both for TNRs and at other DNA sequences. It was also found that RTG2 acts independently of its normal partners RTG1 and RTG3, suggesting a novel function of RTG2 that helps modify CTG.CAG repeat mutation risk.

Instability of CAG-trinucleotide repeats in chronic lymphocytic leukemia

Anticipation--earlier onset and more severe disease in the offspring generation--is a well documented feature of familial chronic lymphocytic leukaemia (CLL). In a number of Mendelian diseases, anticipation is caused by expansion of contiguous triplets of nucleotides. The severity of disease expression and penetrance is related to the extent of the triplet expansion. To investigate whether repeat nucleotide repeat expansion is a feature of CLL, the repeat expansion detection (RED) technique was applied to samples from 17 patients with familial disease and 32 patients with early-onset CLL disease. No potentially pathological CAG expansions were detected. We conclude that unstable CAG repeat expansion is not a feature of CLL and that other processes are likely to be involved in generating anticipation in familial forms of the disease.

Transcription and nuclear transport of CAG/CTG trinucleotide repeats in yeast

Trinucleotide repeats are involved in several neurological disorders in humans. DNA sequences containing CAG/CTG repeats are prone to slippage during replication and double-strand break repair. The effects of trinucleotide repeats on transcription and on nuclear export were analyzed in vivo in yeast. Transcription of a CAG/CTG trinucleotide repeat in the 3'-untranslated region of a URA3 reporter gene leads to transcription of messenger RNAs several kilobases longer than the expected size. These long mRNAs form more readily when CAG rather than CTG repeats are transcribed. CAG- or CUG-containing transcripts show a non-homogeneous cellular localization. We propose that long mRNAs result from transcription slippage, and discuss the possible implications for human diseases.

The androgen receptor CAG repeat: a modifier of carcinogenesis?

The first exon of the human androgen receptor (AR) contains a translated CAG (poly-glutamine) repeat. The repeat length is polymorphic in the normal population ranging from 8 to 35 repeats. Expansions to over 40 repeats lead to spinal bulbar muscular atrophy (SBMA), a late onset neurodegenerative disease. The repeat is located between the two parts of a bipartite amino-terminal transactivation function and the repeat length, also within in the normal range, is inversely correlated to the transactivation power of the receptor. P160 type co-activators bind more strongly to shorter repeats. A correlation between AR CAG repeat length and total risk, age at diagnosis, recurrence after surgery and aggressive growth has been reported for tumors of classical androgen target tissues. In the prostate, where androgens exert a mitogenic effect, the cancer risk increases with decreasing AR-CAG repeat length. In contrast, in the breast, where the hormone probably acts as anti-mitogen, a higher risk and earlier onset of breast cancer has been reported for carriers of BRCA1 mutations who also have long CAG repeats in the receptor gene. Somatic alterations during carcinogenesis appear to be frequent in endometrial and in colon cancer. In the endometrium the AR CAG repeat prevalently undergoes expansions consistent with the putative protective function of androgens in this tissue. Frequent repeat reductions during colon carcinogenesis would be consistent with a mitogenic effect of androgens. Analysis of AR protein expression by Western blot reveals expression of the AR in healthy and neoplastic colon tissues. Normal mucosa of the colon expresses both AR-isoforms of 110 and 87 kDa, while the tumor samples have lost the expression of the 110-kDa isoform. The 87-kDa isoform is devoid of the amino-terminal portion of the receptor molecule that also contains the poly-glutamine tract. The temporal and causal relation between isoform switch and somatic repeat reductions during colon carcinogenesis is as yet unclear, but the two events could both enhance p160 mediated androgen signaling. The recent finding that smad3 interacts with the AR in a way similar to p160 links the AR to TGFbeta signaling. Interruption of this signaling pathway is a frequent event in colon carcinogenesis.

Identification of ter94, Drosophila VCP, as a modulator of polyglutamine-induced neurodegeneration

We have successfully generated a Drosophila model of human polyglutamine (polyQ) diseases by the targeted expression of expanded-polyQ (ex-polyQ) in the Drosophila compound eye. The resulting eye degeneration is progressive and ex-polyQ dosage- and ex-polyQ length-dependent. Furthermore, intergenerational changes in repeat length were observed in homozygotes, with concomitant changes in the levels of degeneration. Through genetic screening, using this fly model, we identified loss-of-function mutants of the ter94 gene that encodes the Drosophila homolog of VCP/CDC48, a member of the AAA+ class of the ATPase protein family, as dominant suppressors. The suppressive effects of the ter94 mutants on ex-polyQ-induced neurodegeneration correlated well with the degrees of loss-of-function, but appeared not to result from the inhibition of ex-polyQ aggregate formation. In the ex-polyQ-expressing cells of the late pupa, an upregulation of ter94 expression was observed prior to cell death. Co-expression of ter94 with ex-polyQ severely enhanced eye degeneration. Interestingly, when ter94 was overexpressed in the eye by increasing the transgene copies, severe eye degeneration was induced. Furthermore, genetical studies revealed that ter94 was not involved in grim-, reaper-, hid-, ced4-, or p53-induced cell death pathways. From these observations, we propose that VCP is a novel cell death effector molecule in ex-polyQ-induced neurodegeneration, where the amount of VCP is critical. Control of VCP expression may thus be a potential therapeutic target in ex-polyQ-induced neurodegeneration.

Partial resistance to malonate-induced striatal cell death in transgenic mouse models of Huntington's disease is dependent on age and CAG repeat length

Transgenic Huntington's disease (HD) mice, expressing exon 1 of the HD gene with an expanded CAG repeat, are totally resistant to striatal lesion induced by excessive NMDA receptor activation. We now show that striatal lesions induced by the mitochondrial toxin malonate are reduced by 70-80% in transgenic HD mice compared with wild-type littermate controls. This occurred in 6- and 12-week-old HD mice with 150 CAG repeats (line R6/2) and in 18-week-old, but not 6-week-old, HD mice with 115 CAG repeats (line R6/1). Therefore, we show for the first time that the resistance to neurotoxin in transgenic HD mice is dependent on both the CAG repeat length and the age of the mice. Importantly, most HD patients develop symptoms in adulthood and exhibit an inverse relationship between CAG repeat length and age of onset. Transgenic mice expressing a normal CAG repeat (18 CAG) were not resistant to malonate. Although endogenous glutamate release has been implicated in malonate-induced cell death, glutamate release from striatal synaptosomes was not decreased in HD mice. Malonate-induced striatal cell death was reduced by 50-60% in wild-type mice when they were treated with either the NMDA receptor antagonist MK-801 or the caspase inhibitor zVAD-fmk. These two compounds did not reduce lesion size in transgenic R6/1 mice. This might suggest that NMDA receptor- and caspase-mediated cell death pathways are inhibited and that the limited malonate-induced cell death still occurring in HD mice is independent of these pathways. There were no changes in striatal levels of the two anti cell death proteins Bcl-X(L) and X-linked inhibitor of apoptosis protein (XIAP), before or after the lesion in transgenic HD mice. We propose that mutant huntingtin causes a sublethal grade of metabolic stress which is CAG repeat length-dependent and results in up-regulation over time of cellular defense mechanisms against impaired energy metabolism and excitotoxicity.

Role for CCG-trinucleotide repeats in the pathogenesis of chronic lymphocytic leukemia

Chromosome 11q deletions are frequently observed in chronic lymphocytic leukemia (CLL) in association with progressive disease and a poor prognosis. A minimal region of deletion has been assigned to 11q22-q23. Trinucleotide repeats have been associated with anticipation in disease, and evidence of anticipation has been observed in various malignancies including CLL. Loss of heterozygosity at 11q22-23 is common in a wide range of cancers, suggesting this is an unstable area prone to chromosome breakage. The location of 8 CCG-trinucleotide repeats on 11q was determined by Southern blot analysis of a 40-Mb YAC and PAC contig spanning 11q22-qter. Deletion breakpoints in CLL are found to co-localize at specific sites on 11q where CCG repeats are located. In addition, a CCG repeat has been identified within the minimal region of deletion. Specific alleles of this repeat are associated with worse prognosis. Folate-sensitive fragile sites are regions of late replication and are characterized by CCG repeats. The mechanism for chromosome deletion at 11q could be explained by a delay in replication. Described here is an association between CCG repeats and chromosome loss suggesting that in vivo "fragile sites" exist on 11q and that the instability of CCG repeats may play an important role in the pathogenesis of CLL.

CUG repeat binding protein (CUGBP1) interacts with the 5' region of C/EBPbeta mRNA and regulates translation of C/EBPbeta isoforms

The transcription factor CCAAT/enhancer binding protein beta, C/EBPbeta, plays a significant role in the regulation of hepatocyte growth and differentiation. A single mRNA coding for C/EBPbeta produces several protein isoforms. Two pathways for generation of low molecular weight C/EBPbeta isoforms have been described: specific proteolytic cleavage and initiation of translation from different AUG codons of C/EBPbeta mRNA. A truncated C/EBPbeta isoform, LIP, is induced in rat livers in response to partial hepatectomy (PH) via the alternative translation mechanism. Here we present evidence that CUG repeat binding protein, CUGBP1, interacts with the 5' region of C/EBPbeta mRNA and regulates translation of C/EBPbeta isoforms. Two binding sites for CUGBP1 are located side by side between the first and second AUG codons of C/EBPbeta mRNA. One binding site is observed in an out of frame short open reading frame (sORF) that has been previously shown to regulate initiation of translation from different AUG codons of C/EBPbeta mRNA. Analysis of cytoplasmic and polysomal proteins from rat liver after PH showed that CUGBP1 is associated with polysomes that translate low molecular weight isoforms of C/EBPbeta. The binding activity of CUGBP1 to the 5' region of C/EBPbeta mRNA shows increased association with these polysomal fractions after PH. Addition of CUGBP1 into a cell-free translation system leads to increased translation of low molecular weight isoforms of C/EBPbeta. Our data demonstrate that CUGBP1 protein is an important component for the regulation of initiation from different AUG codons of C/EBPbeta mRNA.

Trinucleotide repeats and other microsatellites in yeasts

Microsatellites are direct tandem DNA repeats found in all genomes. A particular class of microsatellites, called trinucleotide repeats, is responsible for a number of neurological disorders in humans. We review here our current state of knowledge on trinucleotide repeat instability, and discuss the molecular mechanisms that may be involved in trinucleotide repeat expansions leading to fatal diseases in humans. We also present original data on microsatellite distribution in several microbial genomes, and on the use of microsatellites as physical markers to accurately and easily genotype yeast strains.

Phenotypic involvement in females with the FMR1 gene mutation

Fragile X syndrome is the most common from of inherited mental retardation. Approximately half of females with the full mutation have significant cognitive deficits, whereas females with the premutation do not. Phenotypic effects seen in 281 females (IQs from 64 to 139) were analyzed. Results showed that females with the full mutation differ significantly from controls on selected anthropometric measurements, physical index score, and various behavioral features. Females with the premutation differed significantly from controls in regards to a few anthropometric measurements and the physical index score but not in behavioral features. These results suggest that phenotypic effects of the FMR1 mutation are not only common in females with the full mutation, but in females with the premutation as well.

Correlation of peripheral nerve fiber loss and trinucleotide repeats in Machado-Joseph disease

BACKGROUND

Machado-Joseph disease (MJD) is a dominantly inherited cerebellar ataxia associated with spasticity, ophthalmoplegia and dystonia. There has been no report of electrophysiological or histological alterations of the peripheral nervous system in patients with MJD.

METHODS

Four patients with MJD were identified by polymerase chain reaction. The peripheral nerves of these patients were subjected to electrophysiological testing and histological study. Correlation analyses were made between various clinical parameters and the electrophysiological and histological changes.

RESULTS

Electrophysiological studies demonstrated a marked reduction of sensory action potential, acute denervation changes on needle EMG, as well as mild decrease in the compound motor action potential. Light microscopy of the sural nerves revealed clear loss of myelinated fibers, and morphometry studies showed a loss of large myelinated fibers. Moreover, the severity of these pathological changes was found to be related to the CAG repeat length in the MJD gene.

CONCLUSION

Our findings indicated that the peripheral nervous system was frequently affected in patients with MJD. These findings were similar to those seen in Friedreich's ataxia, suggesting a loss of sensory and motor fibers probably following a lesion of the dorsal root ganglion and the anterior horns in the spinal cord. Furthermore, the number of CAG repeats seems to have an inverse relationship to the extent of pathological changes of the peripheral nerves.

Increased trinucleotide repeat instability with advanced maternal age

Nucleotide repeat instability is associated with an increasing number of cancers and neurological disorders. The mechanisms that govern repeat instability in these biological disorders are not well understood. To examine genetic aspects of repeat instability we have introduced an expanded CAG trinucleotide repeat into transgenic mice. We have detected intergenerational CAG repeat instability in transgenic mice only when the transgene was maternally transmitted. These intergenerational instabilities increased in frequency and magnitude as the transgenic mother aged. Furthermore, triplet repeat variations were detected in unfertilized oocytes and were comparable with those in the offspring. These data show that maternal repeat instability in the transgenic mice occurs after meiotic DNA replication and prior to oocyte fertilization. Thus, these findings demonstrate that advanced maternal age is an important factor for instability of nucleotide repeats in mammalian DNA.

Meiotic instability associated with the CAGR1 trinucleotide repeat at 13q13

CAGR1 is a recently characterised polymorphic trinucleotide repeat localised to 13q13, which has been suggested as a possible candidate gene for neurological disorders that manifest genetic anticipation. To provide evidence in support of this hypothesis, a large number of chromosomes (n = 928) from patients with a wide variety of neurological diseases were screened for evidence of repeat expansion and meiotic instability. One person with a CAGR1 repeat number of 50 was identified (normal range 9-29). Subsequent molecular analyses of CAGR1 repeat number in additional family members showed meiotic instability of a (CAG)45 allele through three generations. While CAGR1 repeat number did not correlate with a readily discernible phenotype in this family, the finding of meiotic stability and mendelian inheritance of normal CAG alleles and meiotic instability of larger repeats fulfil several criteria thought essential for pathologically relevant mutations of this type. Thus, these data strengthen the hypothesis for a role of CAGR1 in the development of an as yet molecularly uncharacterised human neurological disease.