John W. Day

An untranslated CTG expansion causes a novel form of spinocerebellar ataxia (SCA8)

Myotonic dystrophy (DM) is the only disease reported to be caused by a CTG expansion. We now report that a non-coding CTG expansion causes a novel form of spinocerebellar ataxia (SCA8). This expansion, located on chromosome 13q21, was isolated directly from the genomic DNA of an ataxia patient by RAPID cloning. SCA8 patients have expansions similar in size (107-127 CTG repeats) to those found among adult-onset DM patients. SCA8 is the first example of a dominant SCA not caused by a CAG expansion translated as a polyglutamine tract.

Bidirectional expression of CUG and CAG expansion transcripts and intranuclear polyglutamine inclusions in spinocerebellar ataxia type 8

We previously reported that a (CTG)n expansion causes spinocerebellar ataxia type 8 (SCA8), a slowly progressive ataxia with reduced penetrance. We now report a transgenic mouse model in which the full-length human SCA8 mutation is transcribed using its endogenous promoter. (CTG)116 expansion, but not (CTG)11 control lines, develop a progressive neurological phenotype with in vivo imaging showing reduced cerebellar-cortical inhibition. 1C2-positive intranuclear inclusions in cerebellar Purkinje and brainstem neurons in SCA8 expansion mice and human SCA8 autopsy tissue result from translation of a polyglutamine protein, encoded on a previously unidentified antiparallel transcript (ataxin 8, ATXN8 ) spanning the repeat in the CAG direction. The neurological phenotype in SCA8 BAC expansion but not BAC control lines demonstrates the pathogenicity of the (CTG-CAG)n expansion. Moreover, the expression of noncoding (CUG)n expansion transcripts (ataxin 8 opposite strand, ATXN8OS ) and the discovery of intranuclear polyglutamine inclusions suggests SCA8 pathogenesis involves toxic gain-of-function mechanisms at both the protein and RNA levels.

Myotonic dystrophy type 2 Molecular, diagnostic and clinical spectrum

Background: Myotonic dystrophy types 1 (DM1) and 2 (DM2/proximal myotonic myopathy PROMM) are dominantly inherited disorders with unusual multisystemic clinical features. The authors have characterized the clinical and molecular features of DM2/PROMM, which is caused by a CCTG repeat expansion in intron 1 of the zinc finger protein 9 (ZNF9) gene. Methods: Three-hundred and seventy-nine individuals from 133 DM2/PROMM families were evaluated genetically, and in 234 individuals clinical and molecular features were compared. Results: Among affected individuals 90% had electrical myotonia, 82% weakness, 61% cataracts, 23% diabetes, and 19% cardiac involvement. Because of the repeat tract’s unprecedented size (mean ∼5,000 CCTGs) and somatic instability, expansions were detectable by Southern analysis in only 80% of known carriers. The authors developed a repeat assay that increased the molecular detection rate to 99%. Only 30% of the positive samples had single sizeable expansions by Southern analysis, and 70% showed multiple bands or smears. Among the 101 individuals with single expansions, repeat size did not correlate with age at disease onset. Affected offspring had markedly shorter expansions than their affected parents, with a mean size difference of −17 kb (−4,250 CCTGs). Conclusions: DM2 is present in a large number of families of northern European ancestry. Clinically, DM2 resembles adult-onset DM1, with myotonia, muscular dystrophy, cataracts, diabetes, testicular failure, hypogammaglobulinemia, and cardiac conduction defects. An important distinction is the lack of a congenital form of DM2. The clinical and molecular parallels between DM1 and DM2 indicate that the multisystemic features common to both diseases are caused by CUG or CCUG expansions expressed at the RNA level.

Spectrin Mutations Cause Spinocerebellar Ataxia Type 5

We have discovered that β-III spectrin (SPTBN2) mutations cause spinocerebellar ataxia type 5 (SCA5) in an 11-generation American kindred descended from President Lincolns grandparents and two additional families. Two families have separate in-frame deletions of 39 and 15 bp, and a third family has a mutation in the actin/ARP1 binding region. β-III spectrin is highly expressed in Purkinje cells and has been shown to stabilize the glutamate transporter EAAT4 at the surface of the plasma membrane. We found marked differences in EAAT4 and GluRδ2 by protein blot and cell fractionation in SCA5 autopsy tissue. Cell culture studies demonstrate that wild-type but not mutant β-III spectrin stabilizes EAAT4 at the plasma membrane. Spectrin mutations are a previously unknown cause of ataxia and neurodegenerative disease that affect membrane proteins involved in glutamate signaling.

Genetic mapping of a second myotonic dystrophy locus.

We report the mapping of a second myotonic dystrophy locus, myotonic dystrophy type 2 (DM2). Myotonic dystrophy (DM) is a multi-system disease and the most common form of muscular dystrophy in adults. In 1992, DM was shown to be caused by an expanded CTG repeat in the 3´ untranslated region of the dystrophia myotonica-protein kinase gene (DMPK) on chromosome 19 (refs 2, 3, 4, 5, 6). Although several theories have been put forth to explain how the CTG expansion causes the broad spectrum of clinical features associated with DM, it is not understood how this mutation, which does not alter the protein-coding region of a gene, causes an affect at the cellular level. We have identified a five-generation family (MN1) with a genetically distinct form of myotonic dystrophy. Affected members exhibit remarkable clinical similarity to DM (myotonia, proximal and distal limb weakness, frontal balding, cataracts and cardiac arrhythmias) but do not have the chromosome-19 CTG expansion. We have mapped the disease locus (DM2) of the MN1 family to a 10-cM region of chromosome 3q. Understanding the common molecular features of two different forms of the disease should shed light on the mechanisms responsible for the broad constellation of seemingly unrelated clinical features present in both diseases.

Misregulation of miR-1 processing is associated with heart defects in myotonic dystrophy.

Myotonic dystrophy is an RNA gain-of-function disease caused by expanded CUG or CCUG repeats, which sequester the RNA binding protein MBNL1. Here we describe a newly discovered function for MBNL1 as a regulator of pre-miR-1 biogenesis and find that miR-1 processing is altered in heart samples from people with myotonic dystrophy. MBNL1 binds to a UGC motif located within the loop of pre-miR-1 and competes for the binding of LIN28, which promotes pre-miR-1 uridylation by ZCCHC11 (TUT4) and blocks Dicer processing. As a consequence of miR-1 loss, expression of GJA1 (connexin 43) and CACNA1C (Cav1.2), which are targets of miR-1, is increased in both DM1- and DM2-affected hearts. CACNA1C and GJA1 encode the main calcium- and gap-junction channels in heart, respectively, and we propose that their misregulation may contribute to the cardiac dysfunctions observed in affected persons.

Ataluren treatment of patients with nonsense mutation dystrophinopathy

Introduction: Dystrophinopathy is a rare, severe muscle disorder, and nonsense mutations are found in 13% of cases. Ataluren was developed to enable ribosomal readthrough of premature stop codons in nonsense mutation (nm) genetic disorders. Methods: Randomized, double‐blind, placebo‐controlled study; males ≥5 years with nm‐dystrophinopathy received study drug orally 3 times daily, ataluren 10, 10, 20 mg/kg (N = 57); ataluren 20, 20, 40 mg/kg (N = 60); or placebo (N = 57) for 48 weeks. The primary endpoint was change in 6‐Minute Walk Distance (6MWD) at Week 48. Results: Ataluren was generally well tolerated. The primary endpoint favored ataluren 10, 10, 20 mg/kg versus placebo; the week 48 6MWD Δ = 31.3 meters, post hoc P = 0.056. Secondary endpoints (timed function tests) showed meaningful differences between ataluren 10, 10, 20 mg/kg, and placebo. Conclusions: As the first investigational new drug targeting the underlying cause of nm‐dystrophinopathy, ataluren offers promise as a treatment for this orphan genetic disorder with high unmet medical need. Muscle Nerve 50: 477–487, 2014

THUNDERCLAP HEADACHE: SYMPTOM OF UNRUPTURED CEREBRAL ANEURYSM

Many patients with a ruptured berry aneurysm report an intense sentinel headache of sudden onset in the weeks before rupture. Such headaches have been attributed to a leak of blood, which implies that partial rupture has occurred. A case is reported of a patient who had severe headaches which seemed to be caused by an unruptured cerebral aneurysm, accompanied by diffuse cerebral vasospasm. Headache episodes with the thunderclap profile may require angiography for diagnosis even if the cerebrospinal fluid is bloodless.

Motor dysfunction and gliosis with preserved dopaminergic markers in human α-synuclein A30P transgenic mice

Alpha-synuclein is a major component of Lewy bodies (LBs) in the substantia nigra and cortex in Parkinsons disease (PD) and dementia with Lewy bodies (DLB), and in glial inclusions in multiple systems atrophy (MSA). Mutations in alpha-synuclein have been associated with autosomal dominant forms of PD. We investigated the clinical and neuropathological effects of overexpression of human alpha-synuclein, alpha-synuclein A30P, and alpha-synuclein A53T under the control of the hamster prion protein (PrP) promoter; 5-15x endogenous levels of protein expression were achieved with widespread neuronal, including nigral, transgene expression. High expression of alpha-synuclein A30P in the Tg5093 line was associated with a progressive motor disorder with rigidity, dystonia, gait impairment, and tremor. Histological analysis of this line showed aberrant expression of the protein in cell soma and progressive CNS gliosis, but no discrete Lewy body-like alpha-synuclein inclusions could be identified. Biochemical analysis demonstrated alpha-synuclein fragmentation. Despite strong expression of the transgene in the nigra, there was no specific deterioration of the nigrostriatal dopaminergic system as assessed by quantitation of nigral tyrosine hydroxylase (TH) containing neurons, striatal TH immunoreactivity, dopamine levels, or dopamine receptor number and function. Lower expressing lines had no specific behavioral or histopathological phenotype. Thus, high expression of mutant human alpha-synuclein resulted in a progressive motor and widespread CNS gliotic phenotype independent of dopaminergic dysfunction in the Tg5093 line.

Mutational spectrum of DMD mutations in dystrophinopathy patients: application of modern diagnostic techniques to a large cohort

Mutations in the DMD gene, encoding the dystrophin protein, are responsible for the dystrophinopathies Duchenne Muscular Dystrophy (DMD), Becker Muscular Dystrophy (BMD), and X‐linked Dilated Cardiomyopathy (XLDC). Mutation analysis has traditionally been challenging, due to the large gene size (79 exons over 2.2 Mb of genomic DNA). We report a very large aggregate data set comprised of DMD mutations detected in samples from patients enrolled in the United Dystrophinopathy Project, a multicenter research consortium, and in referral samples submitted for mutation analysis with a diagnosis of dystrophinopathy. We report 1,111 mutations in the DMD gene, including 891 mutations with associated phenotypes. These results encompass 506 point mutations (including 294 nonsense mutations) and significantly expand the number of mutations associated with the dystrophinopathies, highlighting the utility of modern diagnostic techniques. Our data supports the uniform hypermutability of CGA>TGA mutations, establishes the frequency of polymorphic muscle (Dp427m) protein isoforms and reveals unique genomic haplotypes associated with “private” mutations. We note that 60% of these patients would be predicted to benefit from skipping of a single DMD exon using antisense oligonucleotide therapy, and 62% would be predicted to benefit from an inclusive multiexonskipping approach directed toward exons 45 through 55. Hum Mutat 30:1657–1666, 2009.