Multiple mutation in an extended Duchenne muscular dystrophy family

Intra-individual plasticity of the TAZ gene leading to different heritable mutations in siblings with Barth syndrome

Infantile-onset skeletal myopathy Barth syndrome (OMIM #302060) is caused by mutations in the X-linked TAZ gene and hence usually manifests itself only in hemizygous males. Confirmatory testing is provided by mutational analysis of the TAZ gene and/or by biochemical dosage of the monolysocardiolipin/tetralinoleoyl cardiolipin ratio. Heterozygous females do not usually display a clinical phenotype but may undergo molecular genetic prenatal diagnosis during pregnancy. We characterized two novel and non-identical TAZ gene rearrangements in the offspring of a single female carrier of Barth syndrome. The hg19chrX:g.153634427_153644361delinsKP_123427.1 TAZ gene rearrangement was identified in her affected son, whereas the NM_000116.3(TAZ)c.-72_109+51del TAZ gene deletion was identified in a male foetus during a subsequent pregnancy. The unaffected mother was surprisingly found to harbour both variants in addition to a wild-type TAZ allele. A combination of breakpoint junction sequencing, linkage analysis and assessment of allelic dosage revealed that the two variants had originated independently from an apparently unstable/mutable TAZ maternal allele albeit via different mutational mechanisms. We conclude that molecular prenatal diagnosis in Barth syndrome families with probands carrying TAZ gene rearrangements should include investigation of the entire coding region of the TAZ gene. The identification of the breakpoint junctions of such gross gene rearrangements is important to ensure accurate ascertainment of carriership with a view to providing appropriate genetic counselling.

A possible bichromatid mutation in a male gamete giving rise to a female mosaic for two different mutations in the X-linked gene WAS

In genetic disorders caused by point mutations or small frameshift mutations, affected members of the same family are expected to have the same mutation in the causative gene. We have recently evaluated a family in which this was not the case. Maternal cousins with Wiskott-Aldrich syndrome (WAS; MIM 301000) had two different but contiguous single base pair deletions in WAS. The proband had an A deletion in codon 242 in exon 7 of WAS; his two cousins had a C deletion in codon 241. The mother of the proband was heterozygous for the A deletion allele, but her three sisters, including the mother of the affected cousins, were heterozygous for the C deletion. Both deletions occurred on the haplotype from the unaffected maternal great-grandfather. The maternal grandmother, who was a carrier of WAS, based on a non-random pattern of X chromosome inactivation in T cells, was mosaic for both deletions. These findings are most consistent with the mutations originating in a male gamete with different mutations on the two strands of DNA, a bichromatid mutation.

The emerging importance of genetics in epidemiologic research II. Issues in study design and gene mapping

PURPOSE

To provide a synthesis of current approaches to the discovery of genes associated with complex human diseases by examining the joint potential of traditional epidemiologic methods and current molecular techniques for gene discovery.

METHODS

A discussion of optimal approaches for defining complex disease phenotypes in genetic epidemiology, ascertainment strategies for estimating genetic influences on disease risk, genomic approaches for localizing complex-disease-susceptibility genes, and the potential synergistic effects of integrating genetic and traditional epidemiologic expertise is provided in the second part of a three-part series on the importance of genetics in epidemiologic research.

RESULTS

The ability to quantify genetic influences on disease risk appears highly dependent on the measurement of specific risk factor traits, ascertainment strategies for recruiting study subjects, and a variety of genomic approaches that are rapidly facilitating our ability to identify genes influencing inherited human diseases and to quantify genetic influences on disease risk.

CONCLUSIONS

Integrating population-based methods of assessing disease risk with human genetics and genome technology is critical for identifying genetic polymorphisms that influence risk of disease and for defining genetic effects on complex disease etiology.

Paternal inheritance or different mutations in maternally related patients occur in about 3% of Duchenne familial cases

Duchenne dystrophy (DMD) is an X-linked lethal condition which affects 1 in 3,500 boys. The DMD gene is deleted in about 60-65% of patients while in the remaining 35-40% the condition is caused by point mutations, small insertions, or duplications. We have ascertained 967 DMD families (680 isolated and 287 familial cases). Screening for deletions showed a molecular deletion in 383 among 615 (62.3%) analyzed cases. However, 10 families were unusual: In 7 of them, 2 or more DMD patients were related through paternal lines while in 3 others, affected boys related through maternal lines carried different mutations or originated through independent new mutation events. The finding of 10 atypical genealogies, which represent about 1% of the sample (10/967) or about 3% of familial cases (10/287) is higher than we would expect by chance. Even so, it is an underestimate because screening of mutations in all the affected DMD relatives from each genealogy is not done in many of the familial cases. It suggests that other mechanisms (such as transposon-like elements, for example) could be responsible for a higher genomic instability leading to novel mutations as reported previously by us and others in DMD and in other genetic disorders such as hemophilia and inherited peripheral neuropathies. On the other hand, it shows the importance of testing all affected patients within each genealogy to prevent possible mistakes in carrier detection, genetic counseling, and prenatal diagnosis.

Effect of adopting a new histological grading system of acute rejection after heart transplantation

BACKGROUND

Treatment policy of acute rejection after heart transplantation has been changed after adopting the ISHLT endomyocardial biopsy grading system in 1991.

OBJECTIVE

To determine the effect of this policy change on clinical outcome after transplantation.

METHODS

The outcome of 147 patients who had a transplant before (early group, median follow up 96 months) and 114 patients who had a transplant after (late group, median follow up 41 months) the introduction of the ISHLT biopsy grading system was studied retrospectively. Initially "moderate rejection" according to Billingham's conventional criteria was treated. From January 1991 grade 3A and higher was considered to require intensification of immunosuppression.

RESULTS

There were some differences between the two groups: recipients (50 v 44 years) as well as donors (28 v 24 years) were older in the "late group" and more patients of this group received early anti-T cell prophylaxis (92% v 56%). Despite more extensive use of early prophylaxis more rejection episodes were diagnosed (2.4 v 1.4) and considerably more courses of rejection treatment were instituted in the late compared with the early group (3.2 v 1.5). There were no deaths because of rejection in the late group, however, more infections occurred within the first year (mean 1.8 v 1.4) and more non-skin malignancies within the first 41 months were diagnosed (8 of 57 v 6 of 147, 95% CIs of difference includes 0). The incidence of graft vascular disease in the late group has been comparable to the early group until now.

CONCLUSION

The interpretation of the ISHLT grading system resulted in lowering of the threshold for the diagnosis of rejection thereby increasing the number of rejections and subsequently the immunosuppressive load and its complications.

The evolution of an intron: analysis of a long, deletion-prone intron in the human dystrophin gene

The sequence of a 112-kb region of the human dystrophin (DMD/BMD) gene encompassing the deletion prone intron 7 (110 kb) and the much shorter intron 8 (1.1 kb) has been determined. Recognizable insertion sequences account for approximately 40% of intron 7. LINE-1 and THE-1/LTR sequences occur in intron 7 with significantly higher frequency than would be expected statistically while Alu sequences are underrepresented. Intron 7 also contains numerous mammalian-wide interspersed repeats, a diverse range of medium reiteration repeats of unknown origin, and a sequence derived from a mariner transposon. By contrast, the shorter intron 8 contains no detectable insertion sequences. Dating of the LI and Alu sequences suggests that intron 7 has approximately doubled in size within the past 130 million years, and comparison with the corresponding intron from the pufferfish (Fugu rubripes) suggests that the intron has expanded some 44-fold over a period of 400 million years. The possible contribution of the insertion elements to the instability of intron 7 is discussed.

High frequency of new mutations in North Indian Duchenne/Becker muscular dystrophy patients

Accurate carrier determination is an important aspect in providing prenatal diagnosis and genetic counselling to families with Duchenne/Becker muscular dystrophy patients. Using quantitative polymerase chain reaction, we have analyzed the carrier status of 31 mothers (5 familial and 23 sporadic) who have an affected son with known deletion in the dystrophin gene. Only four out of 23 mothers of sporadic cases turned out to be heterozygous for the deleted exons. The lower number of carrier mothers in sporadic cases suggests a higher frequency of new mutations in North Indian DMD@BMD patients.

Occurrence of two different intragenic deletions in two male relatives affected with Duchenne muscular dystrophy

The occurrence of 2 different intragenic deletions (exons 10-44 and exon 45, respectively) is reported in 2 male relatives affected with Duchenne muscular dystrophy, both showing the same haplotype for DNA markers not included in the deleted segment. The 2 different deletions seem to have occurred independently in the same X chromosome. This finding, together with other reports, suggests possibly an increased predisposition to mutations within the DMD locus in some families. Therefore, when dealing with prenatal diagnosis, the investigation on fetal DNA cannot be restricted only to the region in which a mutation was previously identified in the family.

Racial distribution of Duchenne muscular dystrophy in the West Midlands region of Britain

In the West Midlands region of Britain, Duchenne muscular dystrophy (DMD) is twice as common as expected in Indians, and is less common than expected in Pakistanis. Although the numbers are small, they cannot be explained by any bias of ascertainment and are considered to be real. One possible mechanism for the high frequency of DMD in Indians is the presence of repetitive elements in the wild type gene which predispose to mutations.