Showing papers by "Jean-Louis Mandel published in 1991"

Instability of a 550-base pair DNA segment and abnormal methylation in fragile X syndrome

Abstract: The fragile X syndrome, a common cause of inherited mental retardation, is characterized by an unusual mode of inheritance. Phenotypic expression has been linked to abnormal cytosine methylation of...

Isolation of sequences that span the fragile X and identification of a fragile X-related CpG island

Abstract: Yeast artificial chromosomes (YACs) were obtained from a 550-kilobase region that contains three probes previously mapped as very close to the locus of the fragile X syndrome. These YACs spanned the fragile site in Xq27.3 as shown by fluorescent in situ hybridization. An internal 200-kilobase segment contained four chromosomal breakpoints generated by induction of fragile X expression. A single CpG island was identified in the cloned region between markers DXS463 and DXS465 that appears methylated in mentally retarded fragile X males, but not in nonexpressing male carriers of the mutation nor in normal males. This CpG island may indicate the presence of a gene involved in the clinical phenotype of the syndrome.

Selection in blood cells from female carriers of the fragile X syndrome: inverse correlation between age and proportion of active X chromosomes carrying the full mutation.

Abstract: We have studied the patterns of mutation and X inactivation in female carriers of a fragile X mutation, to try to correlate them with various phenotypic features. We used a simple assay, which shows simultaneously the size of the mutation, its methylation status, and DNA fragments that represent the normal active and inactive X chromosomes. We have observed an age dependent process, whereby the 'full' fragile X mutation is found preferentially on the inactive X in leucocytes in adult females, but not in younger ones. This phenomenon was not observed in female carriers of a 'premutation', who have little phenotypic expression. Preliminary data suggest that young females who show preferential presence of a full mutation on the active X in leucocytes may be at increased risk for mental retardation. We have also obtained preliminary evidence for an age dependent decrease in the somatic heterogeneity of full mutations, possibly owing to selection for smaller mutated fragments. If confirmed, the latter phenomenon might account for the known decrease with age of the expression of the fragile site. Our observations suggest that a gene whose expression is affected by the presence of a full mutation (possibly the FMR-1 gene) has a cell autonomous function in leucocytes, leading to a slowly progressive selection for cells where the mutation is on the inactive X chromosome.

Four chromosomal breakpoints and four new probes mark out a 10-cM region encompassing the fragile-X locus (FRAXA).

Abstract: We report the validation and use of a cell hybrid panel which allowed us a rapid physical localization of new DNA probes in the vicinity of the fragile-X locus (FRAXA). Seven regions are defined by this panel, two of which lie between DXS369 and DXS296, until now the closest genetic markers that flank FRAXA. Of those two interesting regions, one is just distal to DXS369 and defined by probe 2-71 (DXS476), which is not polymorphic. The next one contains probes St677 (DXS463) and 2-34 (DXS477), which are within 130 kb and both detect TaqI RFLPs. The combined informativeness of these two probes is 30%. We cloned from an irradiation-reduced hybrid line another new polymorphic probe, Do33 (DXS465; 42% heterozygosity). This probe maps to the DXS296 region, proximal to a chromosomal breakpoint that corresponds to the Hunter syndrome locus (IDS). The physical order is thus Cen–DXS369–DXS476–(DXS463,DXS477)–(DXS296,DXS465)–IDS–DXS304–tel. We performed a linkage analysis for five of these markers in both the Centre d'Etude du Polymorphisme Humain families and in a large set of fragile-X families. This establishes that DXS296 is distal to FRAXA. The relative position of DXS463 and DXS477 with respect to FRAXA remains uncertain, but our results place them genetically halfway between DXS369 and DXS304. Thus the DXS463–DXS477 cluster defines presently either the closest proximal or the closest distal polymorphic marker with respect to FRAXA. The three new polymorphic probes described here have a combined heterozygosity of 60% and represent a major improvement for genetic analysis of fragile-X families, in particular for diagnostic applications.

Adrenoleukodystrophy: a complex chromosomal rearrangement in the Xq28 red/green-color-pigment gene region indicates two possible gene localizations.

Abstract: We have characterized a complex chromosomal rearrangement in band Xq28, in an adrenoleukodystrophy patient who also has blue-cone monochromacy. A 130-kb region upstream from the color-vision pigment genes was isolated as yeast artificial chromosome or cosmid clones. Another Xq28 sequence, not included in the above region, was obtained by cloning a deletion breakpoint from the patient. Using probes derived from the cloned sequences, we have shown that the rearrangement affects the color-pigment genes and includes two deletions, most likely separated by a large (greater than 110-kb) inversion. One deletion encompasses part of the pigment gene cluster and 33 kb of upstream sequences and accounts for the patient's blue-cone monochromacy. If this rearrangement also caused ALD, the disease gene would be expected to lie within or close to one of the deletions. However, deletions were not detected in a 50-kb region upstream of the red-color-pigment gene in 81 other ALD patients. Two CpG islands were mapped, at 46 and 115 kb upstream from the pigment genes.

New polymorphism and a new chromosome breakpoint establish the physical and genetic mapping of DXS369 in the DXS98-FRAXA interval.

Abstract: Recently some of us cloned a new probe RN1 (DXS369), which appears a close marker for the fragile X locus (FRAXA) [Oostra et al.: Genomics 1990]. We present here new evidence for its physical and genetic mapping in the DXS98--FRAXA interval. We used 2 different somatic cell hybrid lines with breakpoints in the Xq27-q28 region: L10B Rea and PeCHN, and we established the order: (DXS105, DXS98)-L10B Rea-DXS369-PeCHN- (DXS304, DXS52). We detected an additional TaqI RFLP at the DXS369 locus which increases its informativeness up to 57%. Two point linkage analysis in a large set of families gave high lod scores for the FRAXA-DXS369 linkage (z(theta) = 10.1 at theta = 0.044) and for DXS369-DXS304, a marker distal to FRAXA (z = 19.2 at theta = 0.070). By multipoint analyses we established the localization of DXS369 in the DXS98-FRAXA interval. DXS369 is a much closer proximal marker for FRAXA than DXS105 or DXS98 and any new probe mapping between the breakpoints in L10B Rea and PeCHN will be of potential interest as a marker for FRAXA.