Unequal interchromosomal rearrangements may result in elastin gene deletions causing the Williams-Beuren syndrome
Unequal interchromosomal rearrangements may result in elastin gene deletions causing the Williams-Beuren syndromeFabrizio Dutly and Albert Schinzel*
Institut für Medizinische Genetik der Universität Zürich, Raemistrasse 74, CH-8001 Zürich, Switzerland
Received August 2, 1996;Revised and Accepted September 16, 1996
Williams-Beuren syndrome (WBS) is generally the consequence of an interstitial microdeletion at 7q11.23, which includes the elastin gene, thus causing hemizygosity at the elastin gene locus. The origin of the deletion has been reported by many authors to be maternal in ~60% and paternal in 40% of cases. Segregation analysis of grandparental markers flanking the microdeletion region in WBS patients and their parents indicated that in the majority of cases a recombination between grandmaternal and grandpaternal chromosomes 7 at the site of the deletion had occurred during meiosis in the parent from whom the deleted chromosome stemmed. Thus, the majority of deletions were considered a consequence of unequal crossing-over between homologous chromosomes 7 (interchromosomal rearrangement) while in the remaining cases an intrachromosomal recombination (between the chromatids of one chromosome 7) may have occurred. These results suggest that the majority of interstitial deletions of the elastin gene region occur during meiosis, due to unbalanced recombination while a minority could occur before or during meiosis probably due to intrachromosomal rearrangements. The recurrence risk of the interchromosomal rearrangements for sibs of a proband with non-affected parents must be negligible, which fits well with the observation of sporadic occurrence of almost all cases of WBS.
The Williams-Beuren syndrome (WBS) is characterised by growth and mental retardation with a friendly, outgoing personality, dysmorphic facial features, hypercalcaemia in early infancy, and congenital cardiovascular malformations, in particular supravalvular aortic stenosis (1 -3 ). This, almost always sporadically occurring, disorder (incidence about 1/20 000 newborns) has been shown to result from a contiguous gene deletion syndrome comprising the elastin gene (4 ) and the recently located gene LIM-kinase 1 (5 ). Individuals with classic WBS have a chromosomal deletion >500 kb at 7q11.23 (5 ). Hemizygosity of the elastin gene is probably responsible for some of the characteristic features of the WBS, including supravalvular aortic stenosis, hoarse voice, and some of the WBS facial features. However, mental retardation and hypercalcaemia in WBS may indicate that hemizygosity of other genes adjacent to the elastin locus may also contribute to the phenotype.
The human elastin gene has been mapped to 7q11.23 (6 ). Indik et al. (7 -8 ) found a remarkable abundance of Alu repetitive sequences in the 3' region of the gene and several alternatively spliced isoforms.
Repetitive DNA sequences and Alu sequences flanking deletion breakpoints have been noted in a considerable number of human gene mutations which cause genetic disorders, including defects in the genes for [alpha]-globin (9 ), [beta]-globin (10 ), growth hormone (11 ) and many others. Such repeated and highly homologous sequences may facilitate a misalignment of chromosomes followed by an unequal crossing-over event which increases the homologous recombination and greatly enhances the possibility of a deletion. Mispairing between the repeats might result in the formation of a loop between the two repeats. Subsequent excision of the loop will remove the sequences between the two repeats. One might hypothesize that an unequal recombination involving Alu sequences at 7q11 would generate a deletion resulting in WBS. The particular mechanism(s) involved in rearrangements of proximal 7q remains to be elucidated by molecular analysis of breakpoint sites.
Results of molecular markers studies around the elastin locus in 15 WBS families
Family
Origin of
Microsatellites
deletion
ELN MvaI
ELN dinuc.
D7S489
D7S1870
5
mat
b,ab,ab
b,bc,ab
a,bb,aa
b,ac,bb
9
mat
c,bc,ac
a,ab,ab
a,bb,aa
b,aa,ab
35
mat
a,ab,ab
c,ab,cc
c,ab,bc
70
mat
a,bc,ab
b,ab,bb
81
mat
a,bc,ac
b,ab,ab
a,bb,ac
c,ab,cd
82
mat
c,ab,bc
b,ab,bb
a,bd,ac
84
mat
c,ac,bc
a,bb,ab
a,bb,ac
101
mat
a,ac,aa
b,ac,bc
b,aa,-
b,ac,ab
3
pat
a,ab,cc
c,cc,ab
a,ab,bc
34
pat
b,ab,ac
c,bc,ac
c,cc,ab
38
pat
a,ab,ab
a,ab,bb
a,ab,ab
41
pat
b,ab,bb
a,ab,bb
a,ab,ab
52
pat
a,aa,cc
c,ac,bb
b,ab,bb
63
pat
c,ac,ab
a,ac,bc
a,aa,aa
76
pat
a,ab,aa
b,bb,ab
b,bc,ab
b,bc,ad
The alleles are given in the order: patient, mother, father. Markers informative for the parental origin of the deletion are shown in bold. Pat, paternal deletion; mat, maternal deletion. Families 3, 5, 9, 34, 35, 38, 41, 52, 63, 70 and 76 were also reported by Robinson et al. (30).
The 15 WBS patients were genotyped for the elastin gene or for sequences flanking this gene. The absence of maternal or paternal contribution to the patients in at least one of these loci, demonstrated the origin of the deletion (Table 1 ). In eight cases there was no maternal inheritance of these markers, and in the other seven cases, there was lack of paternal alleles.
Fluorescent in situ hybridization (FISH) analysis of the parents, with the probe WSCR (ONCOR), demonstrated that all the deletions had occured de novo.
The 15 families consisting of the patient, the two parents and the available maternal or paternal grandparents were genotyped using microsatellites centromeric and telomeric adjacent to the elastin gene. The markers allowed us to determine which allele was inherited from the parents and from the grandparents. Haplotype analysis of the flanking markers on each side of the deletion showed a recombination event between the centromeric (D7S663, D7S645, D7S672, D7S653) and the telomeric loci (D7S669, D7S440, D7S634, D7S524, D7S630) in 10 out of the 15 analysed families (Fig. 1 A). As shown in Figure 2 A, in Family 5 the WBS patient inherited the allele `c' from the maternal grandfather for locus D7S672. The allele `c' for locus D7S669 was inherited from the maternal grandmother, suggesting a crossing-over between the two reported loci, which caused the deletion at the elastin gene. In contrast, in Family 82 (Fig. 2 B) the alleles `b' (at D7S672) and `d' (at D7S669) were inherited from the maternal grandmother. A recombination event between the grandmaternal and grandpaternal haplotypes in the deleted region is unlikely.
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*To whom correspondence should be addressed
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