Efficient identification of marker chromosomes in 27 patients by stepwise hybridization with alpha-satellite DNA probes

Small supernumerary marker chromosomes derived from human chromosome 11

Introduction: With only 39 reported cases in the literature, carriers of a small supernumerary marker chromosome (sSMC) derived from chromosome 11 represent an extremely rare cytogenomic condition. Methods: Herein, we present a review of reported sSMC(11), add 18 previously unpublished cases, and closely review eight cases classified as 'centromere-near partial trisomy 11' and a further four suited cases from DECIPHER. Results and discussion: Based on these data, we deduced the borders of the pericentric regions associated with clinical symptoms into a range of 2.63 and 0.96 Mb for chromosome 11 short (p) and long (q) arms, respectively. In addition, the minimal pericentric region of chromosome 11 without triplo-sensitive genes was narrowed to positions 47.68 and 60.52 Mb (GRCh37). Furthermore, there are apparent differences in the presentation of signs and symptoms in carriers of larger sSMCs derived from chromosome 11 when the partial trisomy is derived from different chromosome arms. However, the number of informative sSMC(11) cases remains low, with overlapping presentation between p- and q-arm-imbalances. In addition, uniparental disomy (UPD) of 'normal' chromosome 11 needs to be considered in the evaluation of sSMC(11) carriers, as imprinting may be an influencing factor, although no such cases have been reported. Comprehensively, prenatal sSMC(11) cases remain a diagnostic and prognostic challenge.

Mosaic partial pericentromeric trisomy 8 and maternal uniparental disomy in a male patient with autism spectrum disorder

Various chromosomal anomalies including small supernumerary marker chromosome (sSMC) and Uniparental disomy (UPD) have been described in association with intellectual disability and autism spectrum disorder. Based on our reported findings, we recommend that patients with sSMC(8) be evaluated for autism spectrum disorder (ASD) for early institution of therapy. In the presence of an identifiable sSMC, exploration of UPD is also recommended to further investigate the role of chromosome 8 UPD in ASD.

Identification of a small supernumerary marker chromosome, r(2)(p10q11.2), and the problem of determining prognosis

The identification of small supernumerary marker chromosomes (SMCs) and the elucidation of their clinical significance remain two of the problems in classical human cytogenetics. We observed a small supernumerary ring in amniotic fluid cell cultures and identified its origin as r(2)(p10q11.2) and its extent by means of fluorescent in situ hybridisation (FISH). Uniparental disomy (UPD) was excluded by microsatellite analysis using polymorphic markers localised in the same region. On the basis of normal ultrasonographic checks, the patient decided to continue the pregnancy. A normal female was delivered at term and subsequent neonatal follow-ups confirmed the normal phenotype and development. In the present case, genetic counselling was not helpful because of the absence of reference cases. Detailed characterisation made it possible to correlate the normal baby phenotype with the trisomic 2p10-2q11.2 genomic region. Further molecular cytogenetic investigations of SMCs classified by DNA content and pregnancy outcome data should improve genetic counselling and risk evaluation.

Multiplex-FISH for pre- and postnatal diagnostic applications

For >3 decades, Giemsa banding of metaphase chromosomes has been the standard karyotypic analysis for pre- and postnatal diagnostic applications. However, marker chromosomes or structural abnormalities are often encountered that cannot be deciphered by G-banding alone. Here we describe the use of multiplex-FISH (M-FISH), which allows the visualization of the 22 human autosomes and the 2 sex chromosomes, in 24 different colors. By M-FISH, the euchromatin in marker chromosomes could be readily identified. In cases of structural abnormalities, M-FISH identified translocations and insertions or demonstrated that the rearranged chromosome did not contain DNA material from another chromosome. In these cases, deleted or duplicated regions were discerned either by chromosome-specific multicolor bar codes or by comparative genomic hybridization. In addition, M-FISH was able to identify cryptic abnormalities in patients with a normal G-karyotype. In summary, M-FISH is a reliable tool for diagnostic applications, and results can be obtained in </=24 h. When M-FISH is combined with G-banding analysis, maximum cytogenetic information is provided.

Rapid identification of multiple supernumerary ring chromosomes with a new FISH technique

Multiple supernumerary ring chromosomes are a rare cytogenetic finding which is poorly understood. With the introduction of FISH techniques, their chromosomal origin can now be defined clearly. The techniques described previously are complicated and time consuming. We report a new rapid technique which has been used to investigate two new cases. Multiple probes were hybridised to a single slide by means of marking the underside with a diamond pen to form a grid of squares, pipetting fixed cell suspension into the centre of each square, forming a rubber solution grid on the denatured, dehydrated slide following the lines on the underside, adding a mixture of probes into each square, and sealing the slide with a silicone rubber rim and a covering slide. The type of probe and the size, dimensions, and number of squares in the grid can be tailored to individual cases. The two new cases examined here are mosaic for three (case 1) and four (case 2) supernumerary ring chromosomes derived from different chromosomes. Normal cell lines were also present. The karyotypes were established as 47,XY,+r(4)/47,XY,+r(17)/.../48,XY,+r(17),+r(20)/ 49,XY,+r(4),+r(17),+r(20)/46,XY for case 1 and 47,XX,+r(4)/47,XX,+r(8)/47,XX,+r (10)/48,XX,+r(X),+r(4)/... /49,XX,+r(X),+r (8),+r(10)/46,XX for case 2. Our findings suggest that the ring chromosomes were formed during meiosis, perhaps involving complex rearrangements, resulting in a germ cell containing all markers, with subsequent loss of markers during cell division. Our second case also shows that the outcome is not invariably mental or physical handicap.

FISH and the paediatrician

Cytogenetics with banding techniques has, since the 1970s, identified patients with chromosome abnormalities and has contributed enormously to the understanding of phenotype-karyotype correlations. However, one chromosome band could contain 20-50 genes. Fluorescence in situ hybridization (FISH) bridges the gap in the area between the resolution obtained by conventional chromosome studies and purely DNA studies. Fluorescence in situ hybridization provides paediatricians with the ability to delve more deeply into the aetiology of congenital abnormalities in children. This annotation aims to clarify the current applications of FISH in paediatric practice.

Molecular cytogenetic characterisation of a small ring X chromosome in a Turner patient and in a male patient with congenital abnormalities: role of X inactivation

The association of small accessory marker chromosomes in man with specific abnormalities has been difficult to define owing to variations in the chromosome origin and the size of the markers. In a patient with typical Turner phenotype and a 45,X/46,X, + mar karyotype the marker was shown to be a small portion of the long arm of the X chromosome which included the centromere and XIST, a candidate gene for the X inactivation centre. Therefore the lack of any additional abnormalities was attributed to inactivation of the portion of the X chromosome in the marker. In a patient with a 47,XY, + mar karyotype the mar was a small ring X chromosome which did not contain the XIST gene. For both markers the short arm breakpoints were localised between UBE1 and DXS423E. The congenital abnormalities of the male patient were attributed to the lack of X inactivation of the small ring and therefore disomic expression of normal genes possessed by the marker.

De novo isochromosome 18p in two patients: cytogenetic diagnosis and confirmation by chromosome painting

This report concerns two patients with clinical features typical for tetrasomy 18p syndrome. Chromosomal analysis revealed a male karyotype in both cases, with an additional small metacentric marker chromosome, putatively an i(18p). Fluorescent in situ hybridization with a chromosome 18-specific paint confirmed that the marker chromosome consisted of chromosome 18 material in both cases.