Relationship between selected molecular, cytogenetic and clinical parameters was analysed in a group of 63 individuals (45 males and 18 females) with full fragile X mutation. Significant correlation between the size and somatic instability of fully mutated alleles in both males and females was found. Possible explanations of this result are discussed. With respect to the mutation size, an apparent difference was observed between males with different degree of mental retardation. No such difference appeared when affected and normal females with full mutation were compared. The proportion of mutated active X chromosome was significantly higher in mentally retarded females than in those without any mental impairment.
We present a family with three cases of recombination aneusomy rec(5)dup(5q) originating from a large parental pericentric inversion of chromosome 5. The proband - a 6-year-old girl with mental retardation, speech delay, microcephaly, and slight facial dysmorphism - was referred for subtelomere testing. FISH with a Multiprobe Chromoprobe T System (CytoCell) and with several BAC clones mapping to both subtelomere regions of chromosome 5, revealed a recombinant chromosome rec(5)dup(5q) originating from a paternal pericentric inversion inv(5)(p15.33q35.3). The same inversion was present in the proband’s father’s twin-brother and rec(5)dup(5q) was also identified in his two mentally retarded daughters. The distance of breakpoints from the telomere was: 0.234-1.4 Mb for 5p and 4.1 —4.8 Mb for 5q. HR-CGH analysis confirmed the duplication of the 5q subtelomeric region but did not identify any concomitant deletion in the 5p subtelomere. Precise mapping of the aneusomic regions in the proband enabled mapping the cat cry and speech delay to 5p15.33, making the earlier localizations of these features more precise. Our family shows that the large pericentric inversion with both breakpoints at subtelomeric regions of chromosome 5 is associated with a high risk of rec(5)dup(5q) in the progeny.
Fluorescence in situ hybridization (FISH) is a very useful method for assessing chromosome rearrangements. When neither banding pattern nor clinical symptoms are sufficient to determine the origin of additional chromosomal fragment, FISH with multiple chromosome-specific libraries (chromosome painting), allows to solve this diagnostic problem rapidly. Three chromosomal additions, 7q+, 13p+ and 22q+, found in routine cytogenetic studies performed in children with phenotypic abnormalities were analysed using FISH. This technique documented the origin of the extra material to be derived from chromosome 16[der(7)t(7; 16)(q36.3;p 13.11)], 18[der(13)t(13; 18)(p12;q 12.2)] and 22[dup(22)(q11.2q13.1)], respectively. In two cases the abnormality arose de novo, while in the third case the product of translocation t(13;18) was maternal by origin. It was present in 30% of mother's lymphocytes, and in 70% of them a balanced Robertsonian translocation t(13q;15q) was found. In the presented cases the chromosome analysis with both traditional banding and chromosome painting techniques, allowed to establish final clinical diagnosis.
Prader-Willi syndrome (PWS) is a developmental disorder caused by a deficiency of paternal contribution of the chromosome region 15q11.2-q13 arising from differently sized deletions, maternal disomy, or rarely imprinting mutations. We have analyzed 20 PWS patients using combined cytogenetic high resolution technique (HRT), fluorescence in situ hybridization (FISH) and molecular studies to identify parental origin (uniparental disomy) or molecular defect (deletion) of the Prader-Willi region. Lack of a paternal copy of 15q11.2-q13 resulting from its deletion was found in 16 patients. Using high resolution GTG banding on prometaphase chromosomes, deletion in the 15q11.2-q13 region was detected in only 8 patients. Application of FISH with different sets of PWS specific unique sequence probes (D15S11, SNRPN, D15S10, GABRß3) revealed microdeletions in 12 patients. In 12 out of 20 cases FISH confirmed HRT studies, while in 8 cases inconsistent results were obtained. No discrepancies between results of FISH and molecular studies were found, although the latter had a higher sensitivity. We conclude that FISH appears to be a rapid and reliable method of microdeletion identification and should be performed as a method of choice in cytogenetic diagnosis of Prader-Willi syndrome.
Until recently marker chromosomes have presented a difficult diagnostic problem for cytogeneticists as well as for clinicians. Introduction of FISH to cytogenetic analysis has enabled identification of their origin giving possibility to outline specific phenotypic effects of defined marker chromosomes. Nine marker chromosomes were analysed with FISH using centromeric probes, chromosome- specific libraries and unique DNA sequences probes for PWS/AS critical region. The origin from acrocentric chromosomes was established in 6 cases. One marker was a product of maternal 11;22 translocation and two others were pericentromeric regions of chromosome 2 and 4. Among 6 markers, derived from acrocentric chromosomes, 2 consisted of pericentromeric part of chromosome 15, one was identified as mar (21) and in 3 other cases the origin could not be differentiated between chromosomes 13 and 21 or 14 and 22. Clinical consequences of marker chromosomes including the risk for chromosomal nondisjunction and trisomy 21 as well as the risk for uniparental disomy (UPD) are discussed.
The unstable DNA sequence in the FMR1 gene was analyzed in 85 individuals from Polish families with fragile X syndrome in order to characterize mutations responsible for the disease in Poland. In all affected individuals classified on the basis of clinical features and expression of the fragile site at X(q27.3) a large expansion of the unstable sequence (full mutation) was detected. About 5% (2 of 43) of individuals with full mutation did not express the fragile site. Among normal alleles, ranging in size from 20 to 41 CGG repeats, allele with 29 repeats was the most frequent (37%). Transmission of premutated and fully mutated alleles to the offspring was always associated with size increase. No change in repeat number was found when normal alleles were transmitted.