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Efficient identification and illustration of mechanism of formation of small supernumerary marker chromosome (2) by FISH

We detected an interstitial deletion of chromosome 2 [del(2)(p11.1p12)]in a lymphocyte culture of a patient. Karyotyping of the parents showed that the father had the same deletion. However, apart from this interstitial deletion he also had a small ring shaped Supernumerary Marker Chromosome(SMC), karyotype: 47, XY, del(2)(p11.1p12), + mar. FISH was performed on metaphase chromosomes using satellite DNA probe specific for chromosome 2 and whole chromosome painting specific probe wcp2. Hybridization was performed according to the protocol provided by the manufacturer. Other members of the family had normal karyotypes. FISH analyses revealed that the proximal break was right through the centromere of 2, spliting the chromosome 2 specific alpha satellite centromeric fragments into two smaller units, thus creating the functional centromere of the marker chromosome. The origin of marker choromosome was confirmed by FISH using wcp2: 47, XY, +mar. ish der(2)(wcp2+, D2Z1+). Marker chromosomes are structurally abnormal chromosomes in which no part can be identified. They represent a heterogeneous group of structural anomalies with different phenotypic expression depending on the size and genetic content of the marker and the level of mosaicism. Most of SMC have been studied using centromere specific DNA probes, which allow identification of chromosomal origin, and the chromosome specific libraries, which provide better insight into the structural composition and genetic content of the marker. The present case is a new case of supernumerary ring marker chromosome 2, identified by FISH, consisting of the proximal region of short arm of chromosome 2 and a part of its centromere.

Supernumerary marker chromosome (2); FISH

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