Genetic and biochemical studies with ataxia telangiectasia. A review

Atypical clinical presentation of ataxia telangiectasia

Ataxia telangiectasia (AT) is an autosomal recessive disorder characterized by progressive cerebellar ataxia, oculocutaneous telangiectasias, immunodeficiency with recurrent infections, IgA and IgE deficiency, and increased incidence of malignancies. The pathognomonic biological abnormalities consist of spontaneous chromosomal instability resulting in a high in vivo occurrence of cells with translocations, especially involving chromosomes 7 and 14, and a relative insensitivity of DNA replication in vitro to radiation exposure. We report on a patient with the biological hallmarks of AT but with atypical clinical manifestations. Although progressive cerebellar ataxia was present, the neurological picture was broader than that usually seen in AT and included peripheral polyneuropathy and spinal atrophy. On the other hand, telangiectasias, recurrent infections, malignancies, IgA deficiency, or other immunological abnormalities were not present. This illustrates that the clinical picture of AT is broad and nonspecific, and highlights the diagnostic value of cytogenetic analysis and studies of radioresistance of DNA synthesis.

Unusual features in the inheritance of ataxia telangiectasia

A prevalence study of ataxia telangiectasia was conducted in the West Midlands, with a population of over 5 million. The prevalence in those aged 50 or less was found to be 1 in 514,000 and the birth frequency to be about 1 in 300,000. A genetic study of 47 families ascertained throughout the United Kingdom was carried out concurrently. A low parental consanguinity rate was found, no parents being first cousins or more closely related, whereas 10% had been expected. The incidence of ataxia telangiectasia in the 79 sibs of index cases was 1 in 7. These two features demonstrate that ataxia telangiectasia may not always be an autosomal recessive condition. Other possible explanations are that some cases are double heterozygotes or new dominant mutations.

A derivative of an ataxia-telangiectasia (A-T) cell line with normal radiosensitivity but A-T-like inhibition of DNA synthesis

Ataxia-telangiectasia (A-T) cells are hypersensitive to the lethal effects of ionizing radiation and fail to inhibit DNA synthesis following radiation exposure. A cell line derived from an A-T line following DNA-mediated gene transfer has normal radiation sensitivity, but the kinetics of DNA synthesis after gamma-irradiation are similar to those of A-T cells.

Ataxia telangiectasia with generalized skin pigmentation and early death

A female infant with clinical and laboratory features of ataxia telangiectasia (AT) showed two clinical features exceptional for the disease, i.e. generalized skin pigmentation and an unusually early death at the age of 15 months. Her clinical features supportive of the diagnosis of AT included growth and developmental retardation and muscle weakness. Findings indicating immunodeficiency included recurrent pulmonary infections, failure of PHA stimulation of PB lymphocytes, decreased levels of serum IgM and IgA and on autopsy, an atrophic thymus without Hassall's corpuscles. Her cultured skin fibroblasts showed increased spontaneous chromosome breakages and hypersensitivity to X-ray irradiation, as would be expected for AT fibroblasts. She showed elevated blood HbF levels, macrocytic anaemia, granulocytopenia and thrombocytopenia, findings suggestive of a preleukaemic or leukaemic process. Yet aspirates of her bone marrow revealed no malignant cells. Autopsy revealed bilateral Pneumocystis carinii pneumonia, telangiectatic lesions in all the internal organs studied, sparse and degenerative Purkinje cells in the cerebellar cortex and atrophic ovaries. In view of these findings, it was concluded that the patient had a hitherto undescribed variant of ataxia telangiectasia.

Rapid rejoining of X-ray-induced DNA single-strand breaks in tuberous sclerosis fibroblasts

The formation and rejoining rates of X-ray-induced DNA single-strand breaks (SSBs) were examined in radiosensitive and non-radiosensitive fibroblast lines from patients with tuberous sclerosis (TS), and fibroblasts from normal individuals, using the sensitive and quantitative alkaline elution method. No difference was found between these cell lines in the frequency of DNA SSBs directly produced by X-irradiation at any dose up to 750 rad. Kinetic analysis of the rate of rejoining of DNA SSBs after X-irradiation at 500 rad indicated that the rate of rejoining involved at least two components, an initial fast component and a slower component. TS fibroblast lines, either radiosensitive or nonsensitive, were proficient as to DNA SSB repair, but they showed an increased rate of rejoining in the initial fast repair process, when compared to normal fibroblast lines. Although the molecular basis for the accelerated rejoining of DNA SSBs remains unknown, it is possible that the abnormality may be related to a basic defect in TS.

Cerebellar cortex ultrastructure in ataxia-telangiectasia

An ultrastructural study of a biopsy fragment from the cerebellar cortex of a patient with ataxia-telangiectasia was performed. There was a sharp reduction in the number of Purkinje cells and an increase in the number of Golgi epithelial cells. Despite the degeneration of Purkinje cell dendrites, the volumetric density of parallel fiber varicosities and the number of synapses in the molecular layer were only slightly reduced. No qualitative nor quantitative abnormalities were noted in the granular layer. An analysis is made of the severe changes in cerebellar circuitry of this patient and their probable functional implications.

Repair of potentially lethal and sublethal damage induced by neocarzinostatin in normal and ataxia-telangiectasia skin fibroblasts

Neocarzinostatin is a radiomimetic antibiotic with a potent cytotoxic effect which elicits a hypersensitive response in human cells homozygous or heterozygous for the gene for ataxia-telangiectasia. The extent and the time course of potentially lethal damage repair and sublethal damage repair following neocarzinostatin treatment were investigated in human skin fibroblast strains and were found to be remarkably similar to those obtained following X-irradiation. Ataxia-telangiectasia homozygous cells essentially lacked potentially lethal damage repair, but were able to perform some degree of sublethal damage repair following neocarzinostatin treatment. Ataxia-telangiectasia heterozygous cells which show an intermediate degree of neocarzinostatin sensitivity could perform both processes but with somewhat reduced efficiency as compared to normal cells. These observations provide further evidence for a DNA repair defect in ataxia-telangiectasia cells.

Reduced inhibition of replicon initiation and chain elongation by neocarzinostatin in skin fibroblasts from patients with ataxia telangiectasia

Cells from patients with the genetic disease ataxia telangiectasia are hypersensitive to the DNA-breaking agents X-rays, bleomycin and neocarzinostatin, and show reduced inhibition of DNA synthesis after treatment with these agents, as compared to normal cells. The rate of replicon initiation and chain elongation was measured shortly after brief exposure of two normal and two ataxia telangiectasia fibroblast strains to low doses (0.10-0.30 microgram/ml) of neocarzinostatin, by means of alkaline sucrose gradient analysis. Neocarzinostatin was found to inhibit both initiation and elongation, and both components of DNA synthesis were more resistant to this inhibition in the A-T strains.