A mitochondrial tRNA anticodon swap associated with a muscle disease.

We have identified an unusual mitochondrial (mt) tRNA mutation in a seven year-old girl with a pure myopathy. This G to A transition at mtDNA position 15990 changed the anticodon normally found in proline tRNAs (UGG) to the one found in serine tRNAs (UGA), and is the first pathogenic anticodon alteration described in a higher eukaryote. The mutant mtDNA was heteroplasmic (85% mutant) in muscle but was undetectable in white blood cells from the patient and her mother. Analysis of single muscle fibres indicated that mutant mtDNAs severely impaired mitochondrial protein synthesis and respiratory chain activity, but only when present at greater than 90%. The recessive behaviour of this mtDNA alteration may explain the patient's relatively mild clinical phenotype.

X-linked spastic paraplegia (SPG1), MASA syndrome and X-linked hydrocephalus result from mutations in the L1 gene.

X-linked hydrocephalus, spastic paraplegia type I and MASA syndrome are related disorders with loci in subchromosomal region Xq28. We have previously shown that X-linked hydrocephalus is caused by mutations in the gene for neural cell adhesion molecule L1 (L1CAM), an axonal glycoprotein involved in neuronal migration and differentiation. Here we report mutations of the L1 gene in MASA syndrome and SPG1, in addition to HSAS families. Two of the HSAS mutations would abolish cell surface expression of L1 and represent the first functional null mutations in this disorder. Our results indicate that these three syndromes from part of a clinical spectrum resulting from a heterogeneous group of mutations in the L1 gene.

Deficiency of dystrophin-associated proteins in Duchenne muscular dystrophy patients lacking COOH-terminal domains of dystrophin.

Dystrophin, the protein product of the Duchenne muscular dystrophy (DMD) gene, is a cytoskeletal protein tightly associated with a large oligomeric complex of sarcolemmal glycoproteins including dystroglycan, which provides a linkage to the extracellular matrix component, laminin. In DMD, the absence of dystrophin leads to a drastic reduction in all of the dystrophin-associated proteins, causing the disruption of the linkage between the subsarcolemmal cytoskeleton and the extracellular matrix which, in turn, may render muscle cells susceptible to necrosis. The COOH-terminal domains (cysteine-rich and carboxyl-terminal) of dystrophin have been suggested to interact with the sarcolemmal glycoprotein complex. However, truncated dystrophin lacking these domains was reported to be localized to the sarcolemma in four DMD patients recently. Here we report that all of the dystrophin-associated proteins are drastically reduced in the sarcolemma of three DMD patients in whom dystrophin lacking the COOH-terminal domains was properly localized to the sarcolemma. Our results indicate that the COOH-terminal domains of dystrophin are required for the proper interaction of dystrophin with the dystrophin-associated proteins and also support our hypothesis that the loss of the dystrophin-associated proteins in the sarcolemma leads to severe muscular dystrophy even when truncated dystrophin is present in the subsarcolemmal cytoskeleton.

Drug trial of superoxide dismutase in Duchenne's muscular dystrophy.

A multicenter, randomized, double-blind, placebo-controlled drug trial in Duchenne's muscular dystrophy, evaluating a superoxide dismutase in 51 ambulatory patients for 18 months was conducted. Fourteen aspects of muscle strength and five of functional ability, as well as serum creatine phosphokinase (CPK) level were studied. The total change in strength, function, and CPK level did not differ significantly in the two groups. The testing method used was reliable in assessing the natural history of Duchenne's dystrophy and would, therefore, be useful in future multicenter drug trials.

Dejerine-Sottas disease with de novo dominant point mutation of the PMP22 gene.

We studied a 33-year-old woman with a negative family history. Both of her parents were examined clinically by nerve conduction velocities (NCVs) and EMG, with normal results. The clinical onset of her condition was at 24 months, with severe weakness and atrophy of her feet and hands, but the proximal muscles were relatively spared. She had bilateral pes cavus, distal weakness and hypesthesia for touch and proprioception, areflexia, claw hands, and severe thoracolumbar kyphoscoliosis. NCVs showed absent motor and sensory responses and EMG revealed diffuse fibrillation potentials. Molecular genetic studies indicated a de novo dominant missense point mutation of exon 3 of the peripheral myelin protein 22 gene at nucleotide 264 that caused the replacement of serine with leucine.

Clinical and morphologic features of a myopathy associated with a point mutation in the mitochondrial tRNA(Pro) gene.

We studied a 9-year-old girl with progressive weakness of her extremities for two years. Her neurologic evaluation showed weakness of proximal muscles but no ophthalmoparesis. With the exception of elevated serum lactic acid, the general blood screen, EMG, nerve conduction velocity tests, and ECG were normal. Light and electron microscopy of a muscle biopsy showed proliferation of mitochondria containing disorganized cristae. Activities of respiratory chain enzymes containing mitochondrial DNA (mtDNA)-encoded subunits were significantly impaired in muscle homogenates. A G-->A transition at position 15990 previously detected in this patient's muscle was not present in peripheral blood cells of her mother or sister. However, the patient's WBCs appeared to contain a very small percentage of mutant mtDNAs, indicating that the mutation may have originated during early embryogenesis.

Mapping of the gene for X-linked dominant Charcot-Marie-Tooth neuropathy.

We performed a clinical study and linkage analysis on 278 subjects (66 affected) belonging to eight families with X-linked dominant Charcot-Marie-Tooth (CMT) neuropathy. This form affects 11.8% of CMT patients in Iowa. Motor nerve conduction velocities (MNCVs) were significantly slowed consistent with type 1 CMT. Fifty-six obligate carriers manifested mild distal weakness, localized areflexia, pes cavus, and slowing on MNCVs. Seven X-linked restriction fragment length polymorphisms mapping in the Xp11-q21 region were tested for linkage against CMT. Two-point linkage results showed the highest lod scores with PGK1, DXS159, and DXYS1. Multipoint linkage analysis excluded the CMT gene from being telomeric to either DXS14 or DXYS1, with over 1,000:1 odds. The highest location scores were at PGK1 and 1 cM proximal to DXS159.

Mutations of the noncoding region of the connexin32 gene in X-linked dominant Charcot-Marie-Tooth neuropathy.

We studied two families with X-linked dominant Charcot-Marie-Tooth neuropathy. The clinical findings included onset around age 14 years, with moderate weakness of feet extensors and palmar and dorsal interossei, areflexia, distal hypesthesia, and slow progressivity. Motor nerve conduction velocities showed slowing (20 to 30 m/sec) and EMGs were normal. Genetic linkage analysis revealed positive lod scores with the markers of the Xq13.1 region in family 2, but was noninformative in family 1. There were no point mutations in the connexin32 gene coding region. Instead, family 1 revealed a T-to-G transversion at position -528 relative to the ATG start codon, whereas family 2 showed a C-to-T transition at position -458. The first mutation is located in the nerve-specific connexin32 promoter just upstream of the transcription start site, the second is located in the 5' untranslated region of the mRNA.

Duchenne muscular dystrophy: deficiency of dystrophin-associated proteins in the sarcolemma.

Dystrophin, the protein product of the Duchenne muscular dystrophy (DMD) gene, is a major component of the subsarcolemmal cytoskeleton and exists in a large oligomeric complex tightly associated with several sarcolemmal glycoproteins which provide a linkage to the extracellular matrix protein, laminin. In the present study, we investigated the status of the dystrophin-associated proteins in the skeletal muscle from 17 DMD patients of various ages. The results revealed a dramatic reduction in all of the dystrophin-associated proteins in the sarcolemma of DMD muscle compared with normal muscle and muscle from a variety of other neuromuscular diseases. This abnormality was common in all 17 DMD patients, irrespective of age. Our results indicate that the absence of dystrophin leads to the loss in all of the dystrophin-associated proteins, which renders DMD muscle fibers susceptible to necrosis. The analysis of dystrophin-associated proteins is important in the assessment of experimental therapies that attempt to replace dystrophin in DMD muscle.

Localization of the gene for X-linked spinal muscular atrophy.

We used probes for DNA polymorphisms on the X chromosome to study genetic linkage in seven families with X-linked adult-onset spinal muscular atrophy. We found significant linkage to the marker DXYS1 on the proximal X chromosome long arm and loose linkage or nonlinkage to markers elsewhere. Our analysis localizes the gene defect for this form of anterior horn cell disease.

New point mutations and deletions of the connexin 32 gene in X-linked Charcot-Marie-Tooth neuropathy.

The purpose of this study was the identification of new mutations of the connexin 32 (CX32) gene in CMTX families. We report six new mutations of the CX32 gene including two medium sized (29 and 18 bp) deletions. The clinical phenotype is consistent with CMT peripheral neuropathy in all patients. Four families show both male and female patients, with more severe symptoms in males. The disease is asymptomatic in females in two families. The clinical deficit in CMTX families Nos 1, 2 and 4 with missense mutations of the CX32 gene was mild or moderate. Severe weakness of the feet and hands was present in CMTX family No. 5 with a G insertion and family No. 6 with a 29 bp deletion in the carboxyl terminal region of the CX32 gene. Most likely the severe clinical impact in those families was related to frame shift and premature termination of the protein.

The role of the dystrophin-glycoprotein complex in the molecular pathogenesis of muscular dystrophies.

The dystrophin-glycoprotein complex is considered to be a major trans-sarcolemmal structure which provides a linkage between the subsarcolemmal actin cytoskeleton and the extracellular matrix component laminin. Recently, deficiency of the dystrophin-associated proteins has been shown to play an important role in the molecular pathogenesis of several forms of muscular dystrophy. These include Duchenne muscular dystrophy (DMD), symptomatic DMD carriers, Becker muscular dystrophy and severe childhood autosomal recessive muscular dystrophy with DMD-like phenotype prevalent in North Africa. In Fukuyama-type congenital muscular dystrophy (FCMD), the finding of abnormal expression of the dystrophin-associated proteins may provide a clue to its molecular pathogenesis. These recent findings indicate that the linkage between the subsarcolemmal cytoskeleton and extracellular matrix via the dystrophin-glycoprotein complex is critical for maintaining the integrity of muscle cell function.

Oculogastrointestinal muscular dystrophy.

The author reports on four patients (one male, three females) from the same kindred with a newly recognized autosomal recessive condition involving striated and smooth muscle that has been designated oculogastrointestinal muscular dystrophy. It is characterized by ptosis, ophthalmoplegia, and progressive intestinal pseudo-obstruction leading to malnutrition and death before 30 y. Autopsy studies in two cases showed a severe primary myopathy of smooth muscles of the stomach and intestine with intact myenteric plexus and vagus nerves. The proposita notably had myopathic changes of striated muscles but also involvement of the peripheral nerves and central nervous system characterized by demyelinating and axonal neuropathy and focal spongiform degeneration of the posterior columns.

Frequency of new mutants among boys with Duchenne muscular dystrophy.

Haldane's rule states that one-third of the cases of an X-linked recessive lethal should represent new mutations. This rule is derived under the assumptions that there is equilibrium between mutation and selection, that mutation rates in ova and sperm are equal, and that heterozygous and homozygous normal women have the same fitness. To test this rule for Duchenne muscular dystrophy (DMD), we have examined the mothers of 55 boys with DMD (16 familial and 39 isolated cases) and classified them as carriers or noncarriers on the basis of measures of ribosomal protein synthesis (RPS). Of the 55 mothers, only nine (16.4%) are classified as noncarriers, a figure significantly different from the expected one-third. When the analysis is limited to the 39 mothers of isolated cases, 23.1% (9/39) are classified as noncarriers, still significantly different than expected under Haldane's rule. Violation of any of the assumptions under which Haldane's rule is derived could lead to deviations from the expected one-third new mutants. We find the most likely explantation to be a higher male than female mutation rate. This is supported also by the finding that maternal grandfathers in whom a mutation occurred had higher mean age at birth of the carrier daughter (33.7 +/- 1.6) than did the general population or intrapedigree controls (29.5 +/- 1.3).

Discriminant analysis of ribosomal protein synthesis findings in carrier detection of Duchenne muscular dystrophy.

Previous studies have shown that in vitro muscle ribosomal protein synthesis (RPS) by monomeric ribosomes (MR) and total polyribosomes (TPR) and collagen synthesis (CS) are significantly increased (P less than 0.01) in 47 known carriers of Duchenne muscular dystrophy as compared to 60 age-matched controls. However, there was considerable overlap of the distribution of controls and carriers, particularly for monomeric ribosomal protein synthesis and collagen synthesis. To improve detection of carriers we used discriminant analysis utilizing logs of each measurement as superior to a univariate or bivariate scheme. This study considered four groups: proven carriers (30) (group 1), presumptive carriers (female relatives of Duchenne patients with high serum creatine kinase (CK) levels) (32) (group 2), controls greater than or equal to 20 years old (42) (group 3) and controls less than 20 years (36) (group 4). Comparison of groups, Misclassification (%), (see chart). These results suggest that discriminant analysis reduces the misclassification rates as compared with univariate or bivariate analysis and confirm the superiority of RPS measurements as a carrier test for Duchenne muscular dystrophy.

Correlation between connexin 32 gene mutations and clinical phenotype in X-linked dominant Charcot-Marie-Tooth neuropathy.

We studied the relationship between the genotype and clinical phenotype in 27 families with dominant X-linked Charcot-Marie-Tooth (CMTX1) neuropathy. Twenty-two families showed mutations in the coding region of the connexin32 (cx32) gene. The mutations include four nonsense mutations, eight missense mutations, two medium size deletions, and one insertion. Most missense mutations showed a mild clinical phenotype (five out of eight), whereas all nonsense mutations, the larger of the two deletions, and the insertion that produced frameshifts showed severe phenotypes. Five CMTX1 families with mild clinical phenotype showed no point mutations of the cx32 gene coding region. Three of these families showed positive genetic linkage with the markers of the Xq13.1 region. The genetic linkage of the remaining two families could not be evaluated because of their small size.

Collagen synthesis by Duchenne myogenic clones: pathogenic implications for the disease.

Myogenic cell clones were grown in triplicates from a 3-year-old boy with Duchenne muscular dystrophy and three normal control boys matched for age. The well-differentiated Duchenne myogenic clone had lower creatine kinase activity and lower MM isozyme than the control clones. Analysis of (3H)-proline incorporation demonstrated a 2.2-fold increase in (3H)-hydroxyproline released into the medium of Duchenne myogenic cell clones, whereas intracellular (3H)-hydroxyproline levels showed normal values. These results strongly suggest that the increased amount of collagen present in Duchenne dystrophic muscle is related to the primary defect of the disease and is not due to a secondary replacement fibrosis.

Multiple congenital anomalies syndrome with myopathy in chromosome 16 abnormality.

An abnormality of chromosome 16 in an eight year-old male was associated with a multiple congenital anomalies syndrome characterized by myopathy, cataracts, blepharophimosis, microcephaly, failure to grow, profound mental retardation, moderate sensorineural hearing loss, grand mal seizures, bilateral inguinal hernia, and thoracolumbar kyphoscoliosis. Magnetic resonance imaging of the head demonstrated absence of the corpus callosum and extensive loss of brain parenchyma in the occipital regions. Chromosome analysis from peripheral blood of the patient showed a recombinant chromosome 16 [46, XY, rec (16), dup (p13.1----p13.3) del (q22----q24)]. The mother had a pericentric inversion of chromosome 16 [46, XX, inv(16) (p13.1;q22)]. Independent recombinant DNA studies have shown that the breakpoints of these chromosomal rearrangements flank the alpha-globin gene cluster locus.

Duchenne muscular dystrophy in monozygotic twins: deletion of 5' fragments of the gene.

A recombinant DNA study for deletion evaluation was performed in a 4 generation family with Duchenne muscular dystrophy (DMD) in twins. The patients were 6 years old, had a history of progressive difficulty in walking since age 4, and showed weak gluteals, iliopsoas, latissimus dorsi, rhomboids, lower trapezius, sternocleidomastoids, pseudohypertrophic calves, and tight heelcords. Both patients had high serum creatine kinase of 19,000 and 11,000 IU, respectively, and the muscle biopsy of the left vastus lateralis showed dystrophic alterations. Both twins had the same red cell types for ABO, Rh, CDE, MNSs, Kelly, Lewis, Duffy, and Kidd. HLA typing also detected the same antigens in both twins: A2, B44, DR4, and DR5. Cytogenetic studies were consistent with 46, XY male individuals with normal banding pattern. By cDNA probes the entire DMD gene was surveyed for missing or abnormal-sized restriction fragments. Both twin boys showed absence of 8.5, 8.0, 4.6, 4.2, and 3.1 kb fragments on Hind III blots and absence of 13.5, 3.7, 2.9, and 1.4 kb fragments on Bgl II blots both hybridized with cDNA 1-2a corresponding to most 5' region of the DMD gene. The mother and other relatives of the patient did not show deletion. These findings strongly suggest that the deletion in the DMD monozygotic twins represents a new mutation.