Radiosensitivity of normal tissues in ataxia-telangiectasia heterozygotes

PURPOSE

Approximately 5% of cancer patients given radiation therapy exhibit severe injuries to the noncancerous tissue in the radiation field. Striking clinical sensitivity to ionizing radiation has been observed frequently in ataxia-telangiectasia (A-T) homozygotes. This study was undertaken to test the hypothesis that heterozygous carriers of a mutated gene for A-T may represent a substantial proportion of all patients who suffer severe radiation toxicity.

METHODS

The medical records of all A-T heterozygotes treated with radiation therapy for breast or prostate cancer were compiled from an ongoing study of mortality and cancer incidence in A-T families. Diagnostic, treatment, and follow-up records were reviewed. Acute and long-term radiation complications were scored according to Radiation Therapy and Oncology Group criteria.

RESULTS

There were no instances of soft tissue necrosis or other apparent serious injuries to normal tissues of two A-T heterozygotes with prostate carcinoma and 11 with breast carcinoma who received moderate-to-high doses of conventionally fractionated radiation therapy by megavoltage techniques.

CONCLUSION

There is no evidence that abnormal clinical radiosensitivity occurs in A-T heterozygotes receiving conventionally fractionated radiation therapy for breast or prostate cancer.

Predisposition of Wolfram syndrome heterozygotes to psychiatric illness

Identification of specific genes that predispose to psychiatric illness will lead to more precise psychiatric diagnosis and more effective treatment. Heterozygous carriers of genes for many autosomal recessive syndromes may be 1% or more of the general population. Thus, if mutations at a specific locus produce psychiatric manifestations in homozygous affected individuals, it is important to determine whether mutations at such a locus also predispose heterozygous carriers to psychiatric disorders. The hypothesis that heterozygous carriers of the gene for the Wolfram syndrome (WS) are predisposed to psychiatric illness was supported previously by the finding of an excess of psychiatric hospitalizations and suicides in WS blood relatives compared to spouse controls. This hypothesis has now been tested further by comparing the number of psychiatrically hospitalized blood relatives with the specific marker haplotype associated with the Wolfram syndrome gene in their families to the number expected under the null hypothesis, calculated from Mendelian inheritance principles and the estimated haplotype frequency. The proportion of psychiatrically hospitalized relatives who were WS carriers (10/11) was much higher than expected (3.1/11), leading to the provisional estimate that WS gene carriers are 26-fold more likely to require psychiatric hospitalization than non-carriers.

Molecular genotyping shows that ataxia-telangiectasia heterozygotes are predisposed to breast cancer

About 1.4% of the general population are heterozygous carriers of the gene for ataxiatelangiectasia (A-T), an autosomal recessive progressive neurologic syndrome in which cancer incidence of homozygotes is approximately 100-fold greater than the general population's rates. The hypothesis that A-T heterozygotes are predisposed to breast cancer was tested by the unbiased statistically powerful index-test method based on molecular genotyping. The A-T gene carrier status of 775 blood relatives in 99 A-T families was determined by tracing the A-T gene in each family through tightly linked flanking DNA markers. There were 33 women with breast cancer who could be genotyped; 25 of these were A-T heterozygotes, compared to an expected 14.9 (odds ratio 3.8, 95% confidence limits 1.7-8.4, one-sided p = .0001). This demonstrates that the A-T gene predisposes heterozygotes to breast cancer. For the 21 breast cancers with onset before age 60, the odds ratio was 2.9 (1.1-7.6, p = .009) and for the 12 cases with onset at age 60 or older, the odds ratio was 6.4 (1.4-28.8, p = .002). Thus the breast cancer risk for A-T heterozygous women is not limited to young women but appears even higher at older ages. Of all breast cancers in the United States, 6.6% may occur in women who are A-T heterozygotes. This proportion is several fold greater than the estimated proportion of carriers of BRCA1 mutations in breast cancer cases with onset at any age.

Morbidity and mortality in the Wolfram syndrome

OBJECTIVE

To determine the major causes of morbidity and mortality in the autosomal recessive Wolfram syndrome, which is defined by diabetes and bilateral progressive optic atrophy with onset in childhood or adolescence.

RESEARCH DESIGN AND METHODS

We abstracted and reviewed the medical records of 68 confirmed cases of Wolfram syndrome identified through a nationwide survey of endocrinologists, ophthalmologists, institutes, and homes for the blind. We also reviewed all available autopsy records.

RESULTS

The most common causes of morbidity and mortality were the neurological manifestations of this syndrome and the complications of urinary tract atony. There was a lower frequency of diabetic ketoacidosis, no histologically proven diabetic glomerulosclerosis, and less severe, more slowly progressive, diabetic retinopathy than in classic type I diabetic patients. Mortality in Wolfram syndrome is much higher than in type I diabetes; 60% of Wolfram syndrome patients die by age 35. Recognition of these clinical differences from classic type I diabetes is important for the proper management of Wolfram syndrome patients.

CONCLUSIONS

Identification of Wolfram syndrome patients among all diabetic patients presenting in childhood or adolescence is important because the management of patients with this syndrome is different from that of patients with classic type I diabetes.

Protection against bronchial asthma by CFTR delta F508 mutation: a heterozygote advantage in cystic fibrosis

Cystic fibrosis (CF) is a multisystem autosomal recessive disorder caused by mutations of the cystic fibrosis transmembrane regulator (CFTR), a protein that regulates cyclic-AMP-mediated chloride conductance at the apical membrane of secretory epithelia. Mutations in the CFTR gene are common in many populations. In North America, 4-5% of the general population are heterozygous for a CFTR mutation. Although there are over 400 known CFTR mutations, a single mutation, a deletion of the phenylalanine at position 508 (delta F508) in exon 10, accounts for about 70% of all CF chromosomes worldwide. The reasons for the high frequency of the delta F508 CFTR allele--the selective advantage associated with CF heterozygosity--are unknown. Many physiological abnormalities have been observed in CF heterozygotes, although the clinical significance of these observations is unknown. Preliminary unpublished data and anecdotal information from CF families suggested that, remarkably, the delta F508 allele might protect heterozygotes against bronchial asthma prompted us to further investigate this possibility. Here we present evidence that the delta F508 CF allele protects against asthma in childhood and early adult life.

Single base polymorphism linked to the ataxia-telangiectasia locus is detected by mismatch PCR

A polymorphic site at D11S384, which shows zero recombination with the ataxia-telangiectasia (A-T) locus, was originally detected by Mutation Detection Enhancement Gel and Denaturing Gradient Gel Electrophoresis. In order to increase the throughput and decrease the complexity of the assays, we developed a site directed mutagenesis using primers with mismatched 3' ends, followed by restriction digestion, as a rapid, nonradioactive method for detecting polymorphisms/mutations.

Obesity in heterozygous carriers of the gene for the Bardet-Biedl syndrome

Obesity and renal failure are common manifestations in the autosomal recessive Bardet-Biedl (BB) syndrome. Because obesity and hypertension have been reported frequently in non-homozygous relatives of BB patients, we hypothesized that BB heterozygotes are predisposed to these conditions. Clinical information was collected from 34 patients of BB homozygotes, who are obligate heterozygotes. The proportion of severely overweight fathers (26.7%) was significantly higher than that in comparably aged United States white males (8.9%). We conclude that the BB gene may predispose male heterozygous carriers to obesity. If BB heterozygotes are 1% of the general population, we estimate that approximately 2.9% of all severely overweight white males carry a single BB gene. The BB parents of both sexes were also significantly taller than U.S. white men and women of comparable age.

DNA marker D11S384 shows zero recombination with the ataxia-telangiectasia locus in North American families

At the genetic locus D11S384, one probe detects a Taql RFLP and another detects a RFMP. In 52 pedigrees of North American A-T patients, parental haplotypes based on these two bi-allelic systems have a heterozygosity of 0.69 and a PIC of 0.64. No recombinant events between D11S384 and the A-T locus were detected in the 43 pedigrees in which this marker locus was informative.

Linkage of the gene for Wolfram syndrome to markers on the short arm of chromosome 4

Wolfram syndrome is an autosomal recessive disorder defined by the occurrence of diabetes mellitus and progressive bilateral optic atrophy. Wolfram syndrome homozygotes develop widespread nervous system abnormalities; in particular, they exhibit severe behavioural difficulties that often lead to suicide attempts or psychiatric hospitalizations. The Wolfram syndrome gene also predisposes heterozygous carriers to psychiatric disorders, and may contribute significantly to the overall burden of psychiatric illness. Based on a linkage analysis of 11 families segregating for this syndrome using microsatellite repeat polymorphisms throughout the human genome, we found the Wolfram syndrome gene to be linked to markers on the short arm of human chromosome 4, with Zmax = 6.46 at theta = 0.02 for marker D4S431.