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Chromosome abnormalities without phenotypic consequences

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Kowalczyk M., Srebniak M., Tomaszewska A.

Journal of Applied Genetics

2007

vol. 48

issue 2

157-166

Some changes in chromosome morphology, detected during cytogenetic analysis, are not associated with clinical defects. Therefore a proper discrimination of harmless variants from true abnormalities, especially during prenatal diagnosis, is crucial to allow precise counseling. In this review we described chromosome variants and examples of chromosome anomalies that are considered to be unrelated to phenotypic consequences. The correlation between the presence of marker chromosomes and a risk of clinical signs is also discussed. Structural rearrangements of heterochromatic material, satellite polymorphism, or fragile sites, are well-known examples of common chromosome variation. However, the absence of clinical effects has also been reported in some cases of chromosome abnormalities concerning euchromatin. Such euchromatic anomalies were divided into 2 categories: unbalanced chromosome abnormalities (UBCAs), such as deletions or duplications, and euchromatic variants (EVs). Recently so-called molecular karyotyping, especially whole-genome screening by the use of high-resolution array-CGH technique, contributed to revealing a high number of previously unknown small genomic variations, which seem to be asymptomatic, as they are present in phenotypically normal individuals.

Molecular cytogenetic characterization of eight small supernumerary marker chromosomes originating from chromosomes 2, 4, 8, 18, and 21 in three patients

100%

Pietrzak J., Mrasek K., Obersztyn E., Stankiewicz P., Kosyakova N., Weise A., Cheung S. W., Ca W. W., vo_ E. F., Mazurczak T., Bocian E., Liehr T.

Journal of Applied Genetics

2007

vol. 48

issue 2

167-175

Small supernumerary marker chromosomes (sSMCs) are a morphologically heterogeneous group of additional structurally abnormal chromosomes that cannot be identified unambiguously by conventional banding techniques alone. Molecular cytogenetic methods enable detailed characterization of sSMCs; however, in many cases interpretation of their clinical significance is problematic. The aim of our study was to characterize precisely sSMCs identified in three patients with dysmorphic features, psychomotor retardation and multiple congenital anomalies. We also attempted to correlate the patients' genotypes with phenotypes by inclusion of data from the literature. The sSMCs were initially detected by G-banding analysis in peripheral blood lymphocytes in these patients and were subsequently characterized using multicolor fluorescence in situ hybridization (M-FISH), (sub)centromere-specific multicolor FISH (cenM-FISH, subcenM-FISH), and multicolor banding (MCB) techniques. Additionally, the sSMCs in two patients were also studied by hybridization to whole-genome bacterial artificial chromosome (BAC) arrays (array-CGH) to map the breakpoints on a single BAC clone level. In all three patients, the chromosome origin, structure, and euchromatin content of the sSMCs were determined. In patient RS, only a neocentric r(2)(q35q36) was identified. It is a second neocentric sSMC(2) in the literature and the first marker chromosome derived from the terminal part of 2q. In the other two patients, two sSMCs were found, as M-FISH detected additional sSMCs that could not be characterized in G-banding analysis. In patient MK, each of four cell lines contained der(4)(:p11.1->.q12:) accompanied by a sSMC(18): r(18)(:p11.2->q11.1::p11.2->q11.1:), inv dup(18)(:p11.1->q11.1::q11.1->p11.1:), or der(18) (:p11.2->q11.1::q11.1->p11.1:). In patient NP, with clinical features of trisomy 8p, three sSMCs were characterized: r(8)(:p12->q11.1::q11.1->p21:) der(8) (:p11.22->q11.1::q11.1->p21::p21->p11.22:) and der(21)(:p11.1->q21.3:). The BAC array results confirmed the molecular cytogenetic results and refined the breakpoints to the single BAC clone resolution. However, the complex mosaic structure of the marker chromosomes derived from chromosomes 8 and 18 could only be identified by molecular cytogenetic methods. This study confirms the usefulness of multicolor FISH combined with whole-genome arrays for comprehensive analyses of marker chromosomes.

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2 Journal of Applied Genetics

2 2007

1 Bocian E.

1 Ca W. W.

1 Cheung S. W.

1 Kosyakova N.

1 Kowalczyk M.

1 Liehr T.

1 Mazurczak T.

1 Mrasek K.

1 Obersztyn E.

1 Pietrzak J.

1 Srebniak M.

1 Stankiewicz P.

1 Tomaszewska A.

1 Weise A.

1 vo_ E. F.

Search results

Chromosome abnormalities without phenotypic consequences

Molecular cytogenetic characterization of eight small supernumerary marker chromosomes originating from chromosomes 2, 4, 8, 18, and 21 in three patients