Identification of marker chromosomes in thirteen patients using FISH probing

Another Small Supernumerary Marker Chromosome (sSMC) Derived from Chromosome 2: Towards a Genotype/Phenotype Correlation

Here we report a prenatally detected small supernumerary marker chromosome (sSMC) derived from chromosome 2 as demonstrated by cenM-FISH (centromere-specific multicolor fluorescence in situ hybridization). By application of a recently described subcentromere-specific probe set (subcenM-FISH) for chromosome 2, the presence of a small partial trisomy due to a karyotype 47,XX, + r(2)(::p11.1->q11.2::) was demonstrated. Including this case, a total of 11 patients with sSMC(2) are described throughout the literature. Based on that data, a first genotype/phenotype correlation according to the size and structure of the marker is suggested.

Redefining monosomy 5 by molecular cytogenetics in 23 patients with MDS/AML

Deletion of the long arm of chromosome 5 [del(5q)] or loss of a whole chromosome 5 (-5) is a common finding, arising de novo in 10% of patients with myelodysplastic syndromes (MDS) or acute myeloid leukemia (AML) and in 40% of patients with therapy-related MDS or AML. We investigated by molecular cytogenetics 23 MDS/AML patients for whom conventional cytogenetics detected a monosomy 5. Monosomy 5 was redefined as unbalanced or balanced translocation and ring of chromosome 5. Loss of 5q material was identified in all 23 patients, but one. One copy of EGR1(5q31) or CSF1R(5q33-34) genes was lost in 22 of the 23 patients. Chromosome 5p material was a constant chromosomal component of derivative chromosomes or rings in all patients, but one. Sequential fluorescent in situ hybridization studies with whole chromosome paints and region-specific probes, used as a complement to conventional cytogenetic analysis, allow a better interpretation of karyotypes in MDS/AML patients.

Characterisation of supernumerary chromosomal markers: a study of 13 cases

Marker chromosomes are defined as 'structurally abnormal chromosomes in which no part can be identified' (ISCN 1995). Supernumerary marker chromosomes (SMC) are 'additional markers' whose origin and composition cannot be determined by conventional cytogenetics. Molecular cytogenetic methods are necessary to identify these additional chromosomal markers. In one third, the SMCs are clinically well-defined in the literature, the remaining two thirds present a major problem for genetic counselling in prenatal diagnosis. At present, different molecular cytogenetic methods are used to determine the origin of SMCs. In this work, we studied 13 SMCs detected by RHG-banding, completed by C-banding and/or NOR-staining. 24-color FISH was used as the primary technique when the chromosomal origin was unknown. Targeted FISH procedures with specific probes (whole chromosome painting, centromeric probe, locus-specific identifier, BAC, etc.) were then performed to confirm and/or specify the chromosomal material present in the SMC. Seven SMCs were found to be associated with phenotypic abnormalities. Five derived from autosomes and two from gonosomes; these are: der(12)t(4;12), dic(15), i(18p), r(19), der(22)t(11;22), r(X), and der(Y). Two markers, r(8) and idic(15), were identified during investigations of infertile couples. Three cases seemed to be phenotypically normal. Four were discovered prenatally: r(2) and r(19) referred for elevated maternal serum markers, der(13/21) referred for advanced maternal age. The fourth SMC, der(14/22), was found during familial investigation following the identification of the same marker in an infertile son. The precise characterisation of the SMCs is of utmost importance for genetic counselling, especially in prenatal diagnosis.

Molecular cytogenetic and rapid aneuploidy detection methods in prenatal diagnosis

Cytogenetic analysis is an important component of prenatal diagnosis. The ability to rapidly detect aneuploidy and identify small structural abnormalities of fetal chromosomes has been greatly enhanced by the use of molecular cytogenetic technologies. In this review, we will present some of the molecular cytogenetic techniques available to the clinical cytogenetics laboratory. These include fluorescence in situ hybridization (FISH), quantitative fluorescence PCR (QF-PCR), multiplex ligation-dependent probe amplification (MLPA) and microarray-based comparative genomic hybridization (array CGH). The benefits and limitations of each technology will be discussed in the context of prenatal diagnosis.

Characterization of six marker chromosomes by comparative genomic hybridization

We applied comparative genomic hybridization (CGH) in six patients with de novo prenatal or postnatal extra marker chromosomes (MC). In four cases, MCs were mosaic and in one of them, the MC was detected in less than 50% of the cells. In three cases, CGH identified the origin of the extra MCs. In the other three, two prenatal cases and one child with an abnormal phenotype, CGH showed normal profiles. Among these cases, a normal profile and entirely C-band positive was identified suggesting that MC did not contain euchromatin. Genetic imbalances detected by CGH were as follow: a gain of 8p10-p12 in a boy with facial dysmorphism, hyperactivity and speech delay, a gain of 8q10-q12 in a healthy man with a history of spontaneous abortions, and a gain of 15q11-q13 in a girl with speech delay, and motor skill and object manipulation difficulties. Clinical data of these patients were compared with those reported in the literature. We conclude that CGH is a very useful and powerful tool for characterizing prenatal or postnatal MCs, even when the mosaicism is present and the MCs are present in less than 50% of the cells.

Characterization of a small supernumerary marker chromosome as r(8) at prenatal diagnosis by MFISH

OBJECTIVE

To identify the mosaic marker chromosome detected in amniotic fluid cells of a 26-year-old woman, with raised triple test values and an ultrasound scan, which showed a fetus with echogenic bowels.

METHODS

Routine karyotyping with G- and C-banding was carried out for both, amniotic fluid at 18 weeks of gestation as well as fetal blood at 22(+6) weeks. Peripheral blood of both parents was karytoyped. MFISH and the all centromeric human probe were used on fetal lymphocytes to identify the marker chromosome.

RESULTS

Both parents had a normal karyotype. Amniotic fluid culture showed a de novo supernumerary marker chromosome (SMC) in 14 of the 30 colonies from four different cover slip cultures. The marker was confirmed in 50% of the fetal lymphocytes. G- and C-banding provided little information except that the marker had some heterochromatic material. The all centromeric human probe also showed the presence of a centromere along with a rim of euchromatic material. MFISH identified this ring marker to be belonging to chromosome 8.

CONCLUSIONS

SMCs with chromosome 8 have been shown to be variable phenotypes. Presence of only heterochromatic material seems to have no discernable phenotypic effects, but, with the presence of euchromatic material, mental and physical developmental delay has been reported. The parents opted to go ahead with the pregnancy and an apparently normal female baby was born at 40 weeks with no complications.

Molecular cytogenetic characterization of two small chromosome 8 derived supernumerary mosaic markers

Two small supernumerary mosaic marker chromosomes (SMC) were identified by conventional cytogenetics, one prenatally, the other postnatally. Fluorescence in situ hybridization (FISH) techniques, including 24-color FISH, were applied to identify both SMCs and better characterize their constitution. Patient 1: a 29 year-old man, whose wife had a spontaneous abortion, was found to have a small ring of the pericentromeric region of chromosome 8 (47,XY,+r(8)(p11q11)/46,XY). Patient 2: a 37 year-old woman had amniocentesis. The fetus was found to have a SMC; its presence was confirmed postnatally. Several FISH techniques (24-color, whole chromosome paints, centromeres, telomeres, band 8p22) led to the identification of a small analphoid marker. The marker was an inversion-duplication for part of the short arm of chromosome 8 (47,XY,+inv dup (8)(p23pter)/46,XY). The 24-color FISH allowed us to conclude that both markers originated exclusively from chromosome 8. However, the structure and content of the markers were elucidated using other molecular cytogenetic techniques, showing their complementarity.

Clinical findings and cytogenetic analysis of small supernumerary ring chromosomes 7: report of two new cases

Two new patients, mosaic for a small supernumerary ring chromosome 7 are described. There are only seven published reported concerning supernumerary ring chromosome 7 and we reviewed the previously reported cases in an attempt to establish genotype-phenotype correlations, which are particularly important for genetic counselling and clinical genetics. Our first case was a 20 months old girl who was referred for a mild motor developmental delay, an asymmetric facial appearance, a plagiocephaly and a short nose with anteverted nostrils. Our second case was a 9 years old boy who was referred for a IQ at the lower end of the normal range (? 80), obesity, hyperactivity and some dysmorphic features including hypertelorism and down slanting palpebral fissures. In both cases, chromosome analysis after G and R banding and FISH showed a small ring chromosome 7 in respectively 76% and 50% of consecutively scored metaphases. Both ring chromosomes were labelled by FISH using the Williams Syndrome locus probe (Elastin Gene D7S486). Comparison between these two cases and previously published cases allowed to delineate frequent clinical findings. A mild mental retardation was found in the majority of patients. which is an important data for genetic counselling.

Supernumerary ring chromosome 8: Clinical and molecular cytogenetic characterization in a case report

We report on a 3-year-old male with developmental delay, autistic behavior, and minor abnormalities consistent with trisomy 8 syndrome whose cytogenetic analysis revealed mosaicism for a supernumerary ring chromosome (SRC). Fluorescence in situ hybridization (FISH) studies, using centromeric and yeast artificial chromosome (YAC) probes, were performed to characterize further the supernumerary chromosome. The ring origin has been detected from the short arm of chromosome 8, resulting in r(8)(p10p23.1). Moreover, uniparental disomy (UPD) using microsatellite analysis was excluded. To our knowledge a total of 25 cases, confirmed by FISH, have been reported with either supernumerary marker or ring chromosome 8. We present a detailed clinical and molecular cytogenetic characterization of this additional case in order to better define the genotype-phenotype correlation.

SKY assessment of two karyotypes with 0-6 supernumerary marker/ring chromosomes and review of previously reported cases with two or more markers

A 7-month-old boy with developmental delay and congenital abnormalities and a 58-year-old man with mental retardation, impaired speech, and dysmorphic features were referred for cytogenetic studies. The peripheral blood chromosome studies of Patient 1 had a de novo mosaic karyotype with 2-6 supernumerary marker chromosomes. Patient 2 had a mosaic karyotype with 1-5 supernumerary marker chromosomes and normal cells. All markers appeared to have a centromere by C-banding and also by fluorescence in situ hybridization (FISH) using all centromere probe for Patient 1. The majority of the markers appeared like rings. Except for one marker in Patient 1 and 2-3 markers in Patient 2 with discernible >5 Mb euchromatin, the rest of the markers were minute and some appeared to have barely discernible euchromatin in C-banding or FISH. Spectral karyotyping (SKY) was attempted to determine the origin of the marker chromosomes. Because some markers had barely any euchromatin, their classification was not clear cut and they were identified as derived from more than one chromosome. The SKY classification of the markers in Patient 1 was 1, 3, 5, 7, 11, 15, and 22 and in Patient 2 was 1, 5, 6, or 7. Patient 2 was lost to further follow-up studies. To confirm the recurring SKY classifications in Patient 1, centromere probes for chromosomes 1, 3, 5, 7, 11, 15, and 22 were used. The markers were negative for 1, 3, and 11 but positive for 7, 15, and 22 and probably 5. Since 5 centromere probe cross hybridizes with 1 and 19, the weak signal on the marker/s in successive hybridization did not give a definitive answer. Also, the 5 paint probe was not conclusive because of the minute size of the marker. In some metaphases, two markers were derived from 5 or 22. For clinical considerations, the marker derived from 7, although variable in size, appeared to consistently have euchromatin, followed by 15, while 22 and 5 markers were mostly centromeric heterochromatin. The elastin gene probe that maps to 7q11.23, SNRPN gene that maps to 15q11.2, and TUPLE gene that maps to 22q11.2 did not give a signal on the markers. As expected for a majority of ring chromosomes, the pan telomere probe did not hybridize to any of the markers. This highly unusual karyotype was confirmed in the buccal epithelium using a mix of centromere 7 and 15 probes and the combination 14/22 probe. The ratio of additional FISH signals in the buccal mucosal cells was comparable to the ratios observed in the peripheral blood. In this study, we have attempted to consolidate the data on >/=2 marker cases to understand the analysis constraints, the range of clinical abnormalities, and the mechanisms involved. The literature was surveyed for multiple markers cases. A majority of the reported cases had two markers, either derived from the same chromosome or from two different chromosomes or two cell lines with different markers derived from the same chromosome. Cases with three or more markers were rare. The nature and extent of euchromatin content of the multiple markers appears to determine the phenotype. Frequently, multiple marker cases had small to minute markers. The clinical presentation varied from mild to severe. While two bisatellited markers may be associated with infertility, the phenotype in other cases ranged from borderline intelligence and mild dysmorphism to developmental delay, mental retardation, and congenital abnormalities.

A series of supernumerary small ring marker autosomes identified by FISH with chromosome probe arrays and literature review excluding chromosome 15

Seven supernumerary small ring marker autosomes were studied. The pantelomere probe (Oncor) in conjunction with scoring for dicentric rings was used to confirm ring morphology. The small rings were identified mainly by FISH with chromosome probe arrays (Cytocell) containing representations from all 24 chromosomes and the rings were derived from chromosomes 7, 8 (three cases), 11, 12, and 14. The effectiveness of the array methodology in identifying markers was tested. Microsatellite DNA data showed biparental disomy (BPD) was present for the rings from chromosomes 7 and 14 thereby excluding UPD, both were de novo but the ring 14 was of paternal origin. The literature on supernumerary small ring autosomes was reviewed excluding chromosome 15. The grade and distribution of mosaicism was invoked as the major determinant of the differences in phenotype and, in addition, variation was attributed to the possibility of different contributions from each chromosome arm. There are 88 published supernumerary small ring cases in total, with phenotypic data attributable to the respective rings in 77 cases and all chromosomes being represented except chromosome 17. Of the prenatally ascertained cases, where there was adequate phenotypic data, 30% had an abnormal phenotype attributable to the ring, and there were 44% familial cases in this group. Of the postnatally ascertained small rings, 75% had an abnormal phenotype attributable to the ring and there were 13% familial cases. This higher abnormality rate is concordant with the considerable ascertainment bias of this latter group and the prenatal data are recommended for genetic counseling. Although data are small there were some differences between the rings derived from different chromosomes. Chromosomes 3 and 8 demonstrate the extremes. Of the supernumerary small r(8) cases reviewed including the three presently described, 8/11 had an abnormal phenotype attributable to the marker but of the small r(3) cases, only 1/6 had an abnormal phenotype. Two of the present r(8) were studied with the GATA4 probe at 8p23.1. The r(8) in case 2 (patient moderately retarded) was comprised mostly of an intact 8p whereas the larger r(8) in case 3 (normal phenotype) was missing 8p23.1 --> pter and had more of 8q contributing to the ring. In other supernumerary rings postnatally ascertained, there is mostly insufficient data but there is an abnormal phenotype in 8/11 cases with multiple small rings, in 5/6 cases with r(20), and in 5/10 with r(1). A novel origin for supernumerary small rings is proposed: that they may originate from incompletely digested superfluous (haploid) pronuclei. The small rings presumptively so formed may occasionally be transfected into the zygote nucleus. The high proportion ( approximately 12.5%) of cases with multiple supernumerary small rings almost always of different centromeric origin is consistent with this concept.

Chromosomal fragility in patients with triple A syndrome

Triple A syndrome is a rare, autosomal recessive disorder characterized by alacrima, achalasia, and adrenal insufficiency. Previous studies have shown that the triple A gene (AAAS) maps to chromosomal band 12q13. Mutations in the AAAS gene have been identified in triple A syndrome patients; however, the function of this gene is still obscure. We used classical and high-resolution chromosome analyses along with chromosome painting and DNA sequencing to study patients with triple A syndrome. We observed abnormalities in the heterochromatic region of chromosome 9 that included chromatid breaks, chromosome breaks, whole chromosome arm loss, and marker chromosomes, which occurred at unusually high frequencies in affected patients and heterozygotes. Our study raises the possibility of an association between chromosomal fragility in band 9q12 and triple A syndrome. Further investigation is necessary to understand the biologic basis of this finding in the context of triple A syndrome.

Small familial supernumerary ring chromosome 2: FISH characterization and genotype-phenotype correlation

A child and his mother were found to be mosaic for a small supernumerary marker chromosome (SMC) that was identified and characterized by means of fluorescent in situ hybridization. The marker chromosome was derived from the pericentromeric region of chromosome 2; the involvement of proximal 2q was determined by YAC probes. The proband was referred because of psychotic illness and mild mental retardation, whereas his mother presented only minor dysmorphisms. There are only a few published reports concerning SMC(2) or proximal 2q trisomy. We reviewed the previously reported cases in an attempt to establish genotype-phenotype correlations, which are particularly important when SMCs are identified in prenatal diagnosis.

Trisomy 17p10-p12 resulting from a supernumerary marker chromosome derived from chromosome 17: molecular analysis and delineation of the phenotype

We report a 5-year-old boy with a small de novo marker chromosome derived from the proximal short arm of chromosome 17. His clinical features include hypotonia, global developmental delay, oval face with large nose and prominent ears, and ligamentous laxity of the fingers. Magnetic resonance imaging of the brain demonstrated mildly delayed myelination. G-band chromosome analysis revealed mosaicism for a small marker chromosome in 85% of the peripheral blood cells analyzed. Fluorescence in situ hybridization and microsatellite polymorphism studies showed that the der(17) was of maternal origin and included genetic material from the 17p10-p12 region, but did not contain the PMP22 gene. One breakpoint mapped within the centromere and the second breakpoint mapped adjacent to the Charcot-Marie-Tooth disease type 1A proximal low-copy repeat (CMT1A-REP). We compare the clinical characteristics of our patient with those previously reported to have a duplication involving the proximal short arm region of chromosome 17 to further delineate the phenotype of trisomy 17pl0-p12.

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.

Identification of supernumerary marker chromosomes derived from chromosomes 5, 6, 19, and 20 using FISH

A large number of cases with supernumerary marker chromosomes (SMCs) should be compared to achieve a better delineation of karyotype-phenotype correlations. Here we present four phenotypically abnormal patients with autosomal marker chromosomes analysed by fluorescence in situ hybridisation using centromeric, telomeric, and unique sequence probes, as well as forward and reverse painting. We also report the first case, to the best of our knowledge, of an SMC derived from chromosome 5. Furthermore, a marker chromosome 20 in a patient with sex differentiation abnormalities, a double mar(6) in a boy with psychomotor retardation, and the association of r(19) with dup(21q21.2q22.12) are described. Although the mar(6) was very small, the presence of euchromatin was shown, suggesting that the partial trisomy of pericentric region derived sequences is implicated in the aetiology of the abnormal phenotypes.

Ring chromosome 8 syndrome: further characterization

We describe two de novo cases of extra r(8) confirmed by fluorescent in situ hybridization (FISH). Based on these two and eight additional cases of extra r(8) confirmed by FISH, the phenotype is better documented. One of our patients had minor facial anomalies, near-normal growth, and neurological development. She had a ring in each cell analyzed. The second had minor craniofacial anomalies and growth and mental retardation. He had a small or double-sized ring in each cell. The phenotype of these 10 cases ranges from almost normal in an adult with 10% mosaicism to variable degrees of minor anomalies, growth retardation, and mental retardation overlapping the mosaic +8 syndrome.

Delineation of supernumerary marker chromosomes in 38 patients

We present cytogenetic and clinical data on 38 patients with supernumerary marker chromosomes (SMCs). SMCs were characterized using a strategy combining classical banding techniques and molecular cytogenetic studies. Cases were ascertained prenatally, postnatally, and after fetal death. In 26 patients (68%), the SMC originated entirely from acrocentric chromosomes. Among these, most patients carried a der(15). In 11 patients (29%), they were of nonacrocentric origin, including 9 autosomal and 2 gonosomal marker chromosomes. In 1 patient the SMC was of partially acrocentric origin. Patients with small derivatives of chromosome 15 [der(15)] had a normal phenotype. Those with a larger der(15) showed phenotypical abnormalities. Patients with supernumerary marker chromosomes derived from chromosomes 13 or 21, and 14 appeared to have a low risk of abnormalities. Out of this group only 1 patient who carried an additional r(21) had physical anomalies. Patients with an SMC originating from chromosome 22 showed physical abnormalities in 2 out of 6 cases. Supernumerary marker chromosomes identified as i(9p), i(12p), and der(18) were all associated with an abnormal phenotype. Two of the derivatives of chromosome 20 analyzed were correlated with a normal phenotype, while the carrier of the third one showed physical anomalies and motor retardation. Of 2 patients with an extra der(X), 1 was normal and 1 showed an abnormal phenotype.

FISH and molecular study of autosomal supernumerary marker chromosomes excluding those derived from chromosomes 15 and 22: I. Results of 26 new cases

The chromosomal origins and in some cases the molecular composition of 26 autosomal supernumerary marker chromosomes (SMC) were identified using combined fluorescence in situ hybridization (FISH) and polymerase chain reaction (PCR) techniques. Fifteen were de novo, 4 maternally and 2 paternally transmitted and in 5 cases the parental origin is not known. Eleven cases were non-mosaic and fifteen cases had SMC cell lines ranging from 8-87%. Ten cases were ascertained prenatally, nine postnatally with abnormal phenotypes, three with poor reproductive histories and four co-incidentally. Five SMC were small rings from chromosomes 3, 6 (2 cases), 20 and 21; 8 were bisatellited from chromosomes 13/21 (4 cases), 14 (3 cases) and 14/22 (1 case). The remaining 13 appeared to be minutes comprising centromeric material only from chromosomes 1, 4, 12, 13/21 (2 cases), 14 (3 cases), 16 (2 cases), 19; 5/19, and a centric fusion involving 13 or 21 and 14. Euchromatin was detected in 9 out of 18 SMC tested with paints and/or PCR, and abnormal phenotypes were most commonly observed in patients with small ring shaped SMCs containing euchromatic sequences. Uniparental paternal isodisomy (UPD) for chromosome 6 was detected in one patient but was the only example of UPD for the normal homologues in association with an autosomal SMC in an overall total of 30 cases examined.

A molecular and FISH approach to determining karyotype and phenotype correlations in six patients with supernumerary marker(22) chromosomes

In a cytogenetic, molecular, and clinical study of patients with autosomal supernumerary marker chromosomes (SMC), 6 out of 72 (8.3%) were shown by fluorescence in situ hybridisation (FISH) to be derived from chromosome 22. PCR microsatellite analysis and FISH using primers and cosmids from proximal 22q showed 3 of the 6 to contain euchromatin. The first, a de novo nonmosaic bisatellited, dicentric SMC, was acsertained in a patient with cat eye syndrome and Duane anomaly. Microsatellite analysis showed the SMC was maternal in origin with euchromatin extending to D22S427, i.e., proximal to the DiGeorge syndrome critical region (DGSCR). The second, a nonmosaic bisatellited, dicentric marker, was found in a child with severe hypotonia and developmental delay and had been inherited from the patient's phenotypically normal father. FISH showed the SMC to contain euchromatin extending into the DGSCR. The third, a de novo SMC, was ascertained antenatally and was shown to contain 22q euchromatin extending distal to the DGSCR. The 19-week terminated fetus was phenotypically normal at autopsy. Two of the three SMC(22)s not containing detectable proximal 22q euchromatin were ascertained coincidentally in phenotypically normal individuals, whereas the third, the only mosaic with a minority euploid cell line, was found in a patient with mild developmental delay. These results suggest that SMC(22)s devoid of proximal 22q euchromatin are not associated with adverse phenotypic effects whereas SMC(22)s containing euchromatin may be found in individuals with phenotypes ranging from cat eye syndrome to normal.

Sex chromosome markers: characterization using fluorescence in situ hybridization and review of the literature

Fluorescence in situ hybridization (FISH) using biotin labeled X- and Y-chromosome DNA probes was utilized in the analysis of 23 sex chromosome-derived markers. Specimens were obtained through prenatal diagnosis, because of a presumptive diagnosis of Ullrich-Turner syndrome, mental retardation, and minor anomalies or ambiguous genitalia; three were spontaneous abortuses. Twelve markers were derived from the X chromosome and eleven from the Y chromosome; this demonstrates successfully the value and necessity of FISH utilizing DNA probes in the identification of sex chromosome markers. Both fresh and older slides, some of which had been previously G-banded, were used in these determinations. We have also reviewed the literature on sex chromosome markers identified using FISH.

FISH characterization of small supernumerary marker chromosomes in two Prader-Willi patients

A small supernumerary chromosome was observed in two Prader-Willi syndrome (PWS) patients. The clinical diagnosis of PWS was confirmed by the ascertainment of the deletion of region 15q11-13 in one case and uniparental disomy (UPD) of the same region in the other. The markers were negative for dystamycinA/DAPI banding, did not contain NOR-positive satellites, and had an appearance consistent with a very small ring chromosome. Fluorescent in situ hybridization (FISH) analysis with the "all human centromere" probe indicated the presence of centromeric sequences in both markers. Chromosomal in situ suppression hybridization with chromosome specific libraries demonstrated that the small markers in the deleted and UPD patient originated from chromosome 15 and X, respectively. To the best of our knowledge these are the only PWS patients reported with a supernumerary marker chromosome other than inv dup(15) characterized by FISH.

Small extra ring chromosome derived from chromosome 10p: clinical report and characterisation by FISH

We present a case with a small extra ring chromosome which was found in 66% of lymphocytes on routine cytogenetic examination. FISH analyses, using centromere specific and single copy probes, showed that the extra ring chromosome was derived from the most proximal part of 10p, close to the centromere. The patient has a unilateral cleft lip and palate, mild dysmorphic features, and mild mental retardation. Only a limited number of extra ring chromosomes have been characterised so far. To our knowledge, this is the first reported patient with an extra ring chromosome derived from chromosome 10p.

Two supernumerary marker chromosomes, derived from chromosome 6 and 9, in a boy with mild developmental delay

We report on a boy with two supernumerary marker chromosomes which were identified by fluorescence in situ hybridization and derived from chromosome 6 and 9. In lymphocytes, a mosaic karyotype was found: 46,XY (17%)/ 47,XY,r(6) (24%)/47,XY,r(9) (20%)/48,XY,r(6),r(9) (39%). Only minor dysmorphic features and mild developmental delay were present. Despite extensive fluorescence in situ hybridization studies using a large panel of probes, we were unable to characterize the marker chromosomes in more detail, mainly because no probes for the chromosome regions involved were available to us. In order to reach a better understanding of the clinical relevance of small supernumerary marker chromosomes, it will be necessary to create a widely available set of probes, covering all chromosome regions.

Supernumerary ring chromosome 20 characterized by fluorescence in situ hybridization

We report on a boy with mild dysmorphic features and developmental delay, in whom karyotyping showed an additional minute ring chromosome in 60% of metaphases. Fluorescence in situ hybridization (FISH) with a centromere specific probe demonstrated that the ring chromosome contained the centromeric region of chromosome 20. The ring was highlighted completely using a chromosome 20 painting probe. A cosmid probe for 20p 12-13 gave a positive signal and hybridization with an all-telomere probe showed on signal, suggesting a breakpoint in the 20p telomere. The results suggested that only a small part of 20q was involved in this ring. The ring was also detected in 18% of nuclei of a buccal smear. The phenotypic similarities of symptoms in the proband to patients with a (partial) trisomy 20p and the dissimilarities to symptoms in patients with (partial) trisomy 20q were in agreement with the FISH results.