Human ribosomal RNA genes: orientation of the tandem array and conservation of the 5' end.

The multiple copies of the human ribosomal RNA genes (rDNA) are arranged as tandem repeat clusters that map to the middle of the short arms of chromosomes 13, 14, 15, 21, and 22. Concerted evolution of the gene family is thought to be mediated by interchromosomal recombination between rDNA repeat units, but such events would also result in conservation of the sequences distal to the rDNA on these five pairs of chromosomes. To test this possibility, a DNA fragment spanning the junction between rDNA and distal flanking sequence has been cloned and characterized. Restriction maps, sequence data, and gene mapping studies demonstrate that (i) the rRNA genes are transcribed in a telomere-to-centromere direction, (ii) the 5' end of the cluster and the adjacent non-rDNA sequences are conserved on the five pairs of chromosomes, and (iii) the 5' end of the cluster is positioned about 3.7 kb upstream from the transcription initiation site of the first repeat unit. The data support a model of concerted evolution by interchromosomal recombination.

Paternal inheritance of translocation chromosomes in a t(X;21) patient with X linked muscular dystrophy.

A number of DNA probes from the short arm of the X chromosome have been used to study the inheritance of the translocation chromosomes in a girl with an X; autosome translocation and muscular dystrophy. The two translocation chromosomes were found to be derived from the father's single normal X chromosome, ruling out maternal inheritance of a pre-existent mutation and enhancing the concept that the de novo translocation is responsible for the dystrophic phenotype.

Cloning of the breakpoint of an X;21 translocation associated with Duchenne muscular dystrophy.

Duchenne muscular dystrophy (DMD) is an X-linked recessive disorder which affects approximately 1 in 3,300 males, making it the most common of the neuromuscular dystrophies. The biochemical basis of the disease is unknown and as yet no effective treatment is available. A small number of females are also affected with the disease, and these have been found to carry X; autosome translocations involving variable autosomal sites but always with a breakpoint within band Xp21 of the X chromosome (implicated by other kinds of genetic evidence as the site of the DMD lesion). In these female patients the normal X chromosome is preferentially inactivated, which it is assumed silences their one normal DMD gene, leading to expression of the disease. In one such affected female the autosomal breakpoint lies in the middle of the short arm of chromosome 21, within a cluster of ribosomal RNA genes. Here we have used rRNA sequences as probes to clone the region spanning the translocation breakpoint. A sequence derived from the X-chromosomal portion of the clone detects a restriction fragment length polymorphism (RFLP) which is closely linked to the DMD gene and uncovers chromosomal deletions in some male DMD patients.