Neurogenetics update.
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Chromosome Mapping↗
Biomedical subjects
A E Harding
Publications and source records attributed to A E Harding.
Mitochondrial DNA mutation underlying Leigh's syndrome: clinical, pathological, biochemical, and genetic studies of a patient presenting with progressive myoclonic epilepsy.
An 18-year-old male patient presented with clinical and radiological evidence of Leigh's syndrome (LS), having developed progressive myoclonic epilepsy and ataxia 11 years previously. Muscle biopsy showed cytochrome oxidase deficiency but no ragged red fibres. Autopsy confirmed the diagnosis of LS; there was additional degenerative change in the cerebellum and dentate and olivary nuclei, and an axonal peripheral neuropathy. Biochemical studies showed reduced activity of complexes I and IV of the respiratory chain in mitochondria from heart, liver and kidney. The mutation of mitochondrial DNA (mtDNA) at position 8344, commonly associated with the syndrome of myoclonic epilepsy and ragged red fibres, was detected in the patient's blood and was present in muscle, brain, liver, heart, and kidney in uniformly high amounts. It is clear that LS is genetically heterogeneous and represents one of the most severe phenotypes of a number of different mtDNA defects.
Adolescent↗
HLA class II genotypes in Leber's hereditary optic neuropathy.
There is an association between Leber's hereditary optic neuropathy (LHON) and a multiple sclerosis-like illness, raising the possibility of autoimmune pathogenetic mechanisms in LHON. We therefore investigated the frequency of HLA-DR genotypes in members of 79 families with LHON, defined by the presence of a pathogenic mitochondrial DNA mutation. There was no association between LHON and any HLA-DR genotype. Furthermore, affected relative pairs did not share HLA genotypes more than discordant pairs. We conclude that the HLA-DR locus is not a major genetic determinant for the development of blindness in LHON.
Alleles↗
Autosomal dominant cerebellar ataxia with pigmentary macular dystrophy. A clinical and genetic study of eight families.
We describe 54 members of eight families with a distinct autosomal dominant cerebellar ataxia associated with visual failure secondary to a pigmentary macular dystrophy. The presenting symptom was ataxia in two-thirds of patients and visual failure or both in the remainder. The macular abnormalities were often subtle in early cases, even in some with moderately reduced visual acuity. Other neurological features included pyramidal tract signs and a supranuclear ophthalmoplegia with progressive saccadic palsy. Ages of onset and clinical course were very variable, even within families, and included a rapidly progressive, infantile-onset phenotype. Pedigree analysis showed the existence of non-manifesting obligate carriers and anticipation in the offspring of affected fathers; transmission of the disease to severe, infantile-onset cases was always from an affected father. Similar genetic phenomena have been reported in myotonic dystrophy and Huntington's disease and it is likely that the gene mutation in this condition will similarly consist of an unstable trinucleotide repeat expansion.
Adolescent↗
The trinucleotide repeat expansion on chromosome 6p (SCA1) in autosomal dominant cerebellar ataxias.
Affected members of 73 families with a variety of autosomal dominant late onset cerebellar ataxias (ADCAs) were investigated for the trinucleotide (CAG) repeat expansion which is found in pedigrees exhibiting linkage to the SCA1 locus on chromosome 6. Most of the families were too small for linkage analysis. The mutation was only found in ADCA type I, in 19 out of 38 such kindreds investigated (50%). It was slightly more common in Italian (59%) than British (50%) families, and was also found in Malaysian, Bangladeshi and Jamaican kindreds. Overall, ADCA type I patients with the expansion had a lower incidence of hyporeflexia and facial fasciculation than those without. The trinucleotide expansion was not found in eight families with ADCA and maculopathy or 24 kindreds with a pure type of ADCA, confirming that these syndromes are genetically distinct. It was also not detected in 12 patients with sporadic degenerative ataxias. DNA analysis for the SCA1 mutation is useful diagnostically in single patients or small families, and can be used for presymptomatic testing where appropriate.
Adolescent↗
A study of hereditary essential tremor.
Twenty index patients with hereditary essential tremor and their kindreds were studied to define the phenotype of this condition. Ninety-three first degree and 38 more distant relatives were examined; 53 definite and 18 possible secondary cases were identified. The age of tremor onset was bimodally distributed with a median at approximately 15 years. Segregation analysis indicated autosomal dominant inheritance and penetrance was virtually complete by the age of 65 years. There were no examples of the disease skipping a generation. Men and women were affected in equal proportions. About 50% of cases were alcohol responsive. In the majority of families alcohol responsiveness was either consistently present or did not occur, but in 20% of kindreds definite heterogeneity of responsiveness was encountered within each family. The typical phenotype was a mild symmetrical postural tremor of the upper limbs. Tremor of the legs, head, facial muscles, voice, jaw and tongue occurred but never in isolation and rest, task specific (e.g. primary writing tremor) and primary orthostatic tremors were not found. Head tremor was invariably mild and 75% was of a 'no-no' type. Dystonia (e.g. torticollis and writer's cramp) were not encountered, a finding which strongly suggests that many previous studies of 'essential tremor' were contaminated by cases of idiopathic or hereditary torsion dystonia. No association with Parkinson's disease was found but classical migraine occurred in approximately 26% of cases and co-segregated with tremor. The severity of arm tremor (assessed using a clinical rating scale and by scoring tremor in Archimedes spirals) and disability increased with advancing age and increasing tremor duration, but there was no correlation between age at tremor onset and either tremor severity or disability. Men and women were affected with equal severity. The sex of the affected parent had no influence on the severity of tremor or the degree of disability experienced by an affected child. Disability commenced in the second decade and progressively increased. All the index patients and 59% of the definite secondary cases had tremor induced disabilities. Eighty-five percent of index patients and 38% of secondary cases also reported some degree of social handicap. Twenty-five percent of index patients and 12% of secondary cases had been compelled to change jobs or retire. Biological fitness was normal.
Adolescent↗
Mitochondrial DNA 8993 (NARP) mutation presenting with a heterogeneous phenotype including 'cerebral palsy'.
The mitochondrial DNA (mtDNA) mutation 8993 is an important cause of Leigh's encephalopathy. A family is reported where other affected members have presented with non-specific delayed development or cerebral palsy. The diagnosis should be considered not only in children with Leigh's encephalopathy, but also in those with mild neurological dysfunction (including cerebral palsy) if there is a pigmentary retinopathy or a family history of neurological or ophthalmological disease. There was some correlation in this family between the disease severity and the proportion of mutant mtDNA in the blood. This mutation appears to segregate to high levels of mutant mtDNA rapidly within pedigrees and the mother of a severely affected child has a high risk of having further children with a high proportion of mutant mtDNA and a severe phenotype.
Adolescent↗
Magnetic resonance imaging in degenerative ataxic disorders.
MRI of the brain was performed in 53 patients with a variety of degenerative ataxias and related disorders and 96 control subjects. Atrophy of intracranial structures was not seen in patients with the pure type of hereditary spastic paraplegia, or in early cases of Friedreich's ataxia. In advanced Friedreich's ataxia there was atrophy of the vermis and medulla. The MRI features of early onset cerebellar ataxia with retained reflexes were variable, and suggest heterogeneity. In autosomal dominant cerebellar ataxias, most patients had cerebellar and brainstem atrophy, probably reflecting the pathological process of olivopontocerebellar atrophy; there was no clearly defined group with both clinical and imaging features of isolated cerebellar involvement. The MRI abnormalities in idiopathic late onset cerebellar ataxia were predominantly those of cerebellar and brainstem atrophy or pure cerebellar atrophy. The clinical and imaging features of brainstem abnormalities were discordant in several patients. Pure cerebellar atrophy was associated with slower progression of disability. Cerebral atrophy was common in the late onset ataxias. Cerebral white matter lesions, although usually few in number, were observed in significantly more patients than controls, particularly those aged over 50 years.
Adolescent↗
Debrisoquine hydroxylase gene polymorphism in familial Parkinson's disease.
Recent molecular genetic studies of the cytochrome P-450 system enzyme CYP2D6, which hydroxylates debrisoquine, have indicated an excess of mutant alleles in patients with Parkinson's disease compared with controls. This indicates that the CYP2D6 locus confers genetic susceptibility to Parkinson's disease. CYP2D6 polymorphism has been investigated in 48 patients with familial Parkinson's disease, from 22 families, and 88 of their unaffected relatives. An excess of CYP2D6 mutant alleles in patients compared with healthy relatives was found only in subjects over the age of 60 years, presumably reflecting the age related prevalence of this disease. There was no difference in distribution of genotypes, however, between sib pairs concordant or discordant for Parkinson's disease. Linkage analysis, exclusively with affected family members, yielded negative lod scores. These data indicate that the CYP2D6 locus is not the major determinant of genetic susceptibility in familial Parkinson's disease.
Age Factors↗
Nigral degeneration in neuroacanthocytosis.
We counted nerve cells in different subregions of the substantia nigra in three patients with neuroacanthocytosis and compared the results with those of age-matched Parkinson's disease and control patients. Two patients with neuroacanthocytosis and clinical parkinsonism in life had a reduced neuronal density in the substantia nigra, while in a third patient without parkinsonism, this number was at the lower limit of the control range. In neuroacanthocytosis with parkinsonism and in Parkinson's disease, the ventrolateral region of the substantia nigra was most severely affected, although in neuroacanthocytosis cases, nigral neuronal loss was more widespread.
Aged↗
Inherited movement disorders.
A high proportion of neurological diseases characterized by movement disorders are caused by single genes; in others, such as Parkinson's disease, there appears to be a genetic component. Gene mapping studies have made substantial progress in unravelling the aetiology of dystonias and Huntington's disease, and are underway in other disorders such as essential tremor and Gilles de la Tourette syndrome. These advances are already applicable to clinical practice, particularly in Huntington's disease where identification of the disease mutation has led to the possibility of improved diagnosis and more widespread availability of predictive testing for asymptomatic family members.
Acanthocytes↗
Disorders of the motor neurone.
The spinal muscular atrophies (SMAs) are defined as a group of inherited disorders sharing the common pathological feature of degeneration of the anterior horn cells of the spinal cord and, in some cases, additionally of the bulbar motor nuclei. They are classified according to clinical features, including severity and distribution of muscle weakness and on their modes of inheritance. The SMAs are not uncommon: SMA type I (severe, acute infantile SMA) alone has a gene frequency in the UK estimated at 0.006. Together they represent a significant source of morbidity and mortality. Over the past 3 years, two major advances have been made towards understanding the molecular basis of these clinically heterogeneous disorders. First, in 1990, it was reported that all three forms of childhood-onset proximal SMA were linked to probes mapping to the proximal long arm of chromosome 5. Significant progress has been made towards isolating the gene or genes responsible, and a protocol for prenatal disease prediction in families with a previously severely affected child has been established. Second, in 1991, the molecular defect in adult-onset X-linked spinal and bulbar muscular atrophy was defined as an expansion of a (CAG)n trinucleotide repeat which encodes a string of glutamine residues in the first exon of the androgen receptor. Little progress has been made in elucidating the molecular pathology of the other SMAs. None of them has been linked to chromosome 5q markers, or indeed to markers elsewhere; the conditions are rare and pedigrees generally small. When the gene (or genes) on proximal 5q underlying SMA types I, II and III is cloned, mutations in this can then be sought in the other SMAs. The product of the chromosome 5q gene presumably plays a crucial role in maintaining the integrity of motor neurones, so determination of its structure and function may well have consequences far beyond those pertaining to the inherited SMAs. The identification of pathogenic mutations in the SOD-1 gene in familial amyotrophic lateral sclerosis is of great interest, although it is not clear that similar mechanisms contribute to the more common sporadic form of the disease.
Age of Onset↗
Effect of trinucleotide repeat length and parental sex on phenotypic variation in spinocerebellar ataxia I.
Trinucleotide repeat expansion has been found in 64 subjects from 19 families: 57 patients with SCA1 and 7 subjects predicted, by haplotype analysis, to carry the mutation. Comparison with a large set of normal chromosomes shows two distinct distributions, with a much wider variation among expanded chromosomes. The sex of transmitting parent plays a major role in the size distribution of expanded alleles, those with > 54 repeats being transmitted by affected fathers exclusively. Our data suggest that alleles with > 54 repeats have a reduced chance of survival; these appear to be replaced in each generation by further expansion of alleles in the low- to medium-expanded repeat range, preferentially in male transmissions. Detailed clinical follow-up of a subset of our patients demonstrates significant relationships between increasing repeat number on expanded chromosomes and earlier age at onset, faster progression of the disease, and earlier age at death.
Adult↗
The gene for Huntington's disease.
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Genetic Markers↗
Linkage analysis with chromosome 9 markers in hereditary essential tremor.
Hereditary essential tremor (ET) is an autosomal dominant disorder with variable expression and reduced penetrance. A tremor indistinguishable from ET may be observed in patients with autosomal dominant idiopathic torsion dystonia (ITD), in which the disease locus has been mapped to 9q32-34 in some kindreds, tightly linked to the argininosuccinate synthetase (ASS) locus. We performed linkage analysis in 15 families with ET containing 60 definitely affected individuals, using dinucleotide repeat polymorphisms at the ASS locus and the Abelson locus (ABL). Cumulative lod scores were -19.5 for ASS and -10.8 for ABL at a recombination fraction of 0.01, and tight linkage to ASS was excluded individually in 11 of the families. These data indicate that the ET gene is not allelic to that causing ITD.
Adolescent↗
Evidence for locus heterogeneity in autosomal dominant torsion dystonia.
Idiopathic torsion dystonia (ITD) is most commonly an autosomal dominant disorder with reduced penetrance and variable expression. A locus on the distal long arm of chromosome 9 has been identified in one large non-Jewish and several Jewish families in the United States. Linkage analysis in a large Australian kindred with ITD, also containing two patients with Wilson's disease, excludes a locus for ITD in chromosome 9q34 or the region of chromosome 13 containing the Wilson disease gene. This study provides evidence for locus heterogeneity in autosomal dominant ITD and also gives additional information on gene order in chromosome 9q.
Adolescent↗
Clinical and molecular neurogenetics in neurosurgery.
This chapter describes the basic principles of molecular genetics, particularly in relation to gene mapping and isolation (positional cloning). The application of this technology is illustrated by progress in elucidating the molecular basis of inherited tumour syndromes, including neurofibromatosis types 1 and 2 and von Hippel-Lindau disease, and molecular studies of tumourigenesis in malignant gliomas.
Brain Neoplasms↗