Molecular analysis of three patients with interstitial deletions of chromosome band 14q31.

Two patients and one three generation family with interstitial deletions of distal chromosome band 14q31 are described. The deletions were initially identified by chromosome analysis; we have used highly informative simple sequence repeat polymorphisms to define the deletions at the molecular level. This analysis also establishes the parental origin of the deleted chromosome. One of the patients was initially described as having a terminal deletion of chromosome 14 from 14q31 to 14qter; we show here that this child has instead an interstitial deletion of band 14q31. The smallest deletion involves a single anonymous DNA marker and is associated with an almost normal phenotype. The two patients with larger deletions have phenotypes similar to those seen in previously described cases of interstitial deletions of chromosome 14, including minor dysmorphic features and developmental delay. Delineation of these deletions allows the ordering of markers within the 14q31 region, in which the gene for the degenerative neurological disorder Machado-Joseph disease is localised.

Physical and genetic mapping of the serpin gene cluster at 14q32.1: allelic association and a unique haplotype associated with alpha 1-antitrypsin deficiency.

The alpha 1-antitrypsin (PI) gene is part of a cluster of structurally related serine protease inhibitor genes localized at chromosome 14q32.1, a cluster that includes the alpha 1-antichymotrypsin (AACT), protein C inhibitor (PCI), and corticosteroid-binding globulin (CBG) genes and the alpha 1-antitrypsin-like pseudogene (PIL). The order of the genes is refined here by genetic mapping using simple tandem repeat polymorphisms (STRPs) and by physical mapping in YACs. The order of the genes is (centromere)-CBG-PIL-PI-PCI-AACT-(telomere). Analysis of DNA haplotypes comprising STRP and RFLP markers in the serpin genes reveals considerable allelic association throughout the cluster. Furthermore, the common alpha 1-antitrypsin deficiency allele, PI*Z, has a unique DNA haplotype at the CBG, PIL, and PI loci, which extends over 60 kb in 97% of cases and in 44% of cases includes the PCI and AACT loci. This unique haplotype will be of use in examining a number of other diseases, particularly those with an inflammatory component, thought to be associated with alpha 1-antitrypsin deficiency or partial deficiency.

The utrophin and dystrophin genes share similarities in genomic structure.

Utrophin and dystrophin are highly homologous proteins which are reciprocally expressed in DMD (Duchenne muscular dystrophy) muscle. The remarkable similarity of these proteins suggests that they may play a similar cellular role in some circumstances; if this were the case then utrophin may be capable of replacing dystrophin in DMD patients. In this paper we show that the genomic structure of the utrophin gene is similar to the dystrophin gene, further exemplifying the relatedness of the two genes and their gene products. We have constructed a 1.25 Mb contig of eight yeast artificial chromosome (YAC) clones covering the utrophin gene located on chromosome 6q24. Utrophin is encoded by multiple small exons spanning approximately 900 kb. The distribution of exons within the genomic DNA has similarities to that of the dystrophin gene. In contrast to dystrophin, the utrophin gene has a long 5' untranslated region composed of two exons and a cluster of unmethylated, rare-cutting restriction enzyme sites at the 5' end of the gene. Similarities between the genomic structure suggest that utrophin and dystrophin arose through an ancient duplication event involving a large region of genomic DNA.

Severe nonspecific X-linked mental retardation caused by a proximally Xp located gene: intragenic heterogeneity or a new form of X-linked mental retardation?

X-linked mental retardation (XLMR) can be subdivided into syndromic and nonsyndromic or nonspecific. Patients with non-syndromal XLMR show no characteristic manifestations, biochemical defects, or distinct fragile sites. Nevertheless, nonspecific XLMR seems to be heterogeneous. To determine the number and location of the genes responsible for XLMR, linkage studies in large pedigrees have to be performed. Here we report the data of linkage analysis in a large Brazilian family with 7 patients affected by a severe form of XLMR, with no other associated malformations. All the obligate carriers are normal. A close linkage without recombination (lod scores 1.95 and 3.25) was found between the disease locus and polymorphic DNA loci DXS255 (Xp11.22), DXS14 (Xp11.21). These results suggest that the gene responsible for the disease in this family maps in the Xp11-cent of the X chromosome. Positive lod scores in this region have also been reported for other XLMR genealogies, but with a much milder phenotype. The possibility of intragenic or locus heterogeneity is discussed.

Dystrophin and dystrophin-related proteins: a review of protein and RNA studies.

The analysis of dystrophin gene expression has led to the identification of multiple transcripts and varying isoforms. The data indicate that transcription of the dystrophin gene occurs from several promoters, which involves developmental and tissue-dependent regulation. These discoveries have complicated the interpretation of immunolocalization studies, although there is a strong correlation between the amount and size of dystrophin and the severity of the clinical phenotype. The importance of using protein-specific antibodies for dystrophin analysis has been underscored by the identification of a protein, designated utrophin, which exhibits significant sequence homology with dystrophin. This review addresses the recent studies of dystrophin and utrophin expression in an attempt to illustrate the transcriptional diversity of these large genes and the localization of their protein products within various tissues.

Primary structure of dystrophin-related protein.

Dystrophin-related protein (DRP or 'utrophin') is localized in normal adult muscle primarily at the neuromuscular junction. In the absence of dystrophin in Duchenne muscular dystrophy (DMD) patients, DRP is also present in the sarcolemma. DRP is expressed in fetal and regenerating muscle and may play a similar role to dystrophin in early development, although it remains to be determined whether DRP can functionally replace dystrophin in adult tissue. Previously we described a 3.5-kilobase complementary DNA clone that exhibits 80 per cent homology to the C-terminal domain of dystrophin. This sequence identifies a 13-kilobase transcript that maps to human chromosome 6 (refs 2, 11). Antibodies raised against the gene product identify a polypeptide with a relative molecular mass of about 400K in all tissues examined. To investigate the relationship between DRP and dystrophin in more detail, we have cloned and sequenced the whole DRP cDNA. Homology between DRP and dystrophin extends over their entire length, suggesting that they derive from a common ancestral gene. Comparative analysis of primary sequences highlights regions of functional importance, including those that may mediate the localization of DRP and dystrophin in the muscle cell.

Localization of two new DNA markers on the linkage map of human chromosome 6q.

Recently, an autosomal homolog of the dystrophin gene (DMDL) was identified on chromosome 6q24. As part of our analysis of the DMDL locus, we endeavoured to isolate DNA markers to further define the genetic map of this region. We have isolated and characterized two new genetic markers in the region of the DMDL locus, the RFLP D6S129 and a (CA)n dinucleotide repeat polymorphism within the DMDL gene itself and have positioned them on the existing genetic map of chromosome 6q. These markers will be important in testing the hypothesis that the DMDL gene is the locus responsible for autosomal forms of neuromuscular disease.

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.

An autosomal transcript in skeletal muscle with homology to dystrophin.

The Duchenne muscular dystrophy (DMD) gene has been localized to chromosome Xp21 and codes for a 14-kilobase (kb) transcript and a protein called dystrophin, of relative molecular mass 427,000. Dystrophin is associated with the cytoplasmic face of muscle fibre membranes and its C-terminal domain is thought to mediate membrane attachment. Although N-terminal and central domain structures share common features with other cytoskeletal components, no significant sequence similarity between the C-terminal region of dystrophin and other previously characterized proteins has been described. Here we report that fragments from the C-terminal domain of the DMD complementary DNA detect a closely related sequence which exhibits nucleic-acid and predicted amino-acid identities with dystrophin of approximately 65 and 80%, respectively. The dystrophin-related sequence identifies a 13-kb transcript in human fetal muscle and maps to chromosome 6. Thus, dystrophin may be a member of a family of functionally related large structural proteins in muscle.