A CCG repeat polymorphism adjacent to the CAG repeat in the Huntington disease gene: implications for diagnostic accuracy and predictive testing

Neurophysiological hallmarks of Huntington's disease progression: an EEG and fMRI connectivity study

Electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) can provide corroborative data on neurophysiological alterations in Huntington's disease (HD). However, the alterations in EEG and fMRI resting-state functional connectivity (rsFC), as well as their interrelations, at different stages of HD remain insufficiently investigated. This study aimed to identify neurophysiological alterations in individuals with preclinical HD (preHD) and early manifest HD (EMHD) by analyzing EEG and fMRI rsFC and examining their interrelationships. We found significant differences in EEG power between preHD individuals and healthy controls (HC), with a decrease in power in a specific frequency range at the theta-alpha border and slow alpha activity. In EMHD patients, in addition to the decrease in power in the 7-9 Hz range, a reduction in power within the classic alpha band compared to HC was observed. The fMRI analysis revealed disrupted functional connectivity in various brain networks, particularly within frontal lobe, putamen-cortical, and cortico-cerebellar networks, in individuals with the HD mutation compared to HC. The analysis of the relationship between EEG and fMRI rsFC revealed an association between decreased alpha power, observed in individuals with EMHD, and increased connectivity in large-scale brain networks. These networks include putamen-cortical, DMN-related and cortico-hippocampal circuits. Overall, the findings suggest that EEG and fMRI provide valuable information for monitoring pathological processes during the development of HD. A decrease in inhibitory control within the putamen-cortical, DMN-related and cortico-hippocampal circuits, accompanied by a reduction in alpha and theta-alpha border oscillatory activity, could potentially contribute to cognitive decline in HD.

Characterising the RNA-binding protein atlas of the mammalian brain uncovers RBM5 misregulation in mouse models of Huntington's disease

RNA-binding proteins (RBPs) are key players regulating RNA processing and are associated with disorders ranging from cancer to neurodegeneration. Here, we present a proteomics workflow for large-scale identification of RBPs and their RNA-binding regions in the mammalian brain identifying 526 RBPs. Analysing brain tissue from males of the Huntington's disease (HD) R6/2 mouse model uncovered differential RNA-binding of the alternative splicing regulator RBM5. Combining several omics workflows, we show that RBM5 binds differentially to transcripts enriched in pathways of neurodegeneration in R6/2 brain tissue. We further find these transcripts to undergo changes in splicing and demonstrate that RBM5 directly regulates these changes in human neurons derived from embryonic stem cells. Finally, we reveal that RBM5 interacts differently with several known huntingtin interactors and components of huntingtin aggregates. Collectively, we demonstrate the applicability of our method for capturing RNA interactor dynamics in the contexts of tissue and disease.

Haplotyping SNPs for allele-specific gene editing of the expanded huntingtin allele using long-read sequencing

Huntington's disease (HD) is an autosomal dominant neurodegenerative disease caused by CAG trinucleotide repeat expansions in exon-1 of huntingtin (HTT). Currently, there is no cure for HD, and the clinical care of individuals with HD is focused on symptom management. Previously, we showed allele-specific deletion of the expanded HTT allele (mHTT) using CRISPR-Cas9 by targeting nearby (<10 kb) SNPs that created or eliminated a protospacer adjacent motif (PAM) near exon-1. Here, we comprehensively analyzed all potential PAM sites within a 10.4-kb genomic region flanking exon-1 of HTT in 983 individuals with HD using a multiplex targeted long-read sequencing approach on the Oxford Nanopore platform. We developed computational tools (NanoBinner and NanoRepeat) to de-multiplex the data, detect repeats, and phase the reads on the expanded or the wild-type HTT allele. One SNP common to 30% of individuals with HD of European ancestry emerged through this analysis, which was confirmed as a strong candidate for allele-specific deletion of the mHTT in human HD cell lines. In addition, up to 57% HD individuals may be candidates for allele-specific editing through combinatorial SNP targeting. Cumulatively, we provide a haplotype map of the region surrounding exon-1 of HTT in individuals affected with HD. Our workflow can be applied to other repeat expansion diseases to facilitate the design of guide RNAs for allele-specific gene editing.

Investigation of the Influence of TBP CAG/CAA Repeats in Conjunction with HTT CAG Repeats on Huntington's Disease Age at Onset in a Brazilian Sample

Huntington's disease (HD) is a genetic neurodegenerative progressive and fatal disease characterized by motor disorder, cognitive impairment, and behavioral problems, caused by expanded repeats of CAG trinucleotides in the HTT gene. The aim of this study was to investigate the influence of TBP gene CAG/CAA repeats in conjunction with HTT gene CAG repeats, on the age at HD onset in Brazilian individuals. Individuals diagnosed as molecularly negative for HD presented 29-39 TBP CAG/CAA. Their most frequent allele had 36 repeats. In individuals diagnosed as molecularly positive for HD, a range of 25-40 TBP CAG/ CAA was found. The most frequent TBP allele had 38 repeats. We also conducted TBP direct Sanger sequencing of some samples which demonstrated other four TBP structures different from the basic TBP structure and others reported in the literature. The HTT expanded CAG and TBP CAG/CAA repeat sizes jointly explained 66% of the age at onset (AO) in our HD patients. The strongest variable in the model associated with AO was the number of expanded HTT CAG repeats. The difference between the association of HD AO with HTT expanded CAG together with TBP CAG/CAA and the association of HD AO with HTT expanded CAG was 0.001 (∆R²). Therefore, we found a weak association (0.1%) of TBP CAG/CAA repeats on HD AO, if any.

Comprehensive genetic diagnosis of tandem repeat expansion disorders with programmable targeted nanopore sequencing

More than 50 neurological and neuromuscular diseases are caused by short tandem repeat (STR) expansions, with 37 different genes implicated to date. We describe the use of programmable targeted long-read sequencing with Oxford Nanopore's ReadUntil function for parallel genotyping of all known neuropathogenic STRs in a single assay. Our approach enables accurate, haplotype-resolved assembly and DNA methylation profiling of STR sites, from a list of predetermined candidates. This correctly diagnoses all individuals in a small cohort (n = 37) including patients with various neurogenetic diseases (n = 25). Targeted long-read sequencing solves large and complex STR expansions that confound established molecular tests and short-read sequencing and identifies noncanonical STR motif conformations and internal sequence interruptions. We observe a diversity of STR alleles of known and unknown pathogenicity, suggesting that long-read sequencing will redefine the genetic landscape of repeat disorders. Last, we show how the inclusion of pharmacogenomic genes as secondary ReadUntil targets can further inform patient care.

A Novel Triplet-Primed PCR Assay to Detect the Full Range of Trinucleotide CAG Repeats in the Huntingtin Gene (HTT)

The expanded CAG repeat number in HTT gene causes Huntington disease (HD), which is a severe, dominant neurodegenerative illness. The accurate determination of the expanded allele size is crucial to confirm the genetic status in symptomatic and presymptomatic at-risk subjects and avoid genetic polymorphism-related false-negative diagnoses. Precise CAG repeat number determination is critical to discriminate the cutoff between unexpanded and intermediate mutable alleles (IAs, 27–35 CAG) as well as between IAs and pathological, low-penetrance alleles (i.e., 36–39 CAG repeats), and it is also critical to detect large repeat expansions causing pediatric HD variants. We analyzed the HTT-CAG repeat number of 14 DNA reference materials and of a DNA collection of 43 additional samples carrying unexpanded, IAs, low and complete penetrance alleles, including large (>60 repeats) and very large (>100 repeats) expansions using a novel triplet-primed PCR-based assay, the AmplideX PCR/CE HTT Kit. The results demonstrate that the method accurately genotypes both normal and expanded HTT-CAG repeat numbers and reveals previously undisclosed and very large CAG expansions >200 repeats. We also show that this technique can improve genetic test reliability and accuracy by detecting CAG expansions in samples with sequence variations within or adjacent to the repeat tract that cause allele drop-outs or inaccuracies using other PCR methods.

Missing the pathological expansion in Huntington disease: de novo c.51C>G variant on the expanded allele causing intrafamilial allele dropout

Huntington disease (HD) is an autosomal dominant disease characterized by motor, behavioral, and cognitive symptoms, caused by the pathological expansion of more than 35 CAG/CAA repeats in the HTT gene. We describe the phenotype of a patient compatible with HD. Several family members were reported as affected, and a paternal cousin and his daughter carried 39 and 42 CAG/CAA. HD genetic testing in proband showed homozygosity for a 14 CAG/CAA allele. Considering the phenotype and family history, HTT gene sequence was performed, revealing heterozygosity for the c.51C>G variant that changes the last nucleotide before the CAG tract, causing misannealing of forward primer (HD344) and dropout of the expanded allele. Polymerase chain reaction (PCR) analysis performed with an alternative forward primer demonstrated a 41 CAG/CAA allele. The c.51C>G variant was not detected in the affected cousin, thus suggesting a de novo occurrence. The lack of biological samples from the proband father and grandmother prevented further investigations to establish in which family member the variant occurred. These data indicate that patients presenting HD phenotype, and homozygous for a normal HTT CAG/CAA allele should be thoroughly evaluated for the presence of a genetic variant, even de novo, within the repeat region that may hamper genetic diagnosis.

Integrative Characterization of the R6/2 Mouse Model of Huntington's Disease Reveals Dysfunctional Astrocyte Metabolism

Huntington's disease is a fatal neurodegenerative disease, where dysfunction and loss of striatal and cortical neurons are central to the pathogenesis of the disease. Here, we integrated quantitative studies to investigate the underlying mechanisms behind HD pathology in a systems-wide manner. To this end, we used state-of-the-art mass spectrometry to establish a spatial brain proteome from late-stage R6/2 mice and compared this with wild-type littermates. We observed altered expression of proteins in pathways related to energy metabolism, synapse function, and neurotransmitter homeostasis. To support these findings, metabolic ¹³C labeling studies confirmed a compromised astrocytic metabolism and regulation of glutamate-GABA-glutamine cycling, resulting in impaired release of glutamine and GABA synthesis. In recent years, increasing attention has been focused on the role of astrocytes in HD, and our data support that therapeutic strategies to improve astrocytic glutamine homeostasis may help ameliorate symptoms in HD.

Robust Preimplantation Genetic Testing of Huntington Disease by Combined Triplet-Primed PCR Analysis of the HTT CAG Repeat and Multi-Microsatellite Haplotyping

Huntington disease (HD) is a lethal neurodegenerative disorder caused by expansion of a CAG repeat within the huntingtin (HTT) gene. Disease prevention can be facilitated by preimplantation genetic testing for this monogenic disorder (PGT-M). We developed a strategy for HD PGT-M, involving whole genome amplification (WGA) followed by combined triplet-primed PCR (TP-PCR) for HTT CAG repeat expansion detection and multi-microsatellite marker genotyping for disease haplotype phasing. The strategy was validated and tested pre-clinically in a simulated PGT-M case before clinical application in five cycles of a PGT-M case. The assay reliably and correctly diagnosed all embryos, even where allele dropout (ADO) occurred at the HTT CAG repeat locus or at one or more linked markers. Ten of the 27 embryos analyzed were diagnosed as unaffected. Four embryo transfers were performed, two of which involved fresh cycle double embryo transfers and two were frozen-thawed single embryo transfers. Pregnancies were achieved from each of the frozen-thawed single embryo transfers and confirmed to be unaffected by amniocentesis, culminating in live births at term. This strategy enhances diagnostic confidence for PGT-M of HD and can also be employed in situations where disease haplotype phase cannot be established prior to the start of PGT-M.

Reviewing Biochemical Implications of Normal and Mutated Huntingtin in Huntington's Disease

Huntingtin (Htt) is a multi-function protein of the brain. Normal Htt shows a common alpha-helical structure but conformational changes in the form with beta strands are the principal cause of Huntington's disease. Huntington's disease is a genetic neurological disorder caused by a repeated expansion of the CAG trinucleotide, causing instability in the N-terminal of the gene coding for the Huntingtin protein. The mutation leads to the abnormal expansion of the production of the polyglutamine tract (polyQ) resulting in the form of an unstable Huntingtin protein commonly referred to as mutant Huntingtin. Mutant Huntingtin is the cause of the complex neurological metabolic alteration of Huntington's disease, resulting in both the loss of all the functions of normal Huntingtin and the genesis of abnormal interactions due to the presence of this mutation. One of the problems arising from the misfolded Huntingtin is the increase in oxidative stress, which is common in many neurological diseases such as Alzheimer's, Parkinson's, Amyotrophic Lateral Sclerosis and Creutzfeldt-Jakob disease. In the last few years, the use of antioxidants had a strong incentive to find valid therapies for defence against neurodegenerations. Although further studies are needed, the use of antioxidant mixtures to counteract neuronal damages seems promising.

Enhanced cerebral branched-chain amino acid metabolism in R6/2 mouse model of Huntington's disease

Huntington's disease (HD) is a hereditary and fatal disease causing profound neurodegeneration. Deficits in cerebral energy and neurotransmitter metabolism have been suggested to play a central role in the neuronal dysfunction and death associated with HD. The branched-chain amino acids (BCAAs), leucine, isoleucine and valine, are important for cerebral nitrogen homeostasis, neurotransmitter recycling and can be utilized as energy substrates in the tricarboxylic acid (TCA) cycle. Reduced levels of BCAAs in HD have been validated by several reports. However, it is still unknown how cerebral BCAA metabolism is regulated in HD. Here we investigate the metabolism of leucine and isoleucine in the R6/2 mouse model of HD. Acutely isolated cerebral cortical and striatal slices of control and R6/2 mice were incubated in media containing 15N- or 13C-labeled leucine or isoleucine and slice extracts were analyzed by gas chromatography-mass spectrometry (GC-MS) to determine isotopic enrichment of derived metabolites. Elevated BCAA transamination was found from incubations with [15N]leucine and [15N]isoleucine, in both cerebral cortical and striatal slices of R6/2 mice compared to controls. Metabolism of [U-13C]leucine and [U-13C]isoleucine, entering oxidative metabolism as acetyl CoA, was maintained in R6/2 mice. However, metabolism of [U-13C]isoleucine, entering the TCA cycle as succinyl CoA, was elevated in both cerebral cortical and striatal slices of R6/2 mice, suggesting enhanced metabolic flux via this anaplerotic pathway. To support the metabolic studies, expression of enzymes in the BCAA metabolic pathway was assessed from a proteomic resource. Several enzymes related to BCAA metabolism were found to exhibit augmented expression in the R6/2 brain, particularly related to isoleucine metabolism, suggesting an increase in the BCAA metabolic machinery. Our results show that the capacity for cerebral BCAA metabolism, predominantly of isoleucine, is amplified in the R6/2 brain and indicates that perturbations in cerebral BCAA homeostasis could have functional consequences for HD pathology.

Detailed analysis of HTT repeat elements in human blood using targeted amplification-free long-read sequencing

Amplification of DNA is required as a mandatory step during library preparation in most targeted sequencing protocols. This can be a critical limitation when targeting regions that are highly repetitive or with extreme guanine-cytosine (GC) content, including repeat expansions associated with human disease. Here, we used an amplification-free protocol for targeted enrichment utilizing the CRISPR/Cas9 system (No-Amp Targeted sequencing) in combination with single molecule, real-time (SMRT) sequencing for studying repeat elements in the huntingtin (HTT) gene, where an expanded CAG repeat is causative for Huntington disease. We also developed a robust data analysis pipeline for repeat element analysis that is independent of alignment of reads to a reference genome. The method was applied to 11 diagnostic blood samples, and for all 22 alleles the resulting CAG repeat count agreed with previous results based on fragment analysis. The amplification-free protocol also allowed for studying somatic variability of repeat elements in our samples, without the interference of PCR stutter. In summary, with No-Amp Targeted sequencing in combination with our analysis pipeline, we could accurately study repeat elements that are difficult to investigate using PCR-based methods.

Improved high sensitivity screen for Huntington disease using a one-step triplet-primed PCR and melting curve assay

Molecular diagnosis of Huntington disease (HD) is currently performed by fluorescent repeat-flanking or triplet-primed PCR (TP-PCR) with capillary electrophoresis (CE). However, CE requires multiple post-PCR steps and may result in high cost in high-throughput settings. We previously described a cost-effective single-step molecular screening strategy employing the use of melting curve analysis (MCA). However, because it relies on repeat-flanking PCR, its efficiency in detecting expansion mutations decreases with increasing size of the repeat, which could lead to false-negative results. To address this pitfall, we have developed an improved screening assay coupling TP-PCR, which has been shown in CE-based assays to detect all expanded alleles regardless of size, with MCA in a rapid one-step assay. A companion protocol for rapid size confirmation of expansion-positive samples is also described. The assay was optimized on 30 genotype-known DNAs, and two plasmids pHTT(CAG)26 and pHTT(CAG)33 were used to establish the threshold temperatures (TTs) distinguishing normal from expansion-positive samples. In contrast to repeat-flanking PCR MCA, TP-PCR MCA displayed much higher sensitivity for detecting large expansions. All 30 DNAs generated distinct melt peak Tms which correlated well with each sample's larger allele. Normal samples were clearly distinguished from affected samples. The companion sizing protocol accurately sized even the largest expanded allele of ~180 CAGs. Blinded analysis of 69 clinical samples enriched for HD demonstrated 100% assay sensitivity and specificity in sample segregation. The assay targets the HTT CAG repeat specifically, tolerates a wide range of input DNA, and works well using DNA from saliva and buccal swab in addition to blood. Therefore, rapid, accurate, reliable, and high-throughput detection/exclusion of HD can be achieved using this one-step screening assay, at less than half the cost of fluorescent PCR with CE.

Clinical and genetic investigation of a Brazilian family with Huntington's disease

The aim of this study was to investigate a Brazilian family carrying full penetrance alleles for Huntington's disease (HD) in order to correlate each member's genetic and clinical features. To this end, the following scales were administered in each patient: the Beck Depression Inventory, the Mini-Mental State Examination (MMSE) and the Unified Huntington's Disease Rating Scale (UHDRS). The patterns of CAG and CCG polymorphic regions in the HTT gene were determined, the disease burden score was calculated, and genotypes were correlated with phenotypes within this family. We suggest that HD duration, the number of years of formal education, and UHDRS status variables can explain 96.6% of the MMSE variability in HD patients. A strong significant correlation was found between the disease burden score and the UHDRS (r = 0.76; p-value = 0.049) and the MMSE (r = -0.90; p-value = 0.006). The correlations between CAG allele size and the three clinical evaluations performed in the HD patients were not statistically significant.

A SNP in the HTT promoter alters NF-κB binding and is a bidirectional genetic modifier of Huntington disease

Cis-regulatory variants that alter gene expression can modify disease expressivity, but none have previously been identified in Huntington disease (HD). Here we provide in vivo evidence in HD patients that cis-regulatory variants in the HTT promoter are bidirectional modifiers of HD age of onset. HTT promoter analysis identified a NF-κB binding site that regulates HTT promoter transcriptional activity. A non-coding SNP, rs13102260:G > A, in this binding site impaired NF-κB binding and reduced HTT transcriptional activity and HTT protein expression. The presence of the rs13102260 minor (A) variant on the HD disease allele was associated with delayed age of onset in familial cases, whereas the presence of the rs13102260 (A) variant on the wild-type HTT allele was associated with earlier age of onset in HD patients in an extreme case-based cohort. Our findings suggest a previously unknown mechanism linking allele-specific effects of rs13102260 on HTT expression to HD age of onset and have implications for HTT silencing treatments that are currently in development.

Proteomics of Huntington's disease-affected human embryonic stem cells reveals an evolving pathology involving mitochondrial dysfunction and metabolic disturbances

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by a mutation in the Huntingtin gene, where excessive (≥ 36) CAG repeats encode for glutamine expansion in the huntingtin protein. Research using mouse models and human pathological material has indicated dysfunctions in a myriad of systems, including mitochondrial and ubiquitin/proteasome complexes, cytoskeletal transport, signaling, and transcriptional regulation. Here, we examined the earliest biochemical and pathways involved in HD pathology. We conducted a proteomics study combined with immunocytochemical analysis of undifferentiated HD-affected and unaffected human embryonic stem cells (hESC). Analysis of 1883 identifications derived from membrane and cytosolic enriched fractions revealed mitochondria as the primary dysfunctional organ in HD-affected pluripotent cells in the absence of significant differences in huntingtin protein. Furthermore, on the basis of analysis of 645 proteins found in neurodifferentiated hESC, we show a shift to transcriptional dysregulation and cytoskeletal abnormalities as the primary pathologies in HD-affected cells differentiating along neural lineages in vitro. We also show this is concomitant with an up-regulation in expression of huntingtin protein in HD-affected cells. This study demonstrates the utility of a model that recapitulates HD pathology and offers insights into disease initiation, etiology, progression, and potential therapeutic intervention.

American College of Medical Genetics and Genomics Standards and Guidelines for Clinical Genetics Laboratories, 2014 edition: technical standards and guidelines for Huntington disease

Huntington disease is an autosomal-dominant neurodegenerative disease of mid-life onset caused by expansion of a polymorphic trinucleotide (CAG) repeat. Variable penetrance for alleles carrying 36-39 repeats has been noted, but the disease appears fully penetrant when the repeat numbers are >40. An abnormal CAG repeat may expand, contract, or be stably transmitted when passed from parent to child. Assays used to diagnose Huntington disease must be optimized to ensure the accurate and unambiguous quantitation of CAG repeat length. This document provides an overview of Huntington disease and methodological considerations for Huntington disease testing. Examples of laboratory reports are also included.

Alpha-theta border EEG abnormalities in preclinical Huntington's disease

BACKGROUND

Brain dysfunction precedes clinical manifestation of Huntington's disease (HD) by decades. This study was aimed to determine whether resting EEG is altered in preclinical HD mutations carriers (pre-HD).

METHODS

We examined relative power of broad traditional EEG bands as well as 1-Hz sub-bands of theta and alpha from the resting-state EEG of 29 pre-HD individuals and of 29 age-matched normal controls.

RESULTS

The relative power of the narrow sub-band in the border of theta-alpha (7-8 Hz) was significantly reduced in pre-HD subjects as compared to normal controls, while the alterations in relative power of the broad frequency bands were not significant. In pre-HD subjects, the number of CAG repeats in the huntingtin (HTT) gene as well as the disease burden score (DBS) showed a positive correlation with relative power of the delta and theta frequency bands and their sub-bands and a negative correlation with alpha band relative power and the differences of relative power of the 7-8 Hz and 4-5 Hz frequency sub-bands.

CONCLUSION

The obtained results suggest that EEG alterations in pre-HD individuals may be related to the course of the pathological process and to HD endophenotype. Analysis of the narrow EEG bands was found to be more useful for assessing EEG alterations in pre-HD individuals than a more traditional approach using broad bandwidths.

Advances in Huntington’s disease diagnostics: development of a standard reference material

Huntington's disease (HD) is a neurodegenerative disease that affects four to seven individuals per 100,000. The onset of symptoms usually begins in middle age, although approximately 5% become symptomatic as juveniles. Death occurs approximately 15 years following the onset of symptoms, which include choreic movements, cognitive decline and psychiatric changes. HD is an autosomal dominant inherited disease that is associated with an expansion of a trinucleotide (CAG) repeat located on chromosome 4. Physicians rely on a positive family history, and diagnostic and genetic tests to detect the expansion in the number of CAG trinucleotide repeats in the HD gene to confirm the diagnosis. More than 99% of HD patients have 40 or more CAG triplet repeats and, therefore, targeted mutational analysis is greater than 99% sensitive. Individuals with 26 triplet repeats or less are normal, and while those with 27-35 repeats may not demonstrate symptoms themselves, their offspring may have the disease. Individuals with 36-39 repeats may or may not exhibit symptoms. The College of American Pathology/American College of Medical Genetics Biochemical and Molecular Genetics Resource Committee has emphasized the need to standardize the methodology for the determination of the accurate number of CAG repeats. This will prevent false-positive or -negative results when conducting predictive or prenatal testing of at-risk individuals. The National Institute of Standards and Technology is developing a standard reference material to provide these positive and negative controls needed by clinical testing laboratories. The use of a HD standard reference material will provide the quality control and assurance that data from different laboratories are both comparable and accurate.

High frequency of intermediate alleles on Huntington disease-associated haplotypes in British Columbia's general population

Intermediate alleles (27-35 CAG, IAs) for Huntington disease (HD) usually do not confer the disease phenotype but are prone to CAG repeat instability. Consequently, offspring are at-risk of inheriting an expanded allele in the HD range (≥36 CAG). IAs that expand into a new mutation have been hypothesized to be more susceptible to instability compared to IAs identified on the non-HD side of a family from the general population. Frequency estimates for IAs are limited and have largely been determined using clinical samples of HD or related disorders, which may result in an ascertainment bias. This study aimed to establish the frequency of IAs in a sample of a British Columbia's (B.C.) general population with no known association to HD and examine the haplotype of new mutation and general population IAs. CAG sizing was performed on 1,600 DNA samples from B.C.'s general population. Haplotypes were determined using 22 tagging SNPs across the HTT gene. 5.8% of individuals were found to have an IA, of which 60% were on HD-associated haplotypes. There was no difference in the haplotype distribution of new mutation and general population IAs. These findings suggest that IAs are relatively frequent in the general population and are often found on haplotypes associated with expanded CAG lengths. There is likely no difference in the propensity of new mutation and general population IAs to expand into the disease range given that they are both found on disease-associated haplotypes. These findings have important implications for clinical practice.

Gene expression profile in fibroblasts of Huntington's disease patients and controls

Huntington's disease is an inherited disorder caused by expanded stretch of consecutive trinucleotides (cytosine-adenosine-guanine, CAG) within the first exon of the huntingtin (HTT) gene on chromosome 4 (p16.3). The mutated huntingtin (mHTT) gains toxic function, probably through mechanisms that involve aberrant interactions in several pathways, causing cytotoxicity. Pathophysiology of disease involves several tissues; indeed it has been shown that there is a broad toxic effect of mHTT in the peripheral tissue of patients with HD, not only in the central nervous system. In this study we compared gene expression profiles (GEP) of HD fibroblasts and matched controls using microarray technology. We used RT-PCR to test the consistency of the microarray data and we found four genes up-regulated in HD patients with respect to control individuals. The genes appear to be involved in different pathways that have been shown to be perturbed even in HD models and patients. Although our study is preliminary and has to be extended to a larger cohort of HD patients and controls, nevertheless it shows that gene expression profiles seem to be altered in the fibroblasts of HD patients. Validation of the differential expressions at the protein level is required to ascertain if this cell type can be considered a suitable model for the identification of HD biomarkers.

CAG size-specific risk estimates for intermediate allele repeat instability in Huntington disease

INTRODUCTION

New mutations for Huntington disease (HD) occur due to CAG repeat instability of intermediate alleles (IA). IAs have between 27 and 35 CAG repeats, a range just below the disease threshold of 36 repeats. While they usually do not confer the HD phenotype, IAs are prone to paternal germline CAG repeat instability. Consequently, they may expand into the HD range upon transmission to the next generation, producing a new mutation. Quantified risk estimates for IA repeat instability are extremely limited but needed to inform clinical practice.

METHODS

Using small-pool PCR of sperm DNA from Caucasian men, we examined the frequency and magnitude of CAG repeat instability across the entire range of intermediate CAG sizes. The CAG size-specific risk estimates generated are based on the largest sample size ever examined, including 30 IAs and 18 198 sperm.

RESULTS

Our findings demonstrate a significant risk of new mutations. While all intermediate CAG sizes demonstrated repeat expansion into the HD range, alleles with 34 and 35 CAG repeats were associated with the highest risk of a new mutation (2.4% and 21.0%, respectively). IAs with ≥33 CAG repeats showed a dramatic increase in the frequency of instability and a switch towards a preponderance of repeat expansions over contractions.

CONCLUSIONS

These data provide novel insights into the origins of new mutations for HD. The CAG size-specific risk estimates inform clinical practice and provide accurate risk information for persons who receive an IA predictive test result.

EMQN/CMGS best practice guidelines for the molecular genetic testing of Huntington disease

Huntington disease (HD) is caused by the expansion of an unstable polymorphic trinucleotide (CAG)n repeat in exon 1 of the HTT gene, which translates into an extended polyglutamine tract in the protein. Laboratory diagnosis of HD involves estimation of the number of CAG repeats. Molecular genetic testing for HD is offered in a wide range of laboratories both within and outside the European community. In order to measure the quality and raise the standard of molecular genetic testing in these laboratories, the European Molecular Genetics Quality Network has organized a yearly external quality assessment (EQA) scheme for molecular genetic testing of HD for over 10 years. EQA compares a laboratory's output with a fixed standard both for genotyping and reporting of the results to the referring physicians. In general, the standard of genotyping is very high but the clarity of interpretation and reporting of the test result varies more widely. This emphasizes the need for best practice guidelines for this disorder. We have therefore developed these best practice guidelines for genetic testing for HD to assist in testing and reporting of results. The analytical methods and the potential pitfalls of molecular genetic testing are highlighted and the implications of the different test outcomes for the consultand and his or her family members are discussed.

Huntington disease in the South African population occurs on diverse and ethnically distinct genetic haplotypes

Huntington disease (HD) is a neurodegenerative disorder resulting from the expansion of a CAG trinucleotide repeat in the huntingtin (HTT) gene. Worldwide prevalence varies geographically with the highest figures reported in populations of European ancestry. HD in South Africa has been reported in Caucasian, black and mixed subpopulations, with similar estimated prevalence in the Caucasian and mixed groups and a lower estimate in the black subpopulation. Recent studies have associated specific HTT haplotypes with HD in distinct populations. Expanded HD alleles in Europe occur predominantly on haplogroup A (specifically high-risk variants A1/A2), whereas in East Asian populations, HD alleles are associated with haplogroup C. Whether specific HTT haplotypes associate with HD in black Africans and how these compare with haplotypes found in European and East Asian populations remains unknown. The current study genotyped the HTT region in unaffected individuals and HD patients from each of the South African subpopulations, and haplotypes were constructed. CAG repeat sizes were determined and phased to haplotype. Results indicate that HD alleles from Caucasian and mixed patients are predominantly associated with haplogroup A, signifying a similar European origin for HD. However, in black patients, HD occurs predominantly on haplogroup B, suggesting several distinct origins of the mutation in South Africa. The absence of high-risk variants (A1/A2) in the black subpopulation may also explain the reported low prevalence of HD. Identification of haplotypes associated with HD-expanded alleles is particularly relevant to the development of population-specific therapeutic targets for selective suppression of the expanded HTT transcript.

Triplet repeat primed PCR simplifies testing for Huntington disease

Diagnostic and predictive testing for Huntington disease (HD) requires an accurate determination of the number of CAG repeats in the Huntingtin (HHT) gene. Currently, when a sample appears to be homozygous for a normal allele, additional testing is required to confirm amplification from both alleles. If the sample still appears homozygous, Southern blot analysis is performed to rule out an undetected expanded HTT allele. Southern blot analysis is expensive, time-consuming, and labor intensive and requires high concentrations of DNA. We have developed a chimeric PCR process to help streamline workflow; true homozygous alleles are easily distinguished by this simplified method, and only very large expanded alleles still require Southern blot analysis. Two hundred forty-six HD samples, previously run with a different fragment analysis method, were analyzed with our new method. All samples were correctly genotyped, resulting in 100% concordance between the methods. The chimeric PCR assay was able to identify expanded alleles up to >150 CAG repeats. This method offers a simple strategy to differentiate normal from expanded CAG alleles, thereby reducing the number of samples reflexed to Southern blot analysis. It also provides assurance that expanded alleles are not routinely missed because of allele dropout.

Huntington's disease and its therapeutic target genes: a global functional profile based on the HD Research Crossroads database

Background

Huntington’s disease (HD) is a fatal progressive neurodegenerative disorder caused by the expansion of the polyglutamine repeat region in the huntingtin gene. Although the disease is triggered by the mutation of a single gene, intensive research has linked numerous other genes to its pathogenesis. To obtain a systematic overview of these genes, which may serve as therapeutic targets, CHDI Foundation has recently established the HD Research Crossroads database. With currently over 800 cataloged genes, this web-based resource constitutes the most extensive curation of genes relevant to HD. It provides us with an unprecedented opportunity to survey molecular mechanisms involved in HD in a holistic manner.

Methods

To gain a synoptic view of therapeutic targets for HD, we have carried out a variety of bioinformatical and statistical analyses to scrutinize the functional association of genes curated in the HD Research Crossroads database. In particular, enrichment analyses were performed with respect to Gene Ontology categories, KEGG signaling pathways, and Pfam protein families. For selected processes, we also analyzed differential expression, using published microarray data. Additionally, we generated a candidate set of novel genetic modifiers of HD by combining information from the HD Research Crossroads database with previous genome-wide linkage studies.

Results

Our analyses led to a comprehensive identification of molecular mechanisms associated with HD. Remarkably, we not only recovered processes and pathways, which have frequently been linked to HD (such as cytotoxicity, apoptosis, and calcium signaling), but also found strong indications for other potentially disease-relevant mechanisms that have been less intensively studied in the context of HD (such as the cell cycle and RNA splicing, as well as Wnt and ErbB signaling). For follow-up studies, we provide a regularly updated compendium of molecular mechanism, that are associated with HD, at http://hdtt.sysbiolab.eu Additionally, we derived a candidate set of 24 novel genetic modifiers, including histone deacetylase 3 (HDAC3), metabotropic glutamate receptor 1 (GRM1), CDK5 regulatory subunit 2 (CDK5R2), and coactivator 1ß of the peroxisome proliferator-activated receptor gamma (PPARGC1B).

Conclusions

The results of our study give us an intriguing picture of the molecular complexity of HD. Our analyses can be seen as a first step towards a comprehensive list of biological processes, molecular functions, and pathways involved in HD, and may provide a basis for the development of more holistic disease models and new therapeutics.

Predictors of survival in a Huntington's disease population from southern Italy

BACKGROUND

The primary aim of the present study was to determine the survival rates and identify predictors of disease duration in a cohort of Huntington's disease (HD) patients from Southern Italy.

METHODS

All medical records of HD patients followed between 1977 and 2008 at the Department of Neurological Sciences of Federico II University in Naples were retrospectively reviewed and 135 patients were enrolled in the analysis. At the time of data collection, 41 patients were deceased (19 males and 22 females) with a mean ± SD age at death of 56.6 ± 14.9 years (range 18-83).

RESULTS

The median survival time was 20 years (95% CI: 18.3-21.7). Cox regression analysis showed that the number of CAG in the expanded allele (HR 1.09 for 1 point triplet increase, p=0.002) and age of onset (HR 1.05 for 1 point year increase, p=0.002) were independent and significant predictors of lower survival rates.

CONCLUSIONS

We believe that these findings are important for a better understanding of the natural history of the disease and may be relevant in designing future therapeutic trials.

Variation within the Huntington's disease gene influences normal brain structure

Genetics of the variability of normal and diseased brain structure largely remains to be elucidated. Expansions of certain trinucleotide repeats cause neurodegenerative disorders of which Huntington's disease constitutes the most common example. Here, we test the hypothesis that variation within the IT15 gene on chromosome 4, whose expansion causes Huntington's disease, influences normal human brain structure. In 278 normal subjects, we determined CAG repeat length within the IT15 gene on chromosome 4 and analyzed high-resolution T1-weighted magnetic resonance images by the use of voxel-based morphometry. We found an increase of GM with increasing long CAG repeat and its interaction with age within the pallidum, which is involved in Huntington's disease. Our study demonstrates that a certain trinucleotide repeat influences normal brain structure in humans. This result may have important implications for the understanding of both the healthy and diseased brain.

Repeat associated non-ATG translation initiation: one DNA, two transcripts, seven reading frames, potentially nine toxic entities!

Diseases associated with unstable repetitive elements in the DNA, RNA, and amino acids have consistently revealed scientific surprises. Most diseases are caused by expansions of trinucleotide repeats, which ultimately lead to diseases like Huntington's disease, myotonic dystrophy, fragile X syndrome, and a series of spinocerebellar ataxias. These repeat mutations are dynamic, changing through generations and within an individual, and the repeats can be bi-directionally transcribed. Unsuspected modes of pathogenesis involve aberrant loss of protein expression; aberrant over-expression of non-mutant proteins; toxic-gain-of-protein function through expanded polyglutamine tracts that are encoded by expanded CAG tracts; and RNA-toxic-gain-of-function caused by transcripts harboring expanded CUG, CAG, or CGG tracts. A recent advance reveals that RNA transcripts with expanded CAG repeats can be translated in the complete absence of a starting ATG, and this Repeat Associated Non-ATG translation (RAN-translation) occurs across expanded CAG repeats in all reading frames (CAG, AGC, and GCA) to produce homopolymeric proteins of long polyglutamine, polyserine, and polyalanine tracts. Expanded CTG tracts expressing CUG transcripts also show RAN-translation occurring in all three frames (CUG, UGC, and GCU), to produce polyleucine, polycysteine, and polyalanine. These RAN-translation products can be toxic. Thus, one unstable (CAG)•(CTG) DNA can produce two expanded repeat transcripts and homopolymeric proteins with reading frames (the AUG-directed polyGln and six RAN-translation proteins), yielding a total of potentially nine toxic entities. The occurrence of RAN-translation in patient tissues expands our horizons of modes of disease pathogenesis. Moreover, since RAN-translation counters the canonical requirements of translation initiation, many new questions are now posed that must be addressed. This review covers RAN-translation and some of the pertinent questions.

Cleavage at the 586 amino acid caspase-6 site in mutant huntingtin influences caspase-6 activation in vivo

Caspase cleavage of huntingtin (htt) and nuclear htt accumulation represent early neuropathological changes in brains of patients with Huntington's disease (HD). However, the relationship between caspase cleavage of htt and caspase activation patterns in the pathogenesis of HD remains poorly understood. The lack of a phenotype in YAC mice expressing caspase-6-resistant (C6R) mutant htt (mhtt) highlights proteolysis of htt at the 586 aa caspase-6 (casp6) site as a key mechanism in the pathology of HD. The goal of this study was to investigate how proteolysis of htt at residue 586 plays a role in the pathogenesis of HD and determine whether inhibiting casp6 cleavage of mhtt alters cell-death pathways in vivo. Here we demonstrate that activation of casp6, and not caspase-3, is observed before onset of motor abnormalities in human and murine HD brain. Active casp6 levels correlate directly with CAG size and inversely with age of onset. In contrast, in vivo expression of C6R mhtt attenuates caspase activation. Increased casp6 activity and apoptotic cell death is evident in primary striatal neurons expressing caspase-cleavable, but not C6R, mhtt after NMDA application. Pretreatment with a casp6 inhibitor rescues the apoptotic cell death observed in this paradigm. These data demonstrate that activation of casp6 is an early marker of disease in HD. Furthermore, these data provide a clear link between excitotoxic pathways and proteolysis and suggest that C6R mhtt protects against neurodegeneration by influencing the activation of neuronal cell-death and excitotoxic pathways operative in HD.

Molecular Mechanisms and Potential Therapeutical Targets in Huntington's Disease

Huntington's disease (HD) is a neurodegenerative disorder caused by a CAG repeat expansion in the gene encoding for huntingtin protein. A lot has been learned about this disease since its first description in 1872 and the identification of its causative gene and mutation in 1993. We now know that the disease is characterized by several molecular and cellular abnormalities whose precise timing and relative roles in pathogenesis have yet to be understood. HD is triggered by the mutant protein, and both gain-of-function (of the mutant protein) and loss-of-function (of the normal protein) mechanisms are involved. Here we review the data that describe the emergence of the ancient huntingtin gene and of the polyglutamine trait during the last 800 million years of evolution. We focus on the known functions of wild-type huntingtin that are fundamental for the survival and functioning of the brain neurons that predominantly degenerate in HD. We summarize data indicating how the loss of these beneficial activities reduces the ability of these neurons to survive. We also review the different mechanisms by which the mutation in huntingtin causes toxicity. This may arise both from cell-autonomous processes and dysfunction of neuronal circuitries. We then focus on novel therapeutical targets and pathways and on the attractive option to counteract HD at its primary source, i.e., by blocking the production of the mutant protein. Strategies and technologies used to screen for candidate HD biomarkers and their potential application are presented. Furthermore, we discuss the opportunities offered by intracerebral cell transplantation and the likely need for these multiple routes into therapies to converge at some point as, ideally, one would wish to stop the disease process and, at the same time, possibly replace the damaged neurons.

Unstable familial transmissions of Huntington disease alleles with 27-35 CAG repeats (intermediate alleles)

There are inconsistent reports regarding the likelihood of repeat instability for alleles with 27-35 CAG repeats in the Huntington disease (HD) gene. We have examined the intergenerational stability of such intermediate alleles in 51 families from the University of British Columbia's DNA and Tissue Bank for Huntington Disease Research (UBC-HD Databank). A total of 181 transmissions were identified, with 30% (n = 54/181) of the alleles being unstable upon transmission. The unstable transmissions included both expansions (n = 37) and contractions (n = 17) of CAG size. Of the expanded alleles, 68% (n = 25/37) expanded into the HD range (>36 CAG). Therefore, 14% (n = 25/181) of the 27-35 CAG allele transmissions examined expanded into the disease-associated range resulting in a new mutation for HD. Significantly, of these new mutations, 40% (n = 10/25) originated from an allele with 35 CAG repeats with CAG repeat expansions ranging from +1 CAG to +23 CAG. The proportion of new mutations in the UBC-HD Databank is consistent with the most recent new mutation rate for HD, estimated to be at least 10%. The observed difference in the stability of HD intermediate allele transmissions in this data set and in other studies may be a reflection of a small sample size. Alternately, these inconsistencies may indicate an underlying difference in genetic factors which influence repeat instability between the different populations examined. Additional studies determining the frequency and magnitude of repeat instability in this CAG repeat range and factors that influence instability are urgently needed. Until we understand the clinical implications of HD alleles with 27-35 CAG repeats and establish reliable risks of instability, we should exercise caution when translating these results to the clinic.

Use of capillary electrophoresis for accurate determination of CAG repeats causing Huntington disease. An oligonucleotide design avoiding shadow bands

Huntington disease (HD) is a neurodegenerative disorder associated with the expansion of a polymorphic trinucleotide CAG repeat in the HD gene. We have developed an assay to accurately determine CAG repeats that combines a novel oligonucleotide design and the resolution of capillary electrophoresis. A mismatch in the second nucleotide from the 3' end enhanced specificity by avoiding mispriming and diminishing shadow bands and artifactual PCR products. The coupling of capillary electrophoresis analysis with the assay added the advantages of accuracy, high resolution, semi-automation, rapid analysis and low sample consumption. Analysis of 200 chromosomes in the Spanish population sample studied (control group) gave a peak frequency for 16 CAG repeats and of 7 triplets for CCG repeats. Diagnosis of HD was confirmed in 22 of 34 individuals with a range of CAG repeats from 39 to 52. Predictive testing was also carried out for 19 relatives of the HD families diagnosed at our laboratory. The method proposed in this article provides an accurate sizing of DNA repeats that can be applied to the analysis of DNA size-related disorders.

Improvement in strategies for the non-invasive prenatal diagnosis of Huntington disease

PURPOSE

We focused on the improvements of prenatal diagnosis by the analysis of DNA from maternal plasma, using Huntington disease as a model of disease.

METHODS

We studied plasma from a pregnancy at risk of having a fetus affected with Huntington disease by the use of two direct analysis of the mutation and polymorphic STRs.

RESULTS

Direct methods were not informative. Analysis with STRs revealed the presence of the allele that does not co-segregate with the disease, thus the fetus was healthy.

CONCLUSIONS

This strategy is very useful to face complex cases when the direct study is not informative not only for Huntington disease but also for many other disorders.

SNP analysis using CataCleave probes

CataCleave probes are catalytically cleavable fluorescence probes having a chimeric deoxyribonucleic acid (DNA)-ribonucleic acid (RNA)-DNA structure that can be used for real-time detection of single nucleotide polymorphisms (SNPs), insertions, and deletions. Fluorescent donor emission is normally quenched by Förster resonance energy transfer (FRET). Upon binding to the target, if the RNA/DNA hybrid is correctly base-paired, ribonuclease (RNase) H will cleave the RNA moiety and the probe fragments will dissociate. FRET is lost and the donor fluorescence signal is recovered. A single-base mismatch within the hybrid region causes probe cleavage to be significantly reduced. We designed CataCleave probes to detect SNPs located in the insulin-like growth factor 2 (IGF-2) gene and at position 702 within the NOD2/CARD15 gene. Probes were also designed to detect a six-basepair deletion in the amelogenin gene and a partially methylated target DNA. Discrimination between wild-type and SNP is demonstrated for both genes in homogeneous reactions under isothermal and temperature cycling conditions. These probes were also able to identify a multibase deletion and methylated DNA. Cleavage rates were proportional to target concentration. Probe length and position of fluorescent labels may also be modified for use in multiplexing high-throughput SNP assays. This represents a novel method for the detection of SNPs.

Single-step scalable-throughput molecular screening for Huntington disease

BACKGROUND

Huntington disease (HD) is a fatal autosomal dominant neurodegenerative disorder caused by an unstable expansion of the CAG trinucleotide repeat in exon 1 of the HTT (huntingtin) gene and typically has an adult onset. Molecular diagnosis and screening for HD currently involve separate amplification and detection steps.

METHODS

We evaluated a novel, rapid microplate-based screening method for HD that combines the amplification and detection procedures in a single-step, closed-tube format. We carried out both the PCR for the HTT CAG-repeat region and the subsequent automated melting-curve analysis of the amplicon in the same wells on the plate. To establish cutoff melting temperatures (T(m)s) for each allelic class, we used a panel of reference DNA samples of known CAG-repeat sizes that represent a range of HTT alleles [normal (< or =26 repeats), intermediate (27-35 repeats), reduced penetrance expanded (36-39 repeats), and fully penetrant expanded (> or =40 repeats)]. We also measured well-to-well variation in T(m) across the thermal block and validated cutoff T(m)s with DNA samples from 5 different populations. We also conducted a blinded validation analysis of clinical samples from an additional 40 HD-affected and 30 unaffected individuals.

RESULTS

We observed a strong correlation between CAG-repeat size and amplicon T(m) among the reference DNA samples. Use of the T(m) cutoffs we established revealed that 5 samples from unaffected individuals had been misclassified as affected (1.1% false-positive rate). All samples from HD-affected and unaffected individuals were correctly identified in the blinded analysis.

CONCLUSIONS

This simple and scalable homogeneous assay may serve as a convenient, rapid, and accurate screen to detect the presence of pathologic expanded HD alleles in symptomatic patients.

Huntington disease mutation in Venezuela: age of onset, haplotype analyses and geographic aggregation

The aggregation of patients with Huntington's disease (HD) around Lake Maracaibo, Zulia State, Venezuela, is widely recognized, but the epidemiology of HD in the whole country is relatively unstudied. We have examined 279 individuals from 60 unrelated affected families residing in various areas of Venezuela for the presence of CAG repeats and other features associated with HD. The number of expanded repeats in 139 carriers varied from 35 to 112. Based on our examination of 71 symptomatic individuals, we developed a log-transformed regression equation, y= -0.0238x + 2.6616, to enable the prediction of age of onset in asymptomatic carriers. Intragenic haplotypes were constructed with two VNTRs (variable number of tandem repeats) and two SNPs (single nucleotide polymorphisms) in the promoter region as well as CCG repeat and Delta2642 polymorphisms to assess kinship between families. In 43 of 45 tested families, the haplotype on the mutated chromosome was 1;G;C;7;(A). The other haplotypes observed, 1;G;C;7;(B) and 4;G;C;7;(A), were of Peruvian and French origins, respectively. The geographic source of the first affected ancestor was assessed in 54 families from 15 different states. Residents of the states of Miranda, Lara and Táchira, excluding those of Zulia, had a mutated allele prevalence five- to ninefold higher than that of other areas. A low (approx. 1/200,000) prevalence, a wide-spread distribution with aggregation in some states and a likely remote European Caucasoid origin are defining epidemiologic features of HD in Venezuela.

A common 1317TC polymorphism in MTHFR can lead to erroneous 1298AC genotyping by PCR-RE and TaqMan probe assays

Multiple polymorphisms of the methylenetetrahydrofolate reductase gene (MTHFR) have been documented, and some are associated with decreased enzyme activity. One polymorphism, 677CT, is commonly tested in the context of thrombosis. Recently, consideration has also been extended to 1298AC, which is also associated with reduced catalytic activity. This report describes problems arising during the development of a PCR restriction enzyme assay for 1298AC. In the process of validating a PCR-MboII assay, it was realized that a nearby 1317TC polymorphism rendered a restriction fragment length polymorphism (RFLP) pattern that was virtually indistinguishable from a 1298A allele. An alternate approach, involving primer mutagenesis and Fnu4HI digestion, resolved the problem. To validate the latter assay, samples were obtained from a CLIA-approved facility that had developed a multiplexed real-time PCR using TaqMan probes for simultaneous assessment of 677CT and 1298AC. Interlaboratory results concurred for 10 out of 11 samples; however, one sample was consistently heterozygous by PCR-Fnu4HI and homozygous 1298CC by real-time PCR. Bidirectional sequencing confirmed that the sample was a compound 1298AC/1317TC heterozygote. It is likely that the 1317C variant, residing with 1298A on one chromosome, disrupted primer annealing in the TaqMan assay, leading to preferential amplification of the 1298C/1317T chromosome and hence an aberrant homozygous 1298CC genotype. This validation exercise emphasizes the need for comprehensive appraisal and continual reassessment of the optimal performance of molecular diagnostic assays. It is hoped that laboratories offering MTHFR 1298AC testing are cognizant of some of the inherent problems in published methods.

Clinical and molecular genetic assessment of a chorea-acanthocytosis pedigree

BACKGROUND

Chorea-acanthocytosis (ChAc) is an autosomal recessive hereditary disease characterized by neurodegeneration in the striatum and acanthocytosis that is caused by mutations in the VPS13A gene. There are only few reports that studied clinical status of the obligate carriers of ChAc. Clinical courses with follow-up neuroradiological and neuropsychological evaluations in individuals with ChAc have been rarely reported.

METHODS

We followed an index patient with ChAc and evaluated the clinical features of the pedigree members. Genetic analyses of VPS13A and genes responsible for other neuroacanthocytotic and neurodegenerative diseases were performed.

CONCLUSIONS

The index patient was homozygous for a 3889C>T nonsense mutation in the VPS13A gene and presented with a typical ChAc phenotype. Neuropsychological evaluation with brain imaging in the patient over 3 years revealed atrophy and a decrease in blood flow at the basal ganglia and frontal lobe, and impairment in cognitive function reflecting frontal lobe dysfunction in progressive manners. Four out of five heterozygous mutation carriers in the pedigree showed signs or symptoms potentially attributable to a heterozygous VPS13A mutation.

The Hdh(Q150/Q150) knock-in mouse model of HD and the R6/2 exon 1 model develop comparable and widespread molecular phenotypes

The identification of the Huntington's disease (HD) mutation as a CAG/polyglutamine repeat expansion enabled the generation of transgenic rodent models and gene-targeted mouse models of HD. Of these, mice that are transgenic for an N-terminal huntingtin fragment have been used most extensively because they develop phenotypes with relatively early ages of onset and rapid disease progression. Although the fragment models have led to novel insights into the pathophysiology of HD, it is important that models expressing a mutant version of the full-length protein are analysed in parallel. We have generated congenic C57BL/6 and CBA strains for the HdhQ150 knock-in mouse model of HD so that homozygotes can be analysed on an F1 hybrid background. Although a significant impairment in grip strength could be detected from a very early age, the performance of these mice in the quantitative behavioural tests most frequently used in preclinical efficacy trials indicates that they are unlikely to be useful for preclinical screening using a battery of conventional tests. However, at 22 months of age, the Hdh(Q150/Q150) homozygotes showed unexpected widespread aggregate deposition throughout the brain, transcriptional dysregulation in the striatum and cerebellum and decreased levels of specific chaperones, all well-characterised molecular phenotypes present in R6/2 mice aged 12 weeks. Therefore, when strain background and CAG repeat length are controlled for, the knock-in and fragment models develop comparable phenotypes. This supports the continued use of the more high-throughput fragment models to identify mechanisms of pathogenesis and for preclinical screening.

Neuroacanthocytosis associated with a defect of the 4.1R membrane protein

Background

Neuroacanthocytosis (NA) denotes a heterogeneous group of diseases that are characterized by nervous system abnormalities in association with acanthocytosis in the patients' blood. The 4.1R protein of the erythrocyte membrane is critical for the membrane-associated cytoskeleton structure and in central neurons it regulates the stabilization of AMPA receptors on the neuronal surface at the postsynaptic density. We report clinical, biochemical, and genetic features in four patients from four unrelated families with NA in order to explain the cause of morphological abnormalities and the relationship with neurodegenerative processes.

Case presentation

All patients were characterised by atypical NA with a novel alteration of the erythrocyte membrane: a 4.1R protein deficiency. The 4.1R protein content was significantly lower in patients (3.40 ± 0.42) than in controls (4.41 ± 0.40, P < 0.0001), reflecting weakened interactions of the cytoskeleton with the membrane. In patients IV:1 (RM23), IV:3 (RM15), and IV:6 (RM16) the 4.1 deficiency seemed to affect the horizontal interactions of spectrin and an impairment of the dimer self-association into tetramers was detected. In patient IV:1 (RM16) the 4.1 deficiency seemed to affect the skeletal attachment to membrane and the protein band 3 was partially reduced.

Conclusion

A decreased expression pattern of the 4.1R protein was observed in the erythrocytes from patients with atypical NA, which might reflect the expression pattern in the central nervous system, especially basal ganglia, and might lead to dysfunction of AMPA-mediated glutamate transmission.

Predictive testing for Huntington disease: interpretation and significance of intermediate alleles

Direct mutation analysis for Huntington disease (HD) became possible in 1993 with the identification of an expanded CAG trinucleotide repeat as the mutation underlying the disease. Expansion of CAG length beyond 35 repeats may be associated with the clinical presentation of HD. HD has never been seen in a person with a CAG size of <36 repeats. Intermediate alleles are defined as being below the affected CAG range but have the potential to expand to >35 CAG repeats within one generation. Thus, children of intermediate allele carriers have a low risk of developing HD. Currently, the intermediate allele range for HD is between 27 and 35 CAG repeats. In this study, we review the current knowledge on intermediate alleles for HD including the CAG repeat range, the intermediate allele frequency, and the clinical implications of an intermediate allele predictive test result. The factors influencing CAG repeat expansion, including the CAG size of the intermediate allele, the sex and age of the transmitting parent, the family history, and the HD gene sequence and haplotype, will also be reviewed.