Biomedical subjects
Publications and source records attributed to W Reardon.
A boy is presented with frontofacionasal dysplasia. The severity of his malformations exceeds that encountered in the cases described previously. The range of clinical abnormality seen in this disorder is reviewed and the delineation of the condition as a separate entity is explored.
Evidence for the location of the Saethre-Chotzen acrocephalosyndactyly mutation on 7p21-22 is based on genetic linkage studies in families segregating for this autosomal dominant disorder. Linkage studies were guided by several reports of chromosome deletions in this region giving rise to craniosynostosis and some other manifestations of Saethre-Chotzen syndrome. We report on a family where a father and daughter carry an apparently balanced t(7;10)(p21.2;q21.2) translocation (de novo in the father) and have the Saethre-Chotzen syndrome. These observations support the localization of the Saethre-Chotzen gene to 7p21.2.
We report on sibs, the offspring of a consanguineous mating, whose mesomelic shortness and bowing of limbs with associated skin dimpling, retrognathia, mandibular hypoplasia, cleft palate, and camptodactyly represents a previously apparently unreported syndrome. The radiological findings are discussed, particularly with regard to the main known diagnostic possibilities.
Although the genetic basis of the congenital form of myotonic dystrophy has recently been clarified, data as to outcome in terms of life expectancy and morbidity are scanty. Life table data based on a cohort of 115 patients with a confirmed diagnosis of congenital myotonic dystrophy are presented. The data suggest a 25% chance of death before 18 months of age and a 50% chance of survival into the mid-30s. The profile of disease and complications among survivors is also charted.
Myotonic dystrophy (dystrophia myotonica), the commonest and most variable of the muscular dystrophies of adult life, has long been known to be associated with cataract, while slit-lamp examination for specific lens opacities has been one of the principal methods of presymptomatic detection of gene carriers. The recent discovery that the myotonic dystrophy mutation is an unstable DNA sequence, composed of varying numbers of CTG triplet repeats, now allows a specific molecular test for this disorder, as well as explaining the phenomenon of anticipation. A series of case reports is presented to illustrate the important practical applications of this development in relation to ophthalmic aspects of the disorder. Reassessment of the specificity of the ophthalmic changes may be required and it will be important for molecular analysis to be used alongside ophthalmic studies, when determining whether family members carry the mutation for myotonic dystrophy.
Genetic counselling in the autosomal dominant condition of the Romano-Ward syndrome might be assumed to be relatively straightforward. The problems posed by consanguinity, deafness, and subclinical gene carriers in a pedigree with this condition have caused us to reevaluate this view. The diagnostic and management difficulties which may attend this potentially fatal condition are highlighted by our experience with this family.
Two brothers are documented with an ectodermal dysplasia primarily involving the teeth and hair. Both have developed cerebellar ataxia in the early teens.
Conditions causing familial ataxia, deafness, and developmental delay are considered in the context of describing brothers with a new disorder characterised by these clinical features.
Auditory and vestibular investigations were carried out in 19 affected men and 13 obligate female carriers of 7 pedigrees with nonsyndromic hearing loss segregating as an X-linked trait. In addition, high resolution computerised tomographic scanning was carried out in 24 affected males and 12 obligate female carriers. The neuro-otological results confirm that non syndromic X-linked hearing loss is a clinically heterogeneous condition, but radiological assessment of the cochlea revealed two distinct groups: a normal group, and an abnormal group characterised by a bulbous internal auditory meatus, a dilated facial nerve canal and incomplete separation of the basal coil of the cochlea from the internal auditory meatus. Within a given pedigree there was marked consistency of the presence or absence of the CT scan abnormality in the affected males. One third of the obligate female carriers of the radiologically abnormal pedigrees were shown to have a similar abnormal finding, but as two thirds were normal, radiological examination did not predict carrier status. In the affected men, pure tone audiometric data did not correlate with the radiological abnormality, whereas vestibular function was strikingly correlated, being normal in all but one case in pedigrees with normal radiology and absent, or grossly impaired, in the pedigrees with abnormal radiology. Neuro-otological abnormalities were documented in approximately two thirds of the obligate female carriers, but were insufficiently frequent in occurrence or specific in type to be of predictive value.
A clinical and molecular analysis of 439 individuals affected with myotonic dystrophy, from 101 kindreds, has shown that the size of the unstable CTG repeat detected in nearly all cases of myotonic dystrophy is related both to age at onset of the disorder and to the severity of the phenotype. The largest repeat sizes (1.5-6.0 kb) are seen in patients with congenital myotonic dystrophy, while the minimally affected patients have repeat sizes of < 0.5 kb. Comparison of parent-child pairs has shown that most offspring have an earlier age at onset and a larger repeat size than their parents, with only 4 of 182 showing a definite decrease in repeat size, accompanied by a later age at onset or less severe phenotype. Increase in repeat size from parent to child is similar for both paternal and maternal transmissions when the increase is expressed as a proportion of the parental repeat size. Analysis of congenitally affected cases shows not only that they have, on average, the largest repeat sizes but also that their mothers have larger mean repeat sizes, supporting previous suggestions that a maternal effect is involved in the pathogenesis of this form of the disorder.
Family studies of diabetes mellitus (DM) show that patients are more likely to have affected mothers than affected fathers. Since the inheritance of mitochondrial (mtDNA), unlike nuclear DNA, is exclusively maternal, could it be that defect(s) in mtDNA account for some cases of DM? Such defects have been associated with rare neurological syndromes, in some of which DM has been an accompanying feature. We have looked for glucose intolerance and for a previously known point mutation of mtDNA in a family, some of whose members have a multisystem disorder with DM but not neurological involvement. DNA samples were obtained from fourteen family members. The point mutation (affecting position 3243 in the tRNA leucine mitochondrial gene) was found in all three diabetic patients and post mortem tissues in the proband; it was also found in seven offspring of female patients. It was not found in the two children of the male proband. The contribution of this mutation to DM in general is not known but clinicians ought to be aware of the possibility, especially in families with multisystem disease and maternal transmission.
A structural cochlear abnormality has been observed by high resolution CT scanning in some families where X-linked deafness is segregating. We now present evidence that the same abnormality is present in a deaf patient who has a deletion within Xq21. This observation provides phenotypic evidence that the genotypic basis of deafness is the same in both patient groups. It is also likely that the perilymphatic fluid "gusher" abnormality may be common to both.
We report on a 5 year experience in providing presymptomatic and prenatal molecular diagnostic services for myotonic dystrophy, using closely linked markers, representing 235 completed results in 161 families. Only 10 analyses (4.3%) proved uninformative, but a further 5 requests (1.9%) could not be reported because of uncertainty in clinical status. Seven of 81 (8.6%) patients considered to be at low risk on clinical grounds were found to be at high risk of carrying the gene. The importance of interpreting molecular results in conjunction with clinical findings is emphasised by the illustrative examples provided. Careful clinical examination and appropriate investigation remain a cornerstone of diagnosis in myotonic dystrophy and are crucial if errors in assigning genotype status by molecular means are to be minimised.
A variable DNA sequence has been detected in patients with myotonic dystrophy. We set out to determine whether identification of this specific molecular defect would improve clinical management of patients and families with myotonic dystrophy. 127 affected patients who were studied had an expanded DNA fragment not seen in 73 normal controls. The increase in length of the fragment correlated broadly with disease severity, and we noted expansion of the sequence in successive generations of the same family. Progressive expansion of the affected gene provides a molecular explanation for an apparently earlier onset in successive generations (anticipation) in myotonic dystrophy and supports the role of an unstable repeat sequence as the basis of the defect. The specificity of this finding will assist in accurate diagnosis of myotonic dystrophy and genetic counselling of affected families.
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Myotonic dystrophy is the commonest adult form of muscular dystrophy, with an estimated incidence of 1 per 7,500, although this is likely to be an underestimate because of the difficulty of detecting minimally affected individuals. It is a multisystem autosomal dominant disorder of unknown biochemical basis. No case of new mutation has been proven. We have isolated a human genomic clone that detects novel restriction fragments specific to individuals with myotonic dystrophy. A two-allele EcoRI polymorphism is seen in normal individuals, but in most affected individuals one of the normal alleles is replaced by a larger fragment, which varies in length both between unrelated affected individuals and within families. The unstable nature of this region may explain the characteristic variation in severity and age at onset of the disease. A second polymorphism at this locus is in almost complete linkage disequilibrium with myotonic dystrophy, strongly supporting our earlier results which indicated that most cases are descended from one original mutation.
Usher syndrome is the most commonly recognized cause of combined visual and hearing loss in technologically developed countries. There are several different types and all are inherited in an autosomal recessive manner. There may be as many as five different genes responsible for at least two closely related phenotypes. The nature of the gene defects is unknown, and positional cloning strategies are being employed to identify the genes. This is a report of the localization of one gene for Usher syndrome type I to chromosome 11q, probably distal to marker D11S527. Another USH1 gene had been previously localized to chromosome 14q, and this second localization establishes the existence of a new and independent locus for Usher syndrome.
The Usher syndromes (USH) are autosomal recessive diseases characterized by congenital sensorineural hearing loss and progressive pigmentary retinopathy. While relatively rare in the general population, collectively they account for approximately 6% of the congenitally deaf population. Usher syndrome type II (USH2) has been mapped to chromosome 1q (W. J. Kimberling, M. D. Weston, C. Möller, et al., 1990, Genomics 7: 245-249; R. A. Lewis, B. Otterud, D. Stauffer, et al., 1990, Genomics 7: 250-256), and one form of Usher syndrome type I (USH1) has been mapped to chromosome 14q (J. Kaplan, S. Gerber, D. Bonneau, J. Rozet, M. Briord, J. Dufier, A. Munnich, and J. Frezal, 1990. Cytogenet. Cell Genet. 58: 1988). These loci have been excluded as regions of USH genes in our data set, which is composed of 8 French-Acadian USH1 families and 11 British USH1 families. Both of these sets of families show linkage to loci on chromosome 11. Linkage analysis demonstrates locus heterogeneity between these sets of families, with the French-Acadian families showing linkage to D11S419 (Z = 4.20, theta = 0) and the British families showing linkage to D11S527 (Z = 6.03, theta = 0). Genetic heterogeneity of the data set was confirmed using HOMOG and the M test (log likelihood ratio > 10(5)). These results confirm the presence of two distinct USH1 loci on chromosome 11.