Metabolic myopathies: a clinical approach; part I

Children and adults with metabolic myopathies have underlying deficiencies of energy production, which may result in dysfunction of muscle or other energy-dependent tissues, or both. Patients with disorders of glycogen, lipid, or mitochondrial metabolism in muscle may present with dynamic findings (i.e., exercise intolerance, reversible weakness, and myoglobinuria) or progressive muscle weakness, or both. In this first part of the review, we present a brief description of energy metabolism in muscle, a simplified overview of the clinical and laboratory evaluation of the patient with suspected metabolic myopathy, and a diagnostic algorithm aimed at predicting the nature of the underlying biochemical abnormality. The goal is to simplify a complex field of neuromuscular disease and thus lead to early recognition and treatment of these disorders.

A mutation in the alpha 3 chain of type IX collagen causes autosomal dominant multiple epiphyseal dysplasia with mild myopathy

Multiple epiphyseal dysplasia (MED) is a degenerative cartilage condition shown in some cases to be caused by mutations in genes encoding cartilage oligomeric matrix protein or type IX collagen. We studied a family with autosomal dominant MED affecting predominantly the knee joints and a mild proximal myopathy. Genetic linkage to the COL9A3 locus on chromosome 20q13.3 was established with a peak log(10) odds ratio for linkage score of 3.87 for markers D20S93 and D20S164. Reverse transcription-PCR performed on the muscle biopsy revealed aberrant mRNA lacking exon 3, which predicted a protein lacking 12 amino acids from the COL3 domain of alpha3(IX) collagen. Direct sequencing of genomic DNA confirmed the presence of a splice acceptor mutation in intron 2 of the COL9A3 gene (intervening sequence 2, G-A, -1) only in affected family members. By electron microscopy, chondrocytes from epiphyseal cartilage exhibited dilated rough endoplasmic reticulum containing linear lamellae of alternating electron-dense and electron-lucent material, reflecting abnormal processing of mutant protein. Type IX collagen chains appeared normal in size and quantity but showed defective cross-linking by Western blotting. The novel phenotype of MED and mild myopathy is likely caused by a dominant-negative effect of the exon 3-skipping mutation in the COL9A3 gene. Patients with MED and a waddling gait but minimal radiographic hip involvement should be evaluated for a primary myopathy and a mutation in type IX collagen.

Exclusion of the gastrin-releasing peptide receptor (GRPR) locus as a candidate gene for Rett syndrome

The gene for the gastrin-releasing peptide receptor (GRPR) has been mapped to a candidate region for Rett syndrome (RTT) on the short arm of the X chromosome. The recent report of a translocation that disrupted the gene in an individual with mental retardation and autistic behavior prompted us to examine GRPR as a possible locus for RTT. Genomic polymerase chain reaction amplification of exons followed by single-strand conformation analysis screening in 25 unrelated RTT-affected individuals and by direct sequencing in 12 others has failed to detect any mutation. No gross structural rearrangements were found by Southern analysis of DNA from six unrelated RTT-affected individuals. A high-frequency biallelic polymorphism caused by two single nucleotide substitutions in exon 2 was discovered. The allele frequencies were identical in the RTT population as compared to 100 normal control X chromosomes. This polymorphism will enable future evaluation of the GRPR locus as a candidate for other X-linked mental retardation or neurobehavioral syndromes.

Neuromuscular disorders in the newborn

The main manifestations of neuromuscular disease in the newborn period are hypotonia and weakness. Infants with severe hypotonia but only marginal weakness usually do not have a disorder of the lower motor unit. These infants may have genetic conditions, metabolic disturbances, congenital heart disease, hypothyroidism, sepsis, or other systemic disorders. Early on, neonates with central nervous system pathology may present with profound hypotonia, decreased reflexes, and moderate to severe but transient weakness. However, they also tend to have seizures, obtundation, cranial nerve signs, or history of perinatal asphyxia.

Clonal analysis of meningiomas

Meningiomas are primary brain tumors arising from meningothelial cells. They usually grow slowly and are surgically easy to separate from the brain. A recent clonal analysis of meningiomas, using methylation-sensitive restriction fragment length polymorphisms, suggested a monoclonal origin. Using the same technique but with a highly informative X chromosome probe (M27 beta), we found that 17 (85%) of the 20 meningiomas analyzed were informative. Of the 17 informative tumors, 8 (47%) were monoclonal, 3 (18%) had loss of heterozygosity on the X chromosome, and, unexpectedly, 6 (35%) had a polyclonal pattern. Samples from two areas of one tumor showed a monoclonal pattern and loss of heterozygosity, respectively, on the X chromosome. A review of the histopathological and radiological features of the 17 informative tumors did not help to distinguish the clonal from the polyclonal tumors. We conclude that meningiomas are heterogeneous in clonal composition.

Clonal analysis of human astrocytomas

Clonal analysis of many human cancers have generally confirmed that they are monoclonal. Although astrocytic neoplasms are the most frequently occurring primary tumors in the central nervous system, their clonal composition has not been systematically studied. In this report, the clonal composition of 22 human astrocytomas of all histological grades (2 well-differentiated astrocytomas, 3 anaplastic astrocytomas and 17 glioblastoma multiforme) was determined by analysis of the pattern of X-chromosome inactivation. Leukocyte and non-tumor brain DNA were used as controls. In addition, specimens from different parts of four glioblastoma multiforme were analyzed to determine whether remote areas of the same tumor had the same clonal composition. Eighteen of nineteen informative astrocytomas had a monoclonal pattern of X-chromosome inactivation; one glioblastoma multiforme had loss of heterozygosity on the X chromosome. Specimens from different areas of the same tumor all had identical patterns of X-chromosome inactivation. Leukocytes and non-tumor brain used as controls uniformly had a polyclonal pattern of X-chromosome inactivation. Furthermore, loss of heterozygosity for chromosomes 10 or 17 p loci was found in 64% (9/14) of informative specimens and identical allelic patterns were observed in specimens from different areas of the same tumor. Our results demonstrate that human astrocytomas from low to high-grade are characterized by monoclonal cell populations. The presence of monoclonality in even low-grade neoplasms suggests that in astrocytic tumors the establishment of monoclonality occurs quite early. Also, the finding of a monoclonal pattern in intermediate- and high-grade astrocytomas further supports the hypothesis that clonal expansion underlies astrocytic tumor progression.

Frequency of p53 tumor suppressor gene mutations in human primary brain tumors

Mutations in the p53 tumor suppressor gene are the most common genetic alterations found in diverse types of human cancer, including the primary malignant brain tumor, glioblastoma multiforme. To estimate the frequency of p53 mutations in human brain tumors, we screened 120 human primary brain tumors (59 astrocytic; 61 nonastrocytic) by the polymerase chain reaction-single-strand conformation polymorphism technique. Six astrocytic tumors (one anaplastic astrocytoma and five glioblastoma multiforme) were found to have putative p53 mutations. Direct sequencing of polymerase chain reaction-amplified deoxyribonucleic acid from these six tumors confirmed the presence of different point mutations in the conserved regions of the p53 gene. Allelic losses on chromosome 17p were detected in four (67%) of the six tumors with p53 mutations. p53 mutations were not detected in any of the 61 nonastrocytic brain tumors. Also, polymerase chain reaction-single-strand conformation polymorphism analysis of 74 leukocyte deoxyribonucleic acid samples from patients with astrocytic and nonastrocytic brain tumors failed to detect any germ-line p53 mutations. We conclude from these findings that p53 gene mutations in brain neoplasms are primarily limited to tumors of astrocytic origin and that the p53 gene mutations in sporadic astrocytomas are somatic in origin (i.e., nonprenatally determined).

Sensitivity of single-strand conformation polymorphism (SSCP) analysis in detecting p53 point mutations in tumors with mixed cell populations

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Loss of heterozygosity for alleles on chromosome 10 in human brain tumours

We analysed for loss of alleles on chromosome 10, 25 astrocytomas, 3 ependymomas, 2 medulloblastomas, 2 juvenile pilocytic astrocytomas, 2 gangliogliomas, 1 subependymal giant cell astrocytoma and 1 anaplastic oligoastrocytoma. A battery of 12 DNA markers spanning chromosome 10 was employed. Loss of heterozygosity on chromosome 10 was seen in 16 tumours (13 glioblastoma multiforme, 2 anaplastic astrocytomas, and 1 anaplastic oligoastrocytoma), but not in any of the low-grade astrocytomas examined. High-resolution restriction fragment length polymorphism (RFLP) analysis showed that the loss of alleles in a number of tumours involved two separate large regions of chromosome 10 (10p-proximal 10q and distal 10q). However, a small common region of deletion overlap could not be identified. Our data indicate that the loss of alleles on chromosome 10 is a common finding, seen in over two-thirds of malignant astrocytomas, and may be suggestive of the presence of two or more chromosome 10 tumour suppressor genes involved in astrocytoma formation. Nevertheless, the possibility of these genetic changes being secondary and not causative of the deregulated cell growth cannot be excluded. Regardless of the mechanisms involved, however, chromosome 10 deletions may be a genetic marker for malignant astrocytomas.

Aggressive oligodendroglioma predicted by chromosome 10 restriction fragment length polymorphism analysis. Case study

Oligodendrogliomas are indolent brain tumors with mean postoperative survival of about 5 years. However, the range of postoperative survivals is wide, suggesting that these tumors are heterogeneous in their biologic behavior. Using restriction fragment length polymorphism (RFLP) analysis, we studied a case of an oligodendroglioma with loss of chromosome 10 sequences, a finding that has only been reported in glioblastoma multiforme and anaplastic astrocytomas. Four and a half months after the initial surgery the patient returned with a recurrent tumor having classic radiologic and pathologic features of glioblastoma multiforme. Loss of chromosome 10 alleles in oligodendroglioma may be predictive of aggressive biologic behavior, even in the absence of recognized histopathologic characteristics of anaplasia, and may enable us to select more appropriate treatments for this group of patients.

Eastern equine encephalitis presenting with a focal brain lesion

Eastern equine encephalitis (EEE) virus causes a severe meningoencephalitis with high morbidity and mortality. Despite numerous clinical reports of EEE, there are only 11 patients in whom cranial computed tomographic (CT) findings are described. In 6 patients, CT was normal and in 5 patients diffuse edema was present; none had a focal brain lesion. Based on these reports, it has been suggested that focal findings on CT support the diagnosis of herpes simplex encephalitis rather than EEE. The first patient with serologically-confirmed EEE and a focal lesion demonstrated by cranial CT and magnetic resonance imaging is described; these findings underscore the importance of including EEE in the differential diagnosis of encephalitides that can cause focal brain lesions on neuroimaging.

A search for X-chromosome uniparental disomy and DNA rearrangements in the Rett syndrome

The cause of the Rett syndrome remains unknown but is thought to be related to X-chromosome abnormalities. Restriction fragment length polymorphism analysis was employed to search for X-chromosome DNA rearrangements and uniparental disomy in 16 probands and their families. Eighteen different probes, each specific for an area on either the long or the short arm of the X-chromosome, were used. DNA rearrangements were not detected at any of the tested loci. In addition, at each informative locus evidence of both maternal and paternal contributions was found in all probands. Thus, no evidence of either chromosomal abnormality or uniparental disomy was found in the population studied. If uniparental disomy is indeed a causative genetic mechanism for the Rett syndrome, its occurrence may only be infrequent.

Structure of the human cytochrome c oxidase subunit Vb gene and chromosomal mapping of the coding gene and of seven pseudogenes

Subunit Vb of mammalian cytochrome c oxidase (COX; EC 1.9.3.1) is encoded by a nuclear gene and assembled with the other 12 COX subunits encoded in both mitochondrial and nuclear DNA. We have cloned the gene for human COX subunit Vb (COX5B) and determined the exon-intron structure by both hybridization analysis and DNA sequencing. The gene contains five exons and four introns; the four coding exons span a region of approximately 2.4 kb. The 5' end of the COX5B gene is GC-rich and contains many HpaII sites. Genomic Southern blot analysis of human DNA probed with the human COX Vb cDNA identified eight restriction fragments containing COX Vb-related sequences that were mapped to different chromosomes with panels of human x Chinese hamster somatic cell hybrids. Because only one of these fragments hybridized with a 210-bp probe from intron 4, we conclude that there is a single expressed gene for COX subunit Vb in the human genome. We have mapped this gene to chromosome 2, region cen-q13.

Origin of mutations in two families with X-linked chronic granulomatous disease

The most common X-linked recessive form of chronic granulomatous disease (X-CGD) is characterized by the absence of cytochrome b558 in neutrophils. In a rare variant form of X-CGD, cytochrome b558 is present but not functional. The gene (locus symbol CYBB) was localized to band Xp21 by studies of patients with small chromosome deletions. The gene was cloned based on its location and found to encode the 91-Kd subunit of the cytochrome b558 complex. Most female carriers for X-CGD can be identified by their X-inactivation mosaicism; on average 50% of their neutrophils express the mutant phenotype and fail to reduce nitroblue tetrazolium (NBT). In 2 of 4 families studied, the maternal grandmothers had normal NBT tests, suggesting either nonrandom X-inactivation or new mutations. Restriction fragment length polymorphism analysis using closely linked flanking markers or the NsiI polymorphism detected by the CYBB probe itself, allowed us to identify the X chromosome carrying the mutation as derived from a healthy NBT-positive maternal grandfather. The mothers of the affected boys must have received a paternal X chromosome carrying a new mutation, consistent with the maternal grandmothers' normal NBT tests. In all of eight potential carriers studied, the results of the NBT and DNA marker testing were in complete agreement. Prenatal diagnosis by DNA testing can be performed in early gestation obviating the need for fetal blood sampling.

The gene for the RNA component of the mitochondrial RNA-processing endoribonuclease is located on human chromosome 9p and on mouse chromosome 4

Mitochondrial RNA-processing endoribonuclease (RNAase MRP) has the capacity to cleave mitochondrial RNA complementary to the light strand of the displacement loop at a unique site. The enzyme is a ribonucleoprotein whose RNA component is a nuclear gene product. The 5' flanking region of the primary transcript has control elements characteristic of RNA polymerase II transcription, and the coding region has features of RNA polymerase III transcription signals. The RNA associated with RNAase MRP is the first known RNA encoded by a single-copy gene in the nucleus and believed to be imported into mitochondria. The gene (RMRP) for this RNA component of RNAase MRP was assigned to human chromosome 9 and mouse chromosome 4 by Southern blot analyses of 11 human X rodent hybrids and 11 mouse X rodent hybrids with probe pHM1.0 and probe pSP270, respectively. In situ hybridization of probe pHSTU300 to normal human chromosomes revealed 29 of 100 cells with label on 9p and 9.6% of 302 silver grains located at 9p21--p12.

Novel use of a chimpanzee pseudogene for chromosomal mapping of human cytochrome c oxidase subunit IV

We have isolated a chimpanzee processed pseudogene for subunit IV of cytochrome c oxidase (COX; EC 1.9.3.1) by screening a chimpanzee genomic library in lambda Charon 32 with a bovine liver cDNA encoding COX subunit IV (COX IV), and localized it to a 1.9-kb HindIII fragment. Southern-blot analysis of genomic DNA from five primates showed that DNAs from human, gorilla, and chimpanzee each contained the 1.9-kb pseudogene fragment, whereas orangutan and pigtail macaque monkey DNA did not. This result clearly indicates that the pseudogene arose before the divergence of the chimpanzee and gorilla from the primate lineage. By screening Chinese hamster x human hybrid panels with the human COX4 cDNA, we have mapped COX4 genes to two human chromosomes, 14 and 16. The 1.9-kb HindIII fragment containing the pseudogene, COX4P1, can be assigned to chromosome 14, and by means of rearranged chromosomes in somatic cell hybrids, to 14q21-qter. Similarly, the functional gene, COX4, has been mapped to 16q22-qter.

The molecular basis for Duchenne versus Becker muscular dystrophy: correlation of severity with type of deletion

About 60% of both Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) is due to deletions of the dystrophin gene. For cases with a deletion mutation, the "reading frame" hypothesis predicts that BMD patients produce a semifunctional, internally deleted dystrophin protein, whereas DMD patients produce a severely truncated protein that would be unstable. To test the validity of this theory, we analyzed 258 independent deletions at the DMD/BMD locus. The correlation between phenotype and type of deletion mutation is in agreement with the "reading frame" theory in 92% of cases and is of diagnostic and prognostic significance. The distribution and frequency of deletions spanning the entire locus suggests that many "in-frame" deletions of the dystrophin gene are not detected because the individuals bearing them are either asymptomatic or exhibit non-DMD/non-BMD clinical features.

Assignment of human genes for phosphorylase kinase subunits alpha (PHKA) to Xq12-q13 and beta (PHKB) to 16q12-q13

Phosphorylase kinase (PHK), the enzyme that activates glycogen phosphorylases in muscle, liver, and other tissues, is composed of four different subunits. Recently isolated rabbit muscle cDNAs for the larger two subunits, alpha and beta, have been used to map the location of their cognate sequences on human chromosomes. Southern blot analysis of rodent x human somatic cell hybrid panels, as well as in situ chromosomal hybridization, have provided evidence of single sites for both genes. The alpha subunit gene (PHKA) is located on the proximal long arm of the X chromosome in region Xq12-q13 near the locus for phosphoglycerate kinase (PGK1). X-linked mutations leading to PHK deficiency, known to exist in humans and mice, are likely to involve this locus. This hypothesis is consistent with the proximity of the Phk and Pgk-1 loci on the mouse X chromosome. In contrast, the beta subunit gene (PHKB) was found to be autosomal and was mapped to chromosome 16, region q12-q13 on the proximal long arm. Several different autosomally inherited forms of PHK deficiency for which the PHKB could be a candidate gene have been described in humans and rats.

Brother/sister pairs affected with early-onset, progressive muscular dystrophy: molecular studies reveal etiologic heterogeneity

An autosomal recessive (AR) form of muscular dystrophy that clinically resembles Duchenne/Becker types exists, but its frequency is unknown. We have studied three unrelated affected brother/sister pairs and their families for deletions and polymorphisms with the entire dystrophin cDNA and other DNA probes from the Xp21 region to test for involvement of the DMD locus. In family 1 a large intragenic deletion was found in the affected male. The affected sister was heterozygous for this deletion, but the mother was not, implying germinal mosaicism. In family 2, no deletion was detected in the affected male. RFLP analysis revealed that the affected male and an unaffected sister shared a complete Xp21 haplotype while the affected sister had inherited a recombinant Xp21 region resulting from a crossover between pERT 87-15 and J-Bir. Only the 5' region of the dystrophin gene was shared with the affected boy. X-inactivation studies using a polymorphism in the 5'-flanking region of the HPRT gene, in conjunction with methylation-sensitive enzymes, revealed random X inactivation in the affected girl's leukocytes. In a muscle biopsy from the affected male, the dystrophin protein was present in normal amount and size. Family 3 was informative for four RFLPs detected with dystrophin cDNA probes which span the entire gene. The affected male was found to share the complete dystrophin RFLP haplotype with his unaffected brother, while his affected sister had inherited the other maternal haplotype. It is concluded that the clinical presentation of early-onset, progressive muscular dystrophy in a male and in his karyotypically normal sister can be caused by mutations at different loci. While in family 1 a deletion in the dystrophin gene is responsible, this gene does not appear to be involved in families 2 and 3.

Intermittent dystonia in Hartnup disease

A 6-month-old girl developed intermittent dystonic posture of the legs and eczematous dermatitis without ataxia. Qualitative and quantitative urine amino acid testing confirmed the diagnosis of Hartnup disease. Cranial computed tomography, electroencephalogram, electromyogram/nerve conduction study, posterior tibial somatosensory evoked potentials, 24-hour electroencephalographic telemetry, and metrizamide myelogram were normal. Spinal fluid hydroxy-indoleacetic acid concentration was less than or equal to 2 S.D. of normal; oral tryptophan loading (70 mg/kg) resulted in a two-fold rise in cerebrospinal fluid 5-hydroxy-indoleacetic acid concentration. Tryptophan administered alone or with nicotinic acid failed to improve the dystonia; however, trihexyphenidyl (1-2 mg/kg/day) dramatically improved it. Hartnup disease should be considered in children with unexplained dystonia.

Normal human genomic restriction-fragment patterns and polymorphisms revealed by hybridization with the entire dystrophin cDNA

Since the complete cDNA for the gene that causes X-linked recessive Duchenne/Becker muscular dystrophy (DMD/BMD) when mutated or deleted has recently been cloned and made generally available, DNA-based diagnostic studies of affected males and their families have entered into a new era. This communication sets forth the standard patterns of restriction fragments that are detected when normal human DNA cleaved with either HindIII or BglII is hybridized with seven contiguous segments comprising the entire 14-kb cDNA. Collectively, the more than 60 restriction fragments allow visualization of approximately 350 (HindIII) to 400 (BglII) kbp. This corresponds to the exon-containing one-fifth of the total genomic length of this gene, including the 3' untranslated region. Twelve two-allele restriction-site polymorphisms that span the entire length of the gene were detected with the cDNA probes and allele frequencies determined. A diagnostic approach is proposed that starts with deletion screening of DNA from male probands, includes carrier detection based on relative fragment intensities, and extends to RFLP detection using the same autoradiographs prepared for deletion screening. Our results on deletion analysis of 32 DMD/BMD families are presented in an accompanying paper.

Intragenic deletions in 21 Duchenne muscular dystrophy (DMD)/Becker muscular dystrophy (BMD) families studied with the dystrophin cDNA: location of breakpoints on HindIII and BglII exon-containing fragment maps, meiotic and mitotic origin of the mutations

Following the strategy outlined in an accompanying paper, we studied 32 X-linked muscular dystrophy families (29 Duchenne [DMD] and three Becker [BMD] type) for abnormalities of HindIII and BglII fragments detected by the entire dystrophin cDNA. Twenty-one different single-intragenic deletions, and no duplications, were identified. The deletion endpoints were precisely mapped on the published HindIII fragment map. Detailed analysis of overlapping deletions led to clarification of the fragment order for some previously unsettled regions of the HindIII map and to the construction of a partial map of exon-containing BglII fragments. For the regions involved in deletions, the corresponding HindIII and BglIII fragments could be identified. Noncontiguous comigrating fragments were detected in two regions by careful analysis of the patterns in deletion patients. Four of the 21 deletions generated novel restriction fragments that facilitated detection of female carriers in these families. Twelve of the deletions had a breakpoint in one of the two large introns known to be the sites of breakpoint clusters. By combining deletions and RFLP analyses, we unequivocally identified the gamete that first carried the mutation in 13 families: eight oocytes and five sperm. Germ-line mosaicism previously detected in one male was confirmed by cDNA studies. In two additional families gonadal mosaicism was found in females. As evidence is accumulating for frequent mitotic origin of these deletion mutations, this phenomenon has to be considered when postulating mutational mechanisms and in genetic counseling of DMD/BMD families.