Forty-nine individuals have been identified with deletions or translocations involving the short arm of chromosome 5. While most display the classical phenotype of the cri-du-chat syndrome, several of the patients do not have the syndrome or have only a subset of the clinical features. Somatic cell hybrids containing the deleted chromosome 5 were derived from each patient. Each somatic cell hybrid was analyzed at the DNA level using 136 chromosome 5p-specific DNA fragments. It was possible to unambiguously order most of the chromosomal breakpoints present in the somatic cell hybrids based on the hybridization patterns of Southern blots. Further comparisons between the deletions present in the patients and their clinical features identified several chromosomal regions that were involved in specific clinical features. A critical chromosomal region involved the high-pitched cry mapped to 5p15.3, while the chromosomal region involved in the remaining features of the cri-du-chat syndrome mapped to a small region within 5p15.2. Deletions that did not include these two chromosomal regions presented varying clinical phenotypes from severe mental retardation and microcephaly to a clinically normal phenotype. These results demonstrate the need for careful characterization of a 5p deletion in prenatal cases before clinical predictions are made.

Facioscapulohumeral muscular dystrophy (FSHD) may be a new member of the class of neuromuscular diseases (NMD) due to defects in the nuclear envelope. Unlike other NMDs with primary defects in nuclear envelope proteins, however, FSHD may result from inappropriate chromatin interactions at the envelope. 3D Immuno-FISH and a novel method of 3D by 2D analysis using NucProfile were developed to examine nuclear organization of the FSHD genomic region. In contrast to most other telomeres, the FSHD region at 4q35.2 localizes to the nuclear periphery. This localization is consistent in normal myoblasts, myotubes, fibroblasts and lymphoblasts, does not vary significantly throughout the cell cycle, and is independent of chromosome territory effects. The nuclear lamina protein lamin A/C is required for FSHD region chromatin localization to the nuclear envelope, as the association is lost in lamin A/C null fibroblasts. As both normal and affected alleles (deleted for the subtelomeric repeat D4Z4) localize to the nuclear periphery, FSHD likely arises instead from improper interactions with transcription factors or chromatin modifiers at the nuclear envelope. Interestingly, it is not D4Z4 itself that mediates interaction with the envelope, as sequences proximal to D4Z4 (i.e. D4S139) localize closer to the nuclear periphery, perhaps accounting for the chromosome 4 specificity of the disease.

Facioscapulohumeral muscular dystrophy (FSHD) is an autosomal dominant form of muscular dystrophy. The FSHD locus has been linked to the most distal genetic markers on the long arm of chromosome 4. Recently, a probe was identified that detects an EcoRI fragment length polymorphism which segregates with the disease in most FSHD families. Within the EcoRI fragment lies a tandem array of 3.2 kb repeats. In several familial cases and four independent sporadic FSHD mutations, the variation in size of the EcoRI fragment was due to a decrease in copy number of the 3.2 kb repeats. To gain further insight into the relationship between the tandem array and FSHD, a single 3.2 kb repeat unit was characterized. Fluorescence in situ hybridization (FISH) demonstrates that the 3.2 kb repeat cross-hybridizes to several regions of heterochromatin in the human genome. In addition, DNA sequence analysis of the repeat reveals a region which is highly homologous to a previously identified family of heterochromatic repeats, LSau. FISH on interphase chromosomes demonstrates that the tandem array of 3.2 kb repeats lies within 215 kb of the 4q telomere. Together, these results suggest that the tandem array of 3.2 kb repeats, tightly linked to the FSHD locus, is contained in heterochromatin adjacent to the telomere. In addition, they are consistent with the hypothesis that the gene responsible for FSHD may be subjected to position effect variegation because of its proximity to telomeric heterochromatin.

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