Genetic mapping of chronic childhood-onset spinal muscular atrophy to chromosome 5q11.2-13.3

Molecular studies of spinal muscular atrophy

Spinal muscular atrophy (SMA) is inherited as an autosomal recessive disorder which presents as a severe, intermediate or mild condition. The disease selectively affects the alpha motor neuron but nothing is as yet known about the underlying biochemical defect. Recent genetic studies have mapped all three types of SMA to the same region of human chromosome 5 (5q11.2-q13.3) raising the possibility that the mutations may be allelic. Polymorphic DNA markers have been characterised which are suitable for prenatal diagnosis. This is the first step in the isolation of the mutant gene (or genes) involved in this disorder.

A new approach to the study of the origin of genetic diseases: retroviral etiology of osteopetrosis

The recent evidence of retroviral activity in a patient suffering from an atypical form of benign osteopetrosis suggests that it may represent the ancestral infectious form of a disease recognized until now only as a genetic disease. Therefore, making c-DNA probes from that virus should evidence retroviral-like sequences integrated in the genome of patients suffering from the inherited form of the disease. One of the goals of these investigations is to use these probes to map human "osteopetrosis" mutation(s) in the genome. A new approach is therefore defined for the study of these diseases, which although similar in their clinical manifestations, appear either as "sporadic" or "familial" cases: a retroviral etiology can account for this discrepancy. Another approach is also suggested for these kinds of disease, consisting of the search for retroviral sequences in a gene coding for an enzyme, when the main disease is associated with an apparently unrelated enzymatic deficiency. The insertion of retroviral sequence in a gene coding for an enzyme may result in both the disease and the inactivation of the gene. By comparing the genomic sequence of the enzyme in normal and pathological cases, the integrated retroviral gene(s) sequence will be available.

The molecular basis of genetic disease

The pace of localization and characterization of genes affected in human genetic disorders is quickening. Many important genes were localized or characterized recently: genes for in cystic fibrosis, NF-2, Marfan's syndrome and xeroderma pigmentosum, to name a few. Also, in the past 15 months, the CFTR gene affected in cystic fibrosis has been isolated, the first disease gene to be isolated without use of previous cytogenetic clues, such as deletions or translocations in sporadic cases. Other examples should follow, although we have been disappointed to date by the difficulties encountered in the isolation of Huntington's disease gene which was localized a number of years ago to distal chromosome 4p. It is still very difficult to isolate a disease gene without critical cytogenetic information. New improved techniques for finding the desired expressed sequences in a large cloned segment of human DNA are needed. Our ability to find mutant alleles of a given sequence has expanded greatly with the recent technical advances in denaturing gradient gel electrophoresis, chemical cleavage, and single-stranded conformational electrophoresis. One would predict that information derived from the human genome project will have a major impact upon the isolation of further disease genes. As whole regions of human chromosomes or indeed entire chromosomes are physically mapped and cloned as continuous, overlapping YACs (yeast artificial chromosomes), isolation of disease genes will become easier and easier.(ABSTRACT TRUNCATED AT 250 WORDS)

Location of facioscapulohumeral muscular dystrophy gene on chromosome 4

The autosomal dominant disorder facioscapulohumeral muscular dystrophy (FSHD) is the last of the major progressive muscular dystrophies in which the gene had not been located. In linkage analysis on ten Dutch families with this disorder a lod score of 6.34 at a recombination fraction of 0.13 was obtained with the microsatellite marker Mfd 22 (D4S171). This maps the FSHD gene to chromosome 4. Only one family was uninformative for this marker. We found no evidence of genetic heterogeneity.

Mapping of acute (type I) spinal muscular atrophy to chromosome 5q12-q14. The French Spinal Muscular Atrophy Investigators

Linkage analysis in twenty-five families with acute (type I) spinal muscular atrophy (SMA) showed that the mutant gene responsible for the disorder is tightly linked to the D5S39 locus. The mutation(s) causing the intermediate (type II) and juvenile chronic (type III) forms of SMA were also mapped to DNA marker D5S39 on chromosome 5 (5q12-q14). Thus, the three forms, which have been differentiated clinically on the basis of age of onset and clinical course, are most probably due to different mutations at a single locus on chromosome 5. Prenatal diagnosis of SMA type I will now be possible.

Genetic homogeneity between acute and chronic forms of spinal muscular atrophy

The childhood-onset spinal muscular atrophies (SMAs) describe a heterogeneous group of disorders that selectively affect the alpha motoneuron. We have shown that chronic childhood-onset SMA (SMA II and III) maps to a single locus on chromosome 5q. Acute SMA (SMA Type I/Werdnig-Hoffmann/severe/infantile) is the main cause of heritable infant mortality. Mapping the acute SMA locus by conventional methods is complicated by the rapidly fatal course of the disease and its recessive mode of inheritance. We present here the typing of four inbred acute-SMA families with DNA markers on chromosome 5q and analysis of these together with acute families from our previous study to demonstrate genetic homogeneity between the acute and chronic forms of SMA. The data indicate that the acute SMA locus maps to chromosome 5q11.2-13.3. Two families seem unlinked to 5q markers, raising the possibility of genetic heterogeneity or disease misclassification within the acute and chronic family sets.