Swedish survey on extra structurally abnormal chromosomes in 39 105 consecutive prenatal diagnoses: prevalence and characterization by fluorescence in situ hybridization

Prenatal diagnosis and molecular cytogenetic identification of small supernumerary marker chromosomes: analysis of three prenatal cases using chromosome microarray analysis

Small supernumerary marker chromosomes cannot be accurately identified by G-banding, and the related phenotypes vary greatly. It is essential to specify the origin, size, and gene content of marker chromosomes using molecular cytogenetic techniques. Herein, three fetuses with de novo marker chromosomes were initially identified by G-banding. Single nucleotide polymorphism array and fluorescence in situ hybridization were performed to characterize the origins of the marker chromosomes. The karyotypes of the three fetuses were 47,XY,+mar, 46,X,+mar[32]/45,X[68], and 45,X[62]/46,X,+mar[9]. In case 1, the karyotype was confirmed as 47,XY,+ idic(22)(q11.2). Therefore, the sSMC originated from chromosome 22 and was associated with cat eye syndrome. In case 2, the marker chromosome derived from ring chromosome X, and the karyotype was interpreted as 45,X[68]/46,X,+r(X)(p11.1q21.31)[32]. Meanwhile, the karyotype of case 3 was defined as 45,X[62]/46,X,idic(Y)(q11.2) and the marker chromosome originated from chromosome Y. Case 1 continued the pregnancy, whereas the other two pregnancies underwent elective termination. The detailed characterization of marker chromosomes can facilitate informed decision making, prevent uncertainty, and provide proper prognostic assessments. Our findings emphasize the importance for combining cytogenetic and molecular genetic techniques in marker chromosome characterization.

Chromosomal microarray analysis for pregnancies with or without ultrasound abnormalities in women of advanced maternal age

BACKGROUND

Chromosomal microarray analysis (CMA) has been suggested to be routinely conducted for fetuses with ultrasound abnormalities (UA), especially with ultrasound structural anomalies (USA). Whether to routinely offer CMA to women of advanced maternal age (AMA) without UA when undergoing invasive prenatal testing is inconclusive.

OBJECTIVE

This study aimed to evaluate the efficiency of CMA in detecting clinically significant chromosomal abnormalities in fetuses, with or without UA, of women with AMA.

METHODS

Data from singleton pregnancies referred for prenatal CMA due to AMA, with or without UA were obtained. The enrolled cases were divided into AMA group (group A) and AMA accompanied by UA group (group B). Single nucleotide polymorphism (SNP) array technology and conventional karyotyping were performed simultaneously.

RESULTS

A total of 703 cases were enrolled and divided into group A (N = 437) and group B (N = 266). Clinically significant abnormalities were detected by CMA in 52 cases (7.4%, 52/703; the value in group A was significantly lower than that in group B (3.9% vs 13.2%, P < .05); no statistic difference was observed with respect to submicroscopic variants of clinical significance between the two groups (0.9% vs 2.6%, P > .05).

CONCLUSIONS

Chromosomal microarray analysis should be available to all women with AMA undergoing invasive prenatal testing, regardless of ultrasound findings.

Genomic inversions and GOLGA core duplicons underlie disease instability at the 15q25 locus

Human chromosome 15q25 is involved in several disease-associated structural rearrangements, including microdeletions and chromosomal markers with inverted duplications. Using comparative fluorescence in situ hybridization, strand-sequencing, single-molecule, real-time sequencing and Bionano optical mapping analyses, we investigated the organization of the 15q25 region in human and nonhuman primates. We found that two independent inversions occurred in this region after the fission event that gave rise to phylogenetic chromosomes XIV and XV in humans and great apes. One of these inversions is still polymorphic in the human population today and may confer differential susceptibility to 15q25 microdeletions and inverted duplications. The inversion breakpoints map within segmental duplications containing core duplicons of the GOLGA gene family and correspond to the site of an ancestral centromere, which became inactivated about 25 million years ago. The inactivation of this centromere likely released segmental duplications from recombination repression typical of centromeric regions. We hypothesize that this increased the frequency of ectopic recombination creating a hotspot of hominid inversions where dispersed GOLGA core elements now predispose this region to recurrent genomic rearrangements associated with disease.

Traditional Prenatal Diagnosis: Past to Present

In the nearly 60 years since prenatal diagnosis for genetic disease was first offered, the field of prenatal diagnosis has progressed far past rudimentary uterine puncture to provide fetal material to assess gender and interpret risk. Concurrent with the improvements in invasive fetal sampling came technological advances in cytogenetics and molecular biology that widened both the scope of genetic disorders that could be diagnosed and also the resolution at which the human genome could be interrogated. Nowadays, routine blood work available to all pregnant women can determine the risk for common chromosome abnormalities; chorionic villus sampling (CVS) and amniocentesis can be used to diagnose nearly all conditions with a known genetic cause; and the genome and/or exome of a fetus with multiple anomalies can be sequenced in an attempt to determine the underlying etiology. This chapter will discuss some of the major advances in prenatal sampling and prenatal diagnostic laboratory techniques that have occurred over the past six decades.

Prenatal diagnosis and molecular cytogenetic characterization of low-level mosaicism for tetrasomy 18p at amniocentesis in a pregnancy with a favorable outcome

OBJECTIVE

We present prenatal diagnosis of low-level mosaicism for tetrasomy 18p at amniocentesis in a pregnancy with a favorable outcome.

CASE REPORT

A 40-year-old woman underwent amniocentesis at 17 weeks of gestation because of advanced maternal age. Amniocentesis revealed a de novo supernumerary isochromosome 18p in eight of 39 colonies of cultured amniocytes. The karyotype was 47,XX,+i(18)(p10)[8]/46,XX[31]. Array comparative genomic hybridization (aCGH) analysis using uncultured amniocytes revealed arr 18p11.32p11.21 [hg 19] (148,963-14,081,887) × 2-3. Repeat amniocentesis was performed at 20 weeks of gestation. Interphase fluorescence in situ hybridization (FISH) analysis showed four 18p11.22-specific probe (RP11-918F20) signals in 11.7% (12/103 cells) of uncultured amniocytes. aCGH analysis on uncultured amniocytes did not detect genomic imbalance in chromosome 18. The parental karyotypes were normal. Polymorphic DNA marker analysis excluded uniparental disomy 18. Cytogenetic analysis of cultured amniocytes at repeat amniocentesis revealed a karyotype of 47,XX,+i(18)(p10)[2]/46,XX[12]. Prenatal ultrasound was unremarkable. The pregnancy was carried to 38 weeks of gestation, and a 2742-g phenotypically normal female baby was delivered with a cord blood karyotype of 46,XX. When examined at 8 months of age, the infant was normal in growth and psychomotor development. Interphase FISH analysis on 21 uncultured urinary cells revealed normal signals in all cells and no mosaic tetrasomy 18p.

CONCLUSION

Low-level mosaic tetrasomy 18p at amniocentesis without ultrasound abnormalities can be associated with a favorable outcome.

Characterization of a de novo Supernumerary Neocentric Ring Chromosome Derived from Chromosome 7

Supernumerary ring chromosomes (SRC) are usually derived from regions adjacent to the centromere. Their identification may be challenging, particularly in case of low mosaicism. Here, we report on a patient who was referred for major in utero growth retardation, severe developmental delay, facial dysmorphism, cleft palate, and hypospadias. The karyotype showed a small SRC in mosaic. The combination of FISH, M-FISH and array-CGH was necessary for a complete characterization of this SRC. M-FISH revealed that the SRC originated from chromosome 7. Array-CGH performed with a 400K oligonucleotide array showed a gain in region 7q22.1q31.1 present in low mosaic. This result was confirmed by FISH using BAC probes specific for chromosome 7. The SRC was a neocentric ring derived from 7q22.1q31.1 and was found in only 8% of the cells. This is the first patient carrying a mosaic neocentric SRC derived from the long arm of chromosome 7. Our study emphasizes the need to combine different techniques and to use adapted bioinformatic tools for low-mosaicism marker identification. It also contributes to the delineation of the partial trisomy 7q phenotype.

Paternal Transmission of Small Supernumerary Marker Chromosome 15 Identified in Prenatal Diagnosis Due to Advanced Maternal Age

The detection of supernumerary marker chromosomes (SMCs) in prenatal diagnosis is always a challenge. In this study, we report a paternally inherited case of a small SMC(15) that was identified in prenatal diagnosis due to advanced maternal age. A 39-year-old woman underwent amniocentesis at 16 weeks of gestation. A fetal abnormal karyotype – 47,XX,+mar – with one sSMC was detected in all metaphases. Since this sSMC was critical in the parental decision to continue or interrupt this pregnancy, we proceeded to study the fetus and their parents. Cytogenetic and molecular analyses revealed a fetal karyotype 47,XX,+mar pat.ish idic(15)(ql2)(D15Zl++,SNRPN−), in which the sSMC(15) was a paternally inherited inverted duplicated chromosome and did not contain the critical region of Prader–Willi/Angelman syndromes. Moreover, fetal uniparental disomy was excluded. Based on this information and normal obstetric ultrasounds, the parents decided to proceed with the pregnancy and a phenotypically normal girl was born at 39 weeks of gestation. In conclusion, the clinical effects of sSMCs need to be investigated, especially when sSMCs are encountered at prenatal diagnosis. Here, although the paternal sSMC(15) was not associated with an abnormal phenotype, its characterization allows more accurate genetic counseling for the family progeny.

Co-occurrence of mosaic supernumerary isochromosome 18p and intermittent 2q13 deletions in a child with multiple congenital anomalies

The present study deals with karyotpye-phenotype correlations in a six month old child with multiple congenital abnormalities. Cytogenetic analysis revealed mosaicism of a small metacentric supernumerary marker chromosome with a karyotype mos 47,XY+mar[34]/46,XY[31]. Cytogenetic microarray result showed three copies of chromosome 18p (15,400 kb in size). Moreover, 255 kbp intermittent deletion of chromosome 2q13 involving RGPD5, RGPD6, LIMS3, and LIMS3-LOC440895 was also observed. Correlating microarray data with the mosaic karyotype, the marker chromosome was identified as mosaic isochromosome 18p and was found to be 32,600 kbp in size. Baby resembled clinical characteristics of trisomy chromosome 18p, isochromosome 18p and trisomy chromosome 18. The present study suggested that deletion of evolutionarily conserved developmental genes (RGPD5, RGPD and LIMS3) in the 2q13 region might have contributed to more severity in phenotype as compared to so far such reported cases of 18p trisomy's, as these are involved in nuclear-cytoplasm trafficking, signaling for tissue patterning and differentiation.

Duplication of the 15q11-q13 region: clinical and genetic study of 30 new cases

BACKGROUND

15q11-q13 region is an area of well-known susceptibility to genomic rearrangements, in which several breakpoints have been identified (BP1-BP5). Duplication of this region is observed in two instances: presence of a supernumerary marker chromosome (SMC) derived of chromosome 15, or interstitial tandem duplication. Duplications are clinically characterized by a variable phenotype that includes central hypotonia, developmental delay, speech delay, seizure, minor dysmorphic features and autism.

METHODS

Retrospective clinical and molecular study of 30 unrelated patients who were identified among the patients seen at the genetic clinics of Robert DEBRE hospital with microduplication of the 15q11-q13 region.

RESULTS

Fifteen patients presented with a supernumerary marker derived from chromosome 15. In fourteen cases the SMC was of large size, encompassing the Prader-Willi/Angelman critical region. All but one was maternal in origin. One patient had a PWS-like phenotype in absence of maternal UPD. In one case, the marker had a smaller size and contained only the BP1-BP2 region. Fifteen patients presented with interstitial duplication. Four cases were inherited from phenotypically normal parents (3 maternal and 1 paternal). Phenotypic features were somewhat variable and 57% presented with autism. Twelve patients showed cerebral anomalies and 18 patients had an abnormal EEG with a typical, recognizable pattern of excessive diffuse rapid spikes in the waking record, similar to the pattern observed after benzodiazepine exposure. Duplication of paternally expressed genes MKRN3, MAGEL2 and NDN in two autistic patients without extra material of a neighboring region enhances their likelihood to be genes related to autism.

Prenatal diagnosis of mosaic tetrasomy 18p

OBJECTIVE

To present prenatal diagnosis and molecular cytogenetic characterization of a small supernumerary marker chromosome derived from isochromosome 18p, by interphase fluorescence in situ hybridization (FISH) on uncultured amniocytes.

CASE REPORT

A 41-year-old woman underwent amniocentesis at 18 weeks of gestation, because of advanced maternal age. Amniocentesis revealed a de novo supernumerary isochromosome 18p in two of 14 colonies of cultured amniocytes. Repeated amniocentesis was performed at 22 weeks of gestation. Interphase FISH analysis on uncultured amniocytes showed four 18p11.32-specific probe (RP11-324G2) signals in 5.7% (3/53 cells) of uncultured amniocytes. A multiplex ligation-dependent probe amplification P095 test kit and array comparative genomic hybridization analysis did not detect genomic imbalance in chromosome 18. Cytogenetic analysis of cultured amniocytes at repeated amniocentesis revealed a karyotype of 47,XY,+i(18)(p10)[3]/46,XY[23]. The pregnancy was carried to 38 weeks of gestation, and a healthy 3120 g male baby was delivered. When examined at 2 months of age, the infant was normal in growth and development, without phenotypic abnormalities. The cord blood had a karyotype of 46,XY. Polymorphic DNA marker analysis excluded uniparental disomy 18. Interphase FISH analysis on uncultured urinary cells showed 9.4% (3/32 cells) mosaicism for tetrasomy 18p.

CONCLUSION

There is cytogenetic discrepancy between amniocytes and cord blood lymphocytes in prenatally detected mosaic tetrasomy 18p. Interphase FISH on uncultured amniocytes has the advantage of rapid confirmation of low-level mosaicism for tetrasomy 18p at amniocentesis.

Marker chromosomes

Marker chromosomes are a morphologically heterogeneous group of structurally abnormal chromosomes that pose a significant challenge in prenatal diagnosis. Phenotypes associated with marker chromosomes are highly variable and range from normal to severely abnormal. Clinical outcomes are very difficult to predict when marker chromosomes are detected prenatally. In this review, we outline the classification, etiology, cytogenetic characterization, and clinical consequences of marker chromosomes, as well as practical approaches to prenatal diagnosis and genetic counseling.

Tetrasomy 15q26: a distinct syndrome or Shprintzen-Goldberg syndrome phenocopy?

PURPOSE

The aim of this study was to characterize the clinical phenotype of patients with tetrasomy of the distal 15q chromosome in the form of a neocentric marker chromosome and to evaluate whether the phenotype represents a new clinical syndrome or is a phenocopy of Shprintzen-Goldberg syndrome.

METHODS

We carried out comprehensive clinical evaluation of four patients who were identified with a supernumerary marker chromosome. The marker chromosome was characterized by G-banding, fluorescence in situ hybridization, single nucleotide polymorphism oligonucleotide microarray analysis, and immunofluorescence with antibodies to centromere protein C.

RESULTS

The marker chromosomes were categorized as being neocentric with all showing tetrasomy for regions distal to 15q25 and the common region of overlap being 15q26→qter.

CONCLUSION

Tetrasomy of 15q26 likely results in a distinct syndrome as the patients with tetrasomy 15q26 share a strikingly more consistent phenotype than do the patients with Shprintzen-Goldberg syndrome, who show remarkable clinical variation.

Supernumerary marker chromosomes derived from chromosome 6: cytogenetic, molecular cytogenetic, and array CGH characterization

Supernumerary marker chromosomes (SMC) are relatively common in prenatal diagnosis. As the clinical outcomes vary greatly, a better understanding of the karyotype-phenotype correlation for different SMCs will be important for genetic counseling. We present two cases of prenatally detected de novo, small SMCs. The markers were present in 80% of amniocyte colonies in Case 1 and 38% of the colonies in Case 2. The SMCs were determined to be derived from chromosome 6 during postnatal confirmation studies. Although the sizes and the chromosomal origin of the SMCs in these two cases appeared to be similar, the clinical outcomes varied. The clinical manifestations observed in Case 1 included small for gestational age, feeding difficulty at birth, hydronephrosis, deviated septum and dysmorphic features, while the phenotype is apparently normal in Case 2. Array comparative genomic hybridization (CGH) was performed and showed increase in dosage for approximately 26 Mb of genetic material from the proximal short and long arms of chromosome 6 in Case 1. Results of array CGH were uninformative in Case 2, either due to mosaicism or lack of detectable euchromatin. The difference in the clinical presentation in these two patients may have resulted from the difference in the actual gene contents of the marker chromosomes and/or the differential distribution of the mosaicism.

Clinical utility of chromosomal microarray analysis in invasive prenatal diagnosis

Novel methodologies for detection of chromosomal abnormalities have been made available in the recent years but their clinical utility in prenatal settings is still unknown. We have conducted a comparative study of currently available methodologies for detection of chromosomal abnormalities after invasive prenatal sampling. A multicentric collection of a 1-year series of fetal samples with indication for prenatal invasive sampling was simultaneously evaluated using three screening methodologies: (1) karyotype and quantitative fluorescent polymerase chain reaction (QF-PCR), (2) two panels of multiplex ligation-dependent probe amplification (MLPA), and (3) chromosomal microarray-based analysis (CMA) with a targeted BAC microarray. A total of 900 pregnant women provided informed consent to participate (94% acceptance rate). Technical performance was excellent for karyotype, QF-PCR, and CMA (~1% failure rate), but relatively poor for MLPA (10% failure). Mean turn-around time (TAT) was 7 days for CMA or MLPA, 25 for karyotype, and two for QF-PCR, with similar combined costs for the different approaches. A total of 57 clinically significant chromosomal aberrations were found (6.3%), with CMA yielding the highest detection rate (32% above other methods). The identification of variants of uncertain clinical significance by CMA (17, 1.9%) tripled that of karyotype and MLPA, but most alterations could be classified as likely benign after proving they all were inherited. High acceptability, significantly higher detection rate and lower TAT, could justify the higher cost of CMA and favor targeted CMA as the best method for detection of chromosomal abnormalities in at-risk pregnancies after invasive prenatal sampling.

Prenatal diagnosis and molecular cytogenetic characterization of mosaicism for a small supernumerary marker chromosome derived from ring chromosome 4

OBJECTIVE

To present prenatal diagnosis and molecular cytogenetic characterization of mosaicism for a small supernumerary marker chromosome (sSMC) derived from ring chromosome, or r(4) by spectral karyotyping (SKY), fluorescence in situ hybridization (FISH), and array comparative genomic hybridization (aCGH).

MATERIALS, METHODS, AND RESULTS

A 37-year-old, primigravid woman underwent amniocentesis at 18 weeks of gestation because of advanced maternal age. Amniocentesis revealed a de novo ring-shaped sSMC in 16 of 31 amniocyte colonies. The parental karyotypes were normal. Level II ultrasound findings were unremarkable. Repeated amniocentesis revealed a karyotype of 47,XX,+mar[17]/46,XX[19]. The sSMC was characterized by SKY and FISH, which showed a chromosome 4 origin of the sSMC. aCGH demonstrated a 21.7-Mb gain in the gene dosage encompassing the region of 4p12→q13.2. The sSMC was r(4)(p12q13.2). The fetal karyotype was 47,XX,+r(4)(p12q13.2)[17]/46,XX[19]. The pregnancy was subsequently terminated. The fetus postnatally manifested hypertelorism, epicanthic folds, a prominent nose, a triangular face, low-set ears, clinodactyly of the fingers, and small big toes. Postnatal cytogenetic analyses of fetal and extraembryonic tissues revealed the karyotypes of 47,XX,+r(4)[18]/46,XX[21] in cord blood, 47,XX,+r(4)[20]/48,XX,+r(4),+r(4)[1]/46,XX[9] in umbilical cord, 47,XX,+r(4)[14]/47,XX,+dic r(4)[1]/46,XX[25] in skin, 47,XX,+r(4)[15]/46,XX[25] in amnion, and 47,XX,+r(4)[12]/47,XX,+dic r(4)[1]/46,XX[2] in placenta.

CONCLUSION

SKY, FISH, and aCGH are helpful in genetic counseling of prenatally detected sSMCs by providing the immediate and thorough information on the origin and genetic component of the sSMC.

Mammalian artificial chromosomes and clinical applications for genetic modification of stem cells: an overview

Modifying multipotent, self-renewing human stem cells with mammalian artificial chromosomes (MACs), present a promising clinical strategy for numerous diseases, especially ex vivo cell therapies that can benefit from constitutive or overexpression of therapeutic gene(s). MACs are nonintegrating, autonomously replicating, with the capacity to carry large cDNA or genomic sequences, which in turn enable potentially prolonged, safe, and regulated therapeutic transgene expression, and render MACs as attractive genetic vectors for "gene replacement" or for controlling differentiation pathways in progenitor cells. The status quo is that the most versatile target cell would be one that was pluripotent and self-renewing to address multiple disease target cell types, thus making multilineage stem cells, such as adult derived early progenitor cells and embryonic stem cells, as attractive universal host cells. We will describe the progress of MAC technologies, the subsequent modifications of stem cells, and discuss the establishment of MAC platform stem cell lines to facilitate proof-of-principle studies and preclinical development.

Screening for genomic rearrangements and methylation abnormalities of the 15q11-q13 region in autism spectrum disorders

BACKGROUND

Maternally derived duplications of the 15q11-q13 region are the most frequently reported chromosomal aberrations in autism spectrum disorders (ASD). Prader-Willi and Angelman syndromes, caused by 15q11-q13 deletions or abnormal methylation of imprinted genes, are also associated with ASD. However, the prevalence of these disorders in ASD is unknown. The aim of this study was to assess the frequency of 15q11-q13 rearrangements in a large sample of patients ascertained for ASD.

METHODS

A total of 522 patients belonging to 430 families were screened for deletions, duplications, and methylation abnormalities involving 15q11-q13 with multiplex ligation-dependent probe amplification (MLPA).

RESULTS

We identified four patients with 15q11-q13 abnormalities: a supernumerary chromosome 15, a paternal interstitial duplication, and two subjects with Angelman syndrome, one with a maternal deletion and the other with a paternal uniparental disomy.

CONCLUSIONS

Our results show that abnormalities of the 15q11-q13 region are a significant cause of ASD, accounting for approximately 1% of cases. Maternal interstitial 15q11-q13 duplications, previously reported to be present in 1% of patients with ASD, were not detected in our sample. Although paternal duplications of chromosome 15 remain phenotypically silent in the majority of patients, they can give rise to developmental delay and ASD in some subjects, suggesting that paternally expressed genes in this region can contribute to ASD, albeit with reduced penetrance compared with maternal duplications. These findings indicate that patients with ASD should be routinely screened for 15q genomic imbalances and methylation abnormalities and that MLPA is a reliable, rapid, and cost-effective method to perform this screening.

Prenatal diagnosis of extrastructurally abnormal chromosomes: clinical experience and literature review

BACKGROUND

To evaluate the clinical association of extrastructurally abnormal chromosomes (ESACs) with pregnancy outcome based on the cytogenetic characteristics of the ESACs.

METHODS

We retrospectively reviewed 12 ESAC cases identified from 12,991 cases who received genetic amniocentesis between January 1983 and March 2008. Prenatal ultrasound findings, characteristics of ESACs (karyotypes, special features, origin, inheritance) and pregnancy outcomes were recorded.

RESULTS

The prenatal prevalence of ESACs was 0.092% (12/12,991). Of the 12 ESAC cases, all were de novo. Seven (58.3%) originated from nonacrocentric chromosomes and the other 5 (41.7%) were from acrocentric chromosomes, with 3 originating from chromosome 15. Six of the 12 cases (50%) were large ESACs; however, the other 6 (50%) were medium to small ESACs. All acrocentric ESACs contained dicentric and bisatellite characteristics. Using FISH and SKY techniques, the origins of 2 cases (patients 10 and 12) were clearly identified to be from chromosomes 15 and 10, respectively. Five of the 12 ESAC cases (41.7%) had congenital anomalies found by prenatal ultrasound. All were nonacrocentric in origin that were medium (1/5) to large (4/5) in size. After prenatal genetic counseling, 8 of the 12 (66.7%) couples opted to terminate the pregnancy. The other 4 (33.3%) continued the pregnancy and their babies were delivered at term normally and were followed-up, with normal development ranging from 2 to 17 years.

CONCLUSION

With sophisticated cytogenetic characterization and ultrasound examination, it is possible to precisely categorize most fetuses with ESACs as being either at high risk of abnormality or at a relatively low risk.

[Strategies to identify supernumerary chromosomal markers in constitutional cytogenetics]

Supernumerary marker chromosomes (SMCs) are defined as extrastructurally abnormal chromosomes which origin and composition cannot be determined by conventional cytogenetics. SMCs are an heterogeneous group of abnormalities concerning all chromosomes with variable structure and size and are associated with phenotypic heterogeneity. The characterisation of SMCs is of utmost importance for genetic counselling. Different molecular techniques are used to identify chromosomal material present in markers such as 24-colour FISH (MFISH, SKY), centromere specific multicolour FISH (cenMFISH) and derivatives (acroMFISH, subcenMFISH), comparative genomic hybridisation (CGH), arrayCGH, and targeted FISH techniques (banding techniques, whole chromosome painting...). Based on the morphology of SMC with conventional cytogenetic and clinical data, we tried to set up different molecular strategies with all available techniques.

Five cases of supernumerary small ring chromosomes 1: Heterogeneity and genotype–phenotype correlation

Genetic counselling of patients with small supernumerary ring chromosomes (sSRCs) can be difficult, especially in prenatal testing, due to the complexity in establishing a karyotype-phenotype correlation. In fact, it has been estimated that about 10% of extra ring(1) chromosomes are associated with an unremarkable phenotype. We report on five new cases of extra ring chromosomes(1) manifesting different clinical outcome. One case was familial, segregating from a mother with mosaic karyotype, while the others were de novo. Ring chromosomes were characterised by FISH. In three subjects the involvement of the same euchromatic 1p region was demonstrated. Present observations corroborate previous results and provide some insight into the identification of the harmless ring(1) structures.

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.

Redefining the risks of prenatally ascertained supernumerary marker chromosomes: a collaborative study

BACKGROUND

A marker chromosome is defined as a structurally abnormal chromosome that cannot be identified by routine cytogenetics. The risk for phenotypic abnormalities associated with a marker chromosome depends on several factors, including inheritance, mode of ascertainment, chromosomal origin, and the morphology, content, and structure of the marker.

METHODS

to understand the karyotype-phenotype relationship of prenatally ascertained supernumerary de novo marker chromosomes, we combined data from prenatal cases obtained from 12 laboratories with those from studies in the literature. We were able to obtain cytogenetic and phenotypic data from 108 prenatally ascertained supernumerary de novo marker chromosomes to refine the phenotypic risk associated with these markers. Because of the growing number of cases and because more techniques are available to delineate marker morphology, we have been able to group risk estimates into subcategories, such as by marker type and whether there are ultrasound abnormalities.

RESULTS

If a de novo supernumerary marker chromosome is found prenatally, our data suggest there is a 26% risk for phenotypic abnormality when there is no other information defining the marker (such as chromosomal origin or information about the existing phenotype). However, if high resolution ultrasound studies are normal, this risk reduces to 18%.

CONCLUSIONS

Our findings strongly support the value of additional genetic studies for more precisely defining the risk in individual cases involving marker chromosomes.

Supernumerary marker chromosomes detected in 100 000 prenatal diagnoses: molecular cytogenetic studies and clinical significance

OBJECTIVE

To evaluate the clinical significance of the supernumerary marker chromosomes (SMCs) detected during prenatal diagnosis.

METHODS

We retrospectively studied cytogenetic/fluorescence in situ hybridization (FISH) results and clinical evaluation of 110 marker cases identified from approximately 100,000 cases referred for prenatal diagnosis. The clinical follow-up performed was focused on cases with de novo markers not derived from chromosome 15.

RESULTS

Among the 110 SMCs, 79 (71.8%) were de novo, 24 (21.8%) were familial, and the origin was undetermined in 7 cases. Fifty-eight of the SMCs originated from nonacrocentric chromosomes and 52 SMCs were derived from acrocentric chromosomes, with 27 originating from chromosome 15. Twenty-two of the SMCs from chromosome 15 did not contain the Prader-Willi/Angelman syndrome critical region, and uniparental disomy was ruled out in 19/19 cases. Clinical information ranging from birth to 4 years was obtained for 46 de novo cases with nonchromosome-15-derived SMCs. Of these cases, 11/11 acrocentric SMCs resulted in normal phenotype. In contrast, 4/23 cases with single nonacrocentric SMCs and 3/5 cases with two or more SMCs resulted in an abnormal phenotype.

CONCLUSIONS

Our data suggests an overall low risk for acrocentric SMCs and a higher risk for nonacrocentric SMCs. Phenotypes associated with markers derived from some specific chromosomes are also discussed.

Three new cases with a supernumerary ring chromosome 1

We report on three cases with a cytogenetically identical ring chromosome containing euchromatin from the long arm of chromosome 1 (r[1][::p11.1-->q21.1::]). Two cases were newborn males (Cases 1 and 2) and the third one was prenatally identified as female (Case 3). Mosaicism was present in all three cases in different degrees, i.e. 48%, 25% and 14% of the cells, respectively. Clinical signs and symptoms vary between the three cases. The results of our three cases are compared with those from the literature.

Small supernumerary marker chromosomes – progress towards a genotype-phenotype correlation

Small supernumerary marker chromosomes (sSMC) are still a major problem in clinical cytogenetics as they are too small to be characterized for their chromosomal origin by traditional banding techniques, but require molecular cytogenetic techniques for their identification. Apart from the correlation of about one third of the sSMC cases with a specific clinical picture, i.e. the i(18p), der(22), i(12p) (Pallister Killian syndrome) and inv dup(22) (cat-eye) syndromes, most of the remaining sSMC have not yet been correlated with clinical syndromes. Recently, we reviewed the available >1600 sSMC cases (Liehr T, sSMC homepage: http://mti-n.mti.uni-jena.de/~huwww/MOL_ZYTO/sSMC.htm). A total of 387 cases (including the 45 new cases reported here) have been molecularly cytogenetically characterized with regard to their chromosomal origin, the presence of euchromatin, heterochromatin and satellite material. Based on analysis of these cases we present the first draft of a basic genotype-phenotype correlation for sSMC for all human chromosomes apart from the chromosomes Y, 10, 11 and 13.

Forty-two supernumerary marker chromosomes (SMCs) in 43 273 prenatal samples: chromosomal distribution, clinical findings, and UPD studies

Fluorescence in situ hybridization (FISH) analyses were performed on supernumerary marker chromosomes (SMCs) detected in 43,273 prenatal diagnoses over a period of 11 years, 1993-2003. A total of 42 pregnancies with SMC were identified, indicating a prevalence of one in 1032. A total of 15 SMCs were endowed with detectable euchromatin (prevalence, 1/2884), including six SMCs containing the cat eye critical region (CECR) on chromosome 22q11.21 (1/7212). De novo SMCs were found in 29 pregnancies (1/1492), including 14 euchromatic SMCs (48.2%). Follow-up studies were available for 24 cases. Nine pregnancies (37.5%) were terminated; two children (8.3%) were born with Pallister-Killian syndrome and cat eye syndrome (CES), respectively; 13 children (54.1%) showed apparently normal development. Familial SMCs were identified in 13 pregnancies (1/3328) from 11 unrelated women. They were all acrocentric. In all, 10 were heterochromatic and one was an extra der(22)t(11;22) chromosome. A total of 12 cases were available for follow-up. One pregnancy was terminated due to anhydramnios, spina bifida, and cystic-dysplastic kidneys; one child suffered from a der(22) syndrome; 10 children (83.3%) appeared unaffected. Studies for uniparental disomy were performed on seven pregnancies and revealed a case of maternal heterodisomy for chromosome 22. So far this is the largest FISH study of prenatally ascertained SMCs and the first study with detailed data on the prevalence. Findings illustrate the spectrum and clinical outcomes of prenatally diagnosed SMCs, and indicate a higher frequency of SMCs than generally assumed.

The identification of small supernumerary marker chromosomes; the experiences of 15,792 fetal karyotyping from Turkey

Small supernumerary marker chromosomes (sSMCs) are often associated with developmental abnormalities and malformations are de novo in approximately 60% of the cases. Fluorescence in situ hybridization (FISH) techniques using various probes provided the possibility to analyze and characterize sSMCs, which is highly important for prenatal diagnosis and genetic counseling. We now present the establishment of a specific strategy to identify the origin and structure of the sSMCs using a combination of conventional banding and classical FISH techniques. Based on this strategy, in a series of 15,792 prenatal karyotypes, 20 cases with sSMCs (prevalence 1.26 per 1000) were diagnosed. Eighteen of these cases were completely analyzed by FISH using commercial probes and Chromoprobe Multiprobe-I System. Out of 20 sSMCs 12 were satellited (10 bisatellited and two monosatellited) (60%) and eight were non-satellited (six ring-like and two isochromosomes) (40%). sSMCs were mostly derived from chromosome 15 (10/20) (50%). Euchromatin material was found in 13 cases by various banding and FISH techniques, while in six of 20 sSMCs there was no evidence of euchromatin material. Parental karyotypes could be evaluated in 15 cases and familial inheritance was found in only three of them (20%). We conclude that the proposed strategy for the identification and characterization of sSMCs is accurate and represents a good alternative to novel FISH techniques for modestly equipped cytogenetic laboratories.

Supernumerary marker chromosomes in man: parental origin, mosaicism and maternal age revisited

The details of all cytogenetic abnormalities diagnosed in the Wessex Regional Genetics Laboratory (WRGL) since 1967 to the present day have been recorded in the Salisbury Treasury of Interesting Chromosomes (STOIC). From this resource, we identified 137 patients with constitutional autosomal supernumerary marker chromosomes (SMC) ascertained in four principal groups: (i) 37% with abnormal phenotypes; (ii) 7% couples with reproductive difficulties; (iii) 47% antenatal diagnoses and (iv) 9% miscellaneous. Overall, 81 (59%) SMCs were mosaics and 56 (41%) nonmosaics. Of the 109 cases with known parental origins, 70% were de novo, 19% maternally and 11% paternally inherited. The chromosomal origins of 112/137 (82%) of the SMCs have been determined by fluorescence in situ hybridization (FISH). In all, 36/112 (32%) were derived from nonacrocentric autosomes, and 76/112 (68%) from the acrocentric autosomes 13/21, 14, 15 and 22. Of these acrocentric SMCs, 39 (51%) were derived from chromosome 15, so that SMC(15) constituted 39/112 (35%) of all SMCs with known chromosomal origins. The frequencies with which mosaicism was observed varied considerably according to the chromosomal origin of the SMCs and accounted for 8/39 (20%) SMC(15), 13/37 (35%) SMCs from other acrocentrics and 25/36 (69%) of nonacrocentric SMCs. The data were analysed for parental age effects, and only de novo SMC(15)s were found to be associated with a significantly increased maternal age.

First small supernumerary ring chromosome carrying 10q euchromatin in a patient with mild phenotype characterized by molecular cytogenetic techniques and review of the literature

We report the identification and characterization of the first supernumerary ring chromosome 10 containing a considerable proportion of 10q euchromatin by microdissection and reverse painting in a female patient presenting with short stature. Fluorescence in situ hybridization studies showed that the marker chromosome originates from chromosome 10 and includes the euchromatic bands p11.2 and q11.2. The supernumerary marker chromosome 10 was found in 14% of the peripheral blood lymphocytes analyzed. This constitutional mosaic could be confirmed in oral mucosa cells as a second cell system (16%) by interphase FISH using an alphoid centromeric probe for chromosome 10. Parental karyotypes were normal, uniparental disomy for the normal chromosomes 10 could be excluded by microsatellite analysis. The karyotype of the patient detected in peripheral blood cells can be described as mos 47,XX,+mar.rev ish r(10)(p11.2q11.2)(wcp10+,cep10+)/46,XX.

Development of mammalian artificial chromosomes for the treatment of genetic diseases: Sandhoff and Krabbe diseases

Mammalian artificial chromosomes (MACs) are being developed as alternatives to viral vectors for gene therapy applications, as they allow for the introduction of large payloads of genetic information in a non-integrating, autonomously replicating format. One class of MACs, the satellite DNA-based artificial chromosome expression vehicle (ACE), is uniquely suited for gene therapy applications, in that it can be generated denovo in cells, along with being easily purified and readily transferred into a variety of recipient cell lines and primary cells. To facilitate the rapid engineering of ACEs, the ACE System was developed, permitting the efficient and reproducible loading of pre-existing ACEs with DNA sequences and/or target gene(s). As a result, the ACE System and ACEs are unique and versatile platforms for ex vivo gene therapy strategies that circumvent and alleviate existing safety and delivery limitations surrounding conventional gene therapy vectors. This review will focus on the status of MAC technologies and, in particular, the application of the ACE System towards an ex vivo gene therapy treatment of lysosomal storage diseases, specifically Sandhoff (MIM #268800) and Krabbe (MIM #245200) diseases.

Prenatal diagnosis of minute supernumerary marker chromosomes

The identification of supernumerary marker chromosomes (SMC) at prenatal diagnosis is problematic, particularly for the prediction of phenotype. The assessment of phenotypic risk is based on the size, morphology and origin of the SMC. Fluorescence in situ hybridization (FISH) characterization and family studies are also employed to aid in determining the significance of a prenatally ascertained SMC. Generally, SMC containing euchromatin are more likely to be associated with abnormal phenotypes and SMC without euchromatin are more likely to result in normal phenotypes. The smallest of SMC, minute SMC (minSMC) appear as dot-like or centric fragments and are particularly difficult to identify and characterize. Previous empirical observations suggested that the risk of phenotypic abnormality in prenatally ascertained minSMC was < or = 5%. We identified minSMC in chorionic villus samples (CVS) or amniocytes from 11 unrelated pregnancies. The chromosomal origin of each minSMC was identified by sequential FISH analysis with chromosome-specific centromere probes. Further FISH analysis with whole chromosome paint probes was undertaken to assess each minSMC for the presence or absence of euchromatin, since the presence of euchromatin may be associated with a higher risk of abnormality. Two minSMC were shown to have euchromatin. The first, a minSMC(12) was found in CVS but not confirmed in amniocytes, indicating confined placental mosaicism. The second, a minSMC derived from chromosome 19, was associated with ultrasound abnormalities. Apart from a case with mild speech delay, the remaining minSMC cases without detectable euchromatin had a normal outcome at birth and/or on longer term follow-up. Additional FISH analyses with a telomeric repeat probe showed no signal on any of the minSMC tested, suggesting that they were ring chromosomes in structure. These data further support the concept that minSMC containing euchromatin are more likely to be associated with an abnormal phenotype, although as more data are collected, this may vary by chromosome of origin. The absence of detectable euchromatin, while not guaranteeing a normal result, is most likely to have a normal outcome. The present report and previous studies do not yet allow any significant adjustment of the empirical < or = 5% risk estimate for minSMC identified at prenatal diagnosis. However, reporting of additional cases with characterization of the minSMC and particularly with long-term follow-up will, in time, allow for more accurate risk estimates and provide prognostic information.

Supernumerary ring chromosome 7 mosaicism: Case report, investigation of the gene content, and delineation of the phenotype

We report a girl with severe retardation of expressive speech development carrying a small, supernumerary ring chromosome derived from the proximal region of the long arm of chromosome 7. The r(7) chromosome is present in 50% of lymphocytes. We also review the six additional cases with a supernumerary r(7) chromosome reported in the literature. Among these patients, a severe retardation of productive language capabilities is seen as a shared clinical feature, irrespective of the degree of mosaicism as detected in blood. The dysmorphisms in these patients are minor and no shared congenital abnormalities seen. We, therefore, recommend chromosomal investigations in children with unexplained, disproportionately retarded expressive speech performance. Because speech and language acquisition are subject to genetic influences, we investigated whether there are genes on the r(7) chromosome that may affect brain development or function in a dosage-dependent manner. We found that both in our patient and in four patients described by others, the supernumerary r(7) chromosome contains the region from the centromere up to marker D7S613 located at 7q11.23. We speculate that the effects on speech acquisition are mediated by the supernumerary copies of the STX1A and LIMK1 genes, which are both located in this region and known to suppress neurite growth when overexpressed in vitro.

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.

Autistic spectrum disorder associated with partial duplication of chromosome 15; three case reports

Duplication of part or the entirety of chromosome 15 that involves the Prader-Willi/Angelman syndrome critical region (PWACR) is a genetic disorder which is associated with variable degrees of intellectual impairment, motor co-ordination problems and social and communication disorders. Published case reports indicate that phenotypic expression is dependent on parental origin of the duplication and implicate maternally derived duplications in the pathogenesis of autistic features. This article describes three individuals, two males and one female, aged between 5 and 8 years, all with partial duplication of chromosome 15. Autism (or autistic spectrum disorder) was present in all three instances with varying degrees of cognitive impairment. The aim of this paper is to describe the phenotypic characteristics of this genetic sequence and the possible associations between social and behavioural patterns on the one hand, and degree and nature of genetic impairment on the other.

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.

Reverse painting highlights the origin of chromosome aberrations

Reverse chromosome painting, as the opposite of forward chromosome painting, means that an abnormal chromosome of interest is recovered by flow sorting or by chromosome microdissection, amplified and labelled by DOP-PCR and hybridized onto normal metaphases of optimal quality. This provides rapid and unequivocal information about the chromosomal origin on the aberrant chromosome in one hybridization. Not only will the specific chromosome(s) involved be identified, but also the subchromosomal origin, including the breakpoints. The method has been used for over 10 years and has proven to be very useful for resolving complex chromosome rearrangements in a variety of different applications, both as a research tool and for clinical purposes in pre- and postnatal diagnosis.

A chromosome 21-derived minute marker in a mosaic trisomy 21 background: implications for risk assessments in marker chromosome cases

We report a prenatal case of a chromosome 21-derived minute supernumerary marker, found as a mosaic along with a trisomy 21 cell line at amniocentesis. Follow-up analysis of other fetal tissues confirmed the mosaicism and also disclosed a normal cell line. It is likely that the marker reflects a mutation event that resulted in trisomy rescue early in embryonic development. Had the trisomy 21 cell line not been found at amniocentesis, a low risk of an abnormal phenotype (approximately 5%) would have been assigned. We suggest that the risk associated with minute non-euchromatic marker chromosomes should be revised to account for the possibility of mosaicism with potentially aneuploid populations and/or uniparental disomy (UPD). The finding of any marker chromosome should prompt a thorough investigation for aneuploid cell lines. In the case of small markers with no euchromatin, the given risk of adverse phenotypic effects is not likely to be associated with the marker per se but with the possible presence of a cryptic aneuploid cell line from which the marker may have arisen.

Molecular and cytogenetic characterization of extra-structurally abnormal chromosomes (ESACs) found prenatally: outcome and follow-up

A 40-year-old woman underwent amniocentesis at 15.3 weeks of gestation. Chromosome analysis performed using QFQ, DA-DAPI and CBG banding revealed two de novo extra-chromosomal markers (ESACs) in 11 of the 16 colonies analysed. Fluorescence in situ hybridization (FISH) showed that both chromosomes came from the Yq11.22.1 region of the Y chromosome. PCR analysis of genes and STS localized on the Y chromosome excluded the Yp presence specifically of the SRY gene, and most of the euchromatic region of Yq. After extensive genetic counselling and considering both laboratory and second-level ultrasound data, the couple decided to continue the pregnancy. At 37.4 weeks of gestational age, a girl weighing 2750 g was born with an Apgar score of 9/10. A blood sample taken from the umbilical cord showed three cellular lines: mos47,XX, +mar1 ish.der (Y)(wcpY+) [21%]/48,XX, +mar1 ish.der (Y)(wcpY+), +mar2 ish.der (Y)(wcpY+) [41%]/46,XX [38%]. One year after birth, the baby was developing normally and had normal psychomotorial activity.

Molecular and fluorescence in situ hybridization characterization of the breakpoints in 46 large supernumerary marker 15 chromosomes reveals an unexpected level of complexity

Supernumerary marker chromosomes (SMCs) of chromosome 15, designated "SMC(15)s," are the most common SMC in humans, accounting for as much as 60% of all those observed. We report the characterization of 46 large SMC(15)s, using both fluorescence in situ hybridization and polymerase chain reaction analysis within and distal to the Prader-Willi/Angelman syndrome critical region (PWACR). Our aim was to establish detailed information on origin, content, and breakpoints, to address the formation of SMC(15)s, and to facilitate genotype-phenotype correlations. For all patients in whom we were able to establish the parental origin, the SMC(15)s were maternally derived. Two patients were observed who had familial SMC(15)s, both inherited from the mother; however, in all remaining patients for whom parental samples were available, the SMC(15)s were shown to have arisen de novo. With one exception, all the SMC(15)s were shown to include the entire PWACR. Detailed investigations of the distal breakpoints categorized the SMC(15)s into two groups. Group A, representing approximately two-thirds of the SMC(15)s, had a breakpoint beyond the standard distal PWS/AS deletion breakpoint BP3, at a position close to the microsatellite marker D15S1010 and the bacterial artificial chromosome 10I10. The group B SMC(15)s were shorter, with more variable breakpoints located around BP3. The majority of the SMC(15)s were shown to have asymmetrical breakpoints, with the two inverted arms of the SMC being unequal in length. Our study revealed an unexpected level of complexity and heterogeneity among SMC(15)s that is not seen in other chromosome 15 rearrangements, such as deletions and duplications. This suggests that multiple mechanisms are involved in the formation of large SMC(15)s.

Partial hexasomy of chromosome 15

Marker chromosomes originating from chromosome 15, often referred to as inv dup(15), is the most common marker chromosome found in humans. The large marker 15 that contains the Prader-Willi syndrome (PWS)/Angelman syndrome (AS) chromosome region is usually associated with an abnormal phenotype of moderate to severe mental retardation, seizures, poor motor coordination, behavioral problems, and mild dysmorphic features. We report here an infant boy with two copies of the large inv dup(15). A 10-day-old infant was found to have infantile spasms, microcephaly, hypotonia, and lethargy. Lymphocyte chromosome analysis revealed a 48,XY, +2mar karyotype. Fluorescence in situ hybridization with probes rRNA, D15Z4, D15S11, and GABRB3 demonstrated that both markers were chromosome 15 in origin and contained the Prader-Willi/Angelman syndrome chromosome region. Therefore, this patient is hexasomic for the PWS/AS region. The phenotype of this patient does not appear to be significantly more severe than patients with one copy of the large inv dup(15) at birth, however, follow-up evaluation of the patient at 21 months of age shows that this patient has frequent and severe seizure activity, severe bilateral hearing loss, and cortical blindness.

Characterization of a supernumerary ring chromosome 1 mosaicism in two cell systems by molecular cytogenetic techniques and review of the literature

We report on a 4-year-old boy with developmental delay and microcephaly with an additional small marker chromosome derived from chromosome 1 and detected in 14% of T-lymphocytes by conventional cytogenetics and in 9% of buccal smear cells by interphase FISH. Using molecular cytogenetic techniques, the marker chromosome was characterized as an extra ring chromosome consisting of euchromatic material from the proximal short arm of chromosome 1. We compare the cytogenetic data and the phenotype of our patient to those previously described cases with marker chromosome 1 mosaicism. We conclude that in addition to the straightforward molecular cytogenetic characterization of the euchromatic content of the ring chromosome, the investigation of a second cell system gives additional information about the tissue specific distribution of the supernumerary marker chromosome (SMC) and provides more reliable data for further karyotype/phenotype correlations and the prediction of the phenotypic outcome in prenatal cases.