Fifty probands with extra structurally abnormal chromosomes characterized by fluorescence in situ hybridization

Possible Phenotypic Consequences of Structural Differences in Idic(15) in a Small Cohort of Patients

Among human supernumerary marker chromosomes, the occurrence of isodicentric form of 15 origin is relatively well known due to its high frequency, both in terms of gene content and associated clinical symptoms. The associated epilepsy and autism are typically more severe than in cases with interstitial 15q duplication, despite copy number gain of approximately the same genomic region. Other mechanisms besides segmental aneuploidy and epigenetic changes may also cause this difference. Among the factors influencing the expression of members of the GABA_A gene cluster, the imprinting effect and copy number differences has been debated. Limited numbers of studies investigate factors influencing the interaction of GABA_A cluster homologues. Five isodicentric (15) patients are reported with heterogeneous symptoms, and structural differences of their isodicentric chromosomes based on array comparative genomic hybridization results. Relations between the structure and the heterogeneous clinical picture are discussed, raising the possibility that the structure of the isodicentric (15), which has an asymmetric breakpoint and consequently a lower copy number segment, would be the basis of the imbalance of the GABA_A homologues. Studies of trans interaction and regulation of GABA_A cluster homologues are needed to resolve this issue, considering copy number differences within the isodicentric chromosome 15.

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.

Tetrasomy 18p: case report and review of literature

Tetrasomy 18p syndrome (Online Mendelian Inheritance in Man 614290) is a very rare chromosomal disorder that is caused by the presence of isochromosome 18p, which is a supernumerary marker composed of two copies of the p arm of chromosome 18. Most tetrasomy 18p cases are de novo cases; however, familial cases have also been reported. It is characterized mainly by developmental delays, cognitive impairment, hypotonia, typical dysmorphic features, and other anomalies. Herein, we report de novo tetrasomy 18p in a 9-month-old boy with dysmorphic features, microcephaly, growth delay, hypotonia, and cerebellar and renal malformations. We compared our case with previously reported ones in the literature. Clinicians should consider tetrasomy 18p in any individual with dysmorphic features and cardiac, skeletal, and renal abnormalities. To the best of our knowledge, we report for the first time an association of this syndrome with partial agenesis of cerebellar vermis.

Electroclinical features of epilepsy in patients with InvDup(15)

PURPOSE

InvDup(15) syndrome is one of the most common chromosomal abnormalities associated with epilepsy. Here we review the seizure types described in InvDup(15) patients and the main electroclinical, therapeutic, and prognostic aspects of the syndrome.

METHODS

A literature search of PubMed, MEDLINE, and EMBASE was performed to identify papers examining InvDup(15) syndrome and epilepsy.

RESULTS

About 65% of the InvDup(15) patients described in the literature had multiple seizure types with a predominance (40.4%) of tonic-clonic seizures. Age at seizure onset was before 10 years in more than half of them. Patients suffered from a variety of EEG abnormalities, generalized spike activity being the most frequent. Brain MRI was unremarkable in the majority of patients. Treatment was with several anticonvulsant drugs used as mono- or polytherapy. Valproic acid was the most common treatment against generalized seizures and was often effective, although drug resistance was a major concern in a large number of cases. Finally, more than 30% of the children suffered from infantile spasms, and status epilepticus was described in nearly 20% of patients, occasionally resulting in death.

CONCLUSION

Seizures are very common in InvDup(15) patients, who suffer from a variety of seizure types. Information about EEG and brain MRI findings, seizure treatment, and prognosis is often poor. The overall prognosis is fair. Prospective studies of larger samples are needed, to gain further insights into the natural history of InvDup(15) syndrome.

Small supernumerary marker chromosomes and their correlation with specific syndromes

A small supernumerary marker chromosome (sSMC) is a structurally abnormal chromosome. It is an additional chromosome smaller than one chromosome most often lacking a distinct banding pattern and is rarely identifiable by conventional banding cytogenetic analysis. The origin and composition of an sSMC is recognizable by molecular cytogenetic analysis. These sSMCs are seen in different shapes, including the ring, centric minute, and inverted duplication shapes. The effects of sSMCs on the phenotype depend on factors such as size, genetic content, and the level of the mosaicism. The presence of an sSMC causes partial tris- or tetrasomy, and 70% of the sSMC carriers are clinically normal, while 30% are abnormal in some way. In 70% of the cases the sSMC is de novo, in 20% it is inherited from the mother, and in 10% it is inherited from the father. An sSMC can be causative for specific syndromes such as Emanuel, Pallister-Killian, or cat eye syndromes. There may be more specific sSMC-related syndromes, which may be identified by further investigation. These 10 syndromes can be useful for genetic counseling after further study.

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 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.

The behavioral phenotype of the idic(15) syndrome

Idic(15) syndrome is a neurogenetic disorder clinically delineated by early central hypotonia, developmental delay and intellectual disability (ID), epilepsy, absent or very poor speech, and autistic or autistic-like behavior. It is due to the presence of a supernumerary marker chromosome formed by the inverted duplication of proximal chromosome 15, resulting in tetrasomy 15p and partial tetrasomy 15q, and containing the Prader-Willi/Angelman syndrome critical region (PWS/ASCR). The vast majority of these idic(15) derives from the two homologous maternal chromosomes at meiosis. To better define the behavior profile, we studied 22 idic(15) children (15 males and 7 females) observed at our institute between 1986 and 2010, and present, in detail, case studies of five of them. We have been able to perform standardized and semi-standardized measures of intelligence, and psychopathology in only 13 of our 22 patients, due to the limitations of chronological age, and to the severity of ID (ranging from mild-moderate, in 15%, to severe-profound, in 85% of our sample). The results show a distinct developmental profile in idic(15) patients, that may provide a behavioral signature for autism spectrum disorder (ASD)/ASD-like arising from the susceptibility locus on proximal 15q; and suggest that idic(15) individuals are not "true autistic," but distinct "autistic-like" persons with high score in the third ADOS-G and ADI-R area.

Tetrasomy 18p: report of the molecular and clinical findings of 43 individuals

Thus far, the phenotype of tetrasomy 18p has been primarily delineated by published case series and reports. Findings reported in more than 25% of these cases include neonatal feeding problems, growth retardation, microcephaly, strabismus, muscle tone abnormalities, scoliosis/kyphosis, and variants on brain MRI. Developmental delays and cognitive impairment are universally present. The purpose of this study was to more fully describe tetrasomy 18p at both the genotypic and the phenotypic levels. Array CGH was performed on 43 samples from individuals with tetrasomy 18p diagnosed via routine karyotype. The medical records of 42 of these 43 individuals were reviewed. In order to gain additional phenotypic data, 31 individuals with tetrasomy 18p underwent a series of clinical evaluations at the Chromosome 18 Clinical Research Center. Results from the molecular analysis indicated that 42 of 43 samples analyzed had 4 copies of the entire p arm of chromosome 18; one individual was also trisomic for a section of proximal 18q. The results of the medical records review and clinical evaluations expand the phenotypic description of tetrasomy 18p to include neonatal jaundice and respiratory distress; recurrent otitis media; hearing loss; seizures; refractive errors; constipation and gastroesophageal reflux; cryptorchidism; heart defects; and foot anomalies. Additional findings identified in a small number of individuals include hernias, myelomeningocele, kidney defects, short stature, and failure to respond to growth hormone stimulation testing. Additionally, a profile of dysmorphic features is described. Lastly, a series of clinical evaluations to be considered for individuals with tetrasomy 18p is suggested.

Low grade mosaic for a complex supernumerary ring chromosome 18 in an adult patient with multiple congenital anomalies

Background

Several cases have been reported of patients with a ring chromosome 18 replacing one of the normal chromosomes 18. Less common are patients with a supernumerary ring chromosomes 18. High resolution whole genome examination in patients with multiple congenital abnormalities might reveal cytogenetic abnormalities of an unexpected complexity.

Results

We report a 24 years old male patient with lower spinal anomalies, hypospadia, bifid scrotum, cryptorchism, anal atresia, kidney stones, urethra anomalies, radial dysplasia, and a hypoplastic thumb. Some of the anomalies overlap with the VACTERL association. Chromosome analysis of cultured peripheral blood lymphocytes revealed an additional ring chromosome in 13% of the metaphases. Both parents had a normal karyotype, demonstrating the de novo origin of this ring chromosome. FISH analysis using whole chromosome paints showed that the additional chromosomal material was derived from chromosome 18. Chromosome analysis of cultured fibroblasts revealed only one cell with the supernumerary ring chromosome in the 400 analyzed. To characterize the ring chromosome in more detail peripheral blood derived DNA was analyzed using SNP-arrays. The array results indicated a 5 Mb gain of the pericentromeric region of chromosome 18q10-q11.2. FISH analysis using BAC-probes located in the region indicated the presence of 6 signals on the r(18) chromosome. In addition, microsatellite analysis demonstrated that the unique supernumerary ring chromosome was paternally derived and both normal copies showed biparental disomy.

Conclusions

We report on an adult patient with multiple congenital abnormalities who had in 13% of his cells a unique supernumerary ring chromosome 18 that was composed of 6 copies of the 5 Mb gene rich region of 18q11.

The inv dup (15) or idic (15) syndrome (Tetrasomy 15q)

The inv dup(15) or idic(15) syndrome displays distinctive clinical findings represented by early central hypotonia, developmental delay and intellectual disability, epilepsy, and autistic behaviour. Incidence at birth is estimated at 1 in 30,000 with a sex ratio of almost 1:1. Developmental delay and intellectual disability affect all individuals with inv dup(15) and are usually moderate to profound. Expressive language is absent or very poor and often echolalic. Comprehension is very limited and contextual. Intention to communicate is absent or very limited. The distinct behavioral disorder shown by children and adolescents has been widely described as autistic or autistic-like. Epilepsy with a wide variety of seizure types can occur in these individuals, with onset between 6 months and 9 years. Various EEG abnormalities have been described. Muscle hypotonia is observed in almost all individuals, associated, in most of them, with joint hyperextensibility and drooling. Facial dysmorphic features are absent or subtle, and major malformations are rare. Feeding difficulties are reported in the newborn period.

Chromosome region 15q11q13, known for its instability, is highly susceptible to clinically relevant genomic rearrangements, such as supernumerary marker chromosomes formed by the inverted duplication of proximal chromosome 15. Inv dup(15) results in tetrasomy 15p and partial tetrasomy 15q. The large rearrangements, containing the Prader-Willi/Angelman syndrome critical region (PWS/ASCR), are responsible for the inv dup(15) or idic(15) syndrome. Diagnosis is achieved by standard cytogenetics and FISH analysis, using probes both from proximal chromosome 15 and from the PWS/ASCR. Microsatellite analysis on parental DNA or methylation analysis on the proband DNA, are also needed to detect the parent-of-origin of the inv dup(15) chromosome. Array CGH has been shown to provide a powerful approach for identifying and detecting the extent of the duplication. The possible occurrence of double supernumerary isodicentric chromosomes derived from chromosome 15, resulting in partial hexasomy of the maternally inherited PWS/ASCR, should be considered in the differential diagnosis. Large idic(15) are nearly always sporadic. Antenatal diagnosis is possible. Management of inv dup(15) includes a comprehensive neurophysiologic and developmental evaluation. Survival is not significantly reduced.

The inv dup(15) or idic(15) syndrome can also be termed "tetrasomy 15q". About 160 patients have been reported in the medical literature [1-5].

Identification of a “cryptic mosaicism” involving at least four different small supernumerary marker chromosomes derived from chromosome 9 in a woman without reproductive success

OBJECTIVE

To characterize the small supernumerary marker chromosomes (sSMCs) present in the female member of an infertile couple who has no further clinical symptoms.

DESIGN

Case report.

SETTING(S)

Faculty of medicine and institute of human genetics and anthropology.

PATIENT(S)

A young, healthy, nonconsanguineous couple asked for genetic evaluation for infertility.

INTERVENTION(S)

Intracytoplasmic sperm injection, conventional and molecular cytogenetic analyses.

MAIN OUTCOME MEASURE(S)

We characterized the sSMCs present in a woman, who was a member of an infertile couple, by molecular cytogenetic techniques.

RESULT(S)

The G-banding technique showed that a marker chromosome was present in some of the examined cells describing the 47,XX,+mar[30]/46,XX[70] karyotype. Subsequently, using new fluorescence in situ hybridization (FISH) techniques, four distinguishable sSMCs (cryptic mosaicism), all derived from chromosome 9, were observed, including minute and ring chromosomes. This heterogeneity was impossible to detect by the conventional G-banding technique or conventional FISH technique that were used before the new FISH techniques (subcentromere-specific multicolor-FISH [subcenM-FISH]) and specific probe for the 9q12 band. In each metaphase with sSMCs, only one or two markers were observed. On the basis of the FISH analyses, the patient's karyotype was defined as 47,XX,+min(9)(:p12-->q12:)/47,XX,+min(9)(:p12-->q12::q12-->p12:)/47,XX,+r(9)(::p12-->q12::)/47,XX,+r(9)(::p12-->q12::p12-->q12::)x2/46,XX.

CONCLUSION(S)

The presence of sSMCs derived from chromosome 9 could influence the couple's infertility. The new subcenM-FISH techniques are very useful in the characterization of cryptic mosaicisms of marker chromosomes. Additionally, the hypothesis that the 9p12 chromosomal band is an euchromatic variant region without any phenotypic impact other than possible infertility is supported by this case study since the woman shows a normal phenotype.

Flow analysis and sorting of microchromosomes (<3 Mb)

BACKGROUND

The analysis and isolation of high numbers of chromosomes smaller than 3 Mb in size (microchromosomes) with good purity is dependent primarily on the detection sensitivity of the flow cytometer and the precision of the sort unit. The aim of this study was to investigate the capability of using a conventional flow cytometer for the detection and sorting at high purity microchromosomes with an estimated size of 2.7 Mb.

METHODS

Chromosomes were isolated from a human cell line containing a pair of X-derived microchromosomes, using a modified polyamine isolation buffer. The chromosome preparation was labeled with Hoechst and Chromomycin and analyzed and purified using a MoFlo sorter (DAKO) configured for high-speed sorting. The purity of the flow-sorted microchromosomes was assessed by reverse chromosome painting.

RESULTS

Improved resolution of the peak of microchromosomes in a bivariate plot of Hoechst versus Chromomycin fluorescence was obtainable after discriminating clumps and debris based on gating data within a FSC versus pulse width plot.

CONCLUSIONS

Chromosomes of smaller size, less than 3 Mb, can be detected with high resolution and flow-sorted with high purity using a conventional flow sorter.

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.

Accommodating chromosome inversions in linkage analysis

This work develops a population-genetics model for polymorphic chromosome inversions. The model precisely describes how an inversion changes the nature of and approach to linkage equilibrium. The work also describes algorithms and software for allele-frequency estimation and linkage analysis in the presence of an inversion. The linkage algorithms implemented in the software package Mendel estimate recombination parameters and calculate the posterior probability that each pedigree member carries the inversion. Application of Mendel to eight Centre d'Etude du Polymorphisme Humain pedigrees in a region containing a common inversion on 8p23 illustrates its potential for providing more-precise estimates of the location of an unmapped marker or trait gene. Our expanded cytogenetic analysis of these families further identifies inversion carriers and increases the evidence of linkage.

Characterization of mosaic supernumerary ring chromosomes by array-CGH: segmental aneusomy for proximal 4q in a child with tall stature and obesity

We report on a 6-year-old girl with developmental delay, tall stature, and obesity. G-banded chromosome analysis revealed mosaicism for one to three small de novo rings in 82% of peripheral lymphocytes. Fluorescence in situ hybridization (FISH) studies and metaphase comparative genomic hybridization (CGH) demonstrated that the rings were derived from 4q10-4q13. A higher resolution investigation was initiated using array-CGH analysis and revealed a gain of 11 adjacent clones spanning a 16 Mb region at 4q11-q13.2 and including the insulin-like growth factor binding protein 7 (IGFBP7) gene. This finding suggests that postnatal overgrowth observed in our patient might be related to a dosage effect of the IGFBP7 gene.

Identification, characterization and clinical implications of two markers detected at prenatal diagnosis

OBJECTIVES

Marker chromosomes are relatively rare in the general population as its identification at prenatal diagnosis. In this article, we identified and characterized two de novo supernumerary marker chromosomes in a mosaic form at prenatal diagnosis.

METHODS

The two cases presented were detected during prenatal diagnosis at 17 and 15 weeks of gestation. The analyses were performed due to the advanced maternal age. In both cases, parent's karyotypes were normal. The identification of the marker chromosomes was possible by FISH techniques.

RESULTS

One marker chromosome was derived from chromosome 5 and the other from chromosome 6. Both children are well at the moment.

CONCLUSION

The two cases described in the present paper, join to the ones already described in the literature. However these results are the first ones without any phenotypical anomalies, at least until the present. Every new characterization of marker chromosomes at prenatal diagnosis should be reported for determining a genotype-phenotype correlation, and thus be used for genetic counselling and risk evaluation.

Molecular cytogenetic characterization of a de novo supernumerary ring chromosome 7 resulting in partial trisomy, tetrasomy, and hexasomy in a child with dysmorphic signs, congenital heart defect, and developmental delay

We report on a girl with mosaicism (65%) of a de novo supernumerary ring chromosome 7. The main clinical features were delayed psychomotor development, congenital heart defect, facial dysmorphisms, and long hands, fingers, feet and toes. Molecular cytogenetic analysis revealed that the ring chromosome was duplicated in 20% of the analyzed metaphases with marker chromosome and quadruplicated in 5% thereof. Uniparental disomy (UPD) of the two normal sister chromosomes 7 was excluded. This is, to our knowledge, the first report of a partial tetrasomy to hexasomy due to a ring chromosome 7. Additionally, the ring evolution could be reconstructed according to the FISH-results.

Microscopy and image analysis

This unit provides an overview of light microscopy, including objectives, light sources, filters, film, and color photography for fluorescence microscopy and fluorescence in situ hybridization (FISH). Computerized image-analysis systems currently used in clinical cytogenetics are also discussed.

The inv dup(15) or idic(15) syndrome: a clinically recognisable neurogenetic disorder

The chromosome region 15q11q13 is known for its instability, and many rearrangements may occur in this imprinted segment: deletions associated either with Angelman syndrome (AS) or with Prader-Willi syndrome (PWS), according to parental origin; translocations; inversions; and supernumerary marker chromosomes formed by the inverted duplication of proximal chromosome 15. Inv dup(15) constitute the most common of the heterogeneous group of the extra structurally abnormal chromosomes, and their presence results in tetrasomy 15p and partial tetrasomy 15q. Inv dup(15), containing the Prader-Willi/Angelman syndrome region, are associated with altered behaviour, developmental delay/mental retardation, and seizures/epilepsy. Clinicians should suspect this syndrome in any infant/child with early central hypotonia, minor dysmorphic features, developmental delay, autism or autistic-like behaviour, and who subsequently develops hard to control seizures/epilepsy. Diagnosis is confirmed by standard cytogenetic techniques and FISH analysis. Although, about 100 cases have been reported to date, limited data are available on the natural history. To obtain better information on diagnosis and outcome in a clinical setting, we reviewed the available literature on clinical and behavioural phenotype of inv dup(15) syndrome.

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.

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.

Mosaic trisomy (8)(p22 --> pter) in a fetus caused by a supernumerary marker chromosome without alphoid sequences

OBJECTIVE

Our objective was to characterise a marker chromosome in cultured amniocytes of a fetus with a mos 47,XX,+mar[3]/46,XX[14] karyotype.

METHODS

The indication for prenatal cytogenetic analysis of cultured amniocytes was advanced maternal age. Classic banding techniques (GTG- and C-banding) were performed. Microdissection combined with reverse painting was used to disclose the exact origin of the marker; the result was confirmed by chromosome painting and FISH with band-specific probes.

RESULTS

Analysis of GTG-banded chromosomes showed a small marker chromosome in 3 of the 17 colonies analysed. Subsequently, C-banding showed no alphoid sequences, suggesting the presence of a neocentromere. The parent's karyotypes were normal. After normal ultrasound findings, the parents decided to continue the pregnancy. Chromosome analysis in peripheral blood after birth demonstrated that the marker chromosome was present in 50% of the lymphocytes. Using microFISH, the marker was further characterised and appeared to be derived from chromosome region (8)(p22 --> pter).

CONCLUSION

Accurate identification of the marker chromosome was very important for prenatal counselling. Combining the results of GTG- and C-banding analysis with the results of the (micro)FISH, we concluded that the patient's karyotype is: mos 47,XX,+mar.rev ish der(8)(p22 --> pter)[50]/46,XX[50].

Molecular cytogenetic characterization of an insertional translocation, ins(6;7)(p25;q33q34): Deletion/duplication of 7q33-34 and clinical correlations

A balanced insertional translocation between chromosomes 6 and 7, ins(6;7)(p25;q33q34) has been extensively investigated. The insertional translocation was found in several members of a three-generation family, where some were healthy balanced carriers while others had clinical symptoms due to deletion or duplication of 7q33-34. The deleted/duplicated segment could only be detected using high resolution banding and fluorescent in situ hybridization. A number of BAC/PAC clones located on chromosome 6 and 7 were used to characterize the breakpoint regions in detail and to determine the size of the deletion, which was 7.6 Mb, containing up to 68 genes. However, the insert on chromosome 6 was only 7.4 Mb, due to a deletion of 227 kb at the distal breakpoint on 7q. This small deletion was also found in the "balanced" carriers, and although the chromosome segment contains at least eight genes, none of the carriers seem to be affected by haploinsufficiency, since the phenotype is apparently normal. This is the first detailed characterization and phenotype correlation of such a deletion/duplication of distal 7q.

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.

Identification and Molecular Characterization of a de novo Supernumerary Ring Chromosome 18 in a Patient with Klippel-Trenaunay Syndrome

Klippel-Trenaunay syndrome (KTS) is a congenital vascular disorder comprised of capillary, venous and lymphatic malformations associated with overgrowth of the affected tissues. In this study, we report the identification of a de novo supernumerary ring chromosome in a patient with mild mental retardation, long tapering fingers, elongated, thin feet and Klippel-Trenaunay syndrome (KTS). The ring marker chromosome was found to be mosaic, present in 24% of cells, and was later shown to be derived from chromosome 18, r(18). Fluorescence in situ hybridization (FISH) was used to define the breakpoints involved in the formation of r(18). The chromosome 18p breakpoint was localized between the markers WI-9619 and D18S1150, which is less than 10 cM to the centromere. The 18q breakpoint was localized between the centromere and BAC clone 666n19, which is a region of less than 40 kb. These data suggest that the r(18) mostly originated from 18p, with an estimated size of less than 10 cM. These studies identify and characterize a new marker chromosome 18, provide insights into the understanding of the relationships between the clinical phenotypes and marker chromosomes, and establish a framework for finding a potential vascular and/or overgrowth gene located on chromosome 18.

A patient with Prader-Willi syndrome and a supernumerary marker chromosome r(15)(q11.1-13p11.1)pat and maternal heterodisomy

We report on a Prader-Willi patient with a de novo supernumerary marker chromosome (SMC) in 16% of the cells. The SMC was a ring chromosome and it included the PWS/AS critical region as was demonstrated by FISH. Segregation analysis indicated that the SMC originated from a paternal chromosome 15 and the two normal chromosomes 15 of the patients were of the maternal homologues. Namely, the patient had maternal heterodisomy in 85% of the cells and triplication of the PWS/AS region in 15% of the cells. The Prader-Willi features were the result of the low mosaicism of the SMC. The evolution of the maternal heterodisomy and the SMC were two unrelated events, the occurrence of both events in the same embryo rescued it from lethality.

Characterization of psu dic(6;5)(p21.3;q13) with reverse chromosome painting in a patient with secondary myelodysplastic syndrome following treatment for multiple myeloma

We report a case of a psu dic(6;5)(p21.3;q13) in a patient with secondary myelodysplastic syndrome (sMDS) following treatment for multiple myeloma. The abnormal chromosome was isolated by flow karyotyping and initially identified by reverse chromosome painting. The findings were then confirmed by forward painting. The value of flow karyotyping as a diagnostic technique in hematologic malignancies is discussed.

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.

Diagnostic evaluation of developmental delay/mental retardation: An overview

Mental retardation (MR) is one of the few clinically important disorders for which the etiopathogenesis is still poorly understood. It is a condition of great concern for public health and society. MR is currently defined as a significant impairment of cognitive and adaptive functions, with onset before age 18 years. It may become evident during infancy or early childhood as developmental delay (DD), but it is best diagnosed during the school years. MR is estimated to occur in 1-10% of the population, and research on its etiology has always been a challenge in medicine. The etiopathogenesis encompasses so many different entities that the attending physician can sometimes feel a "virtual panic," starting a wide-range diagnostic evaluation. The Consensus Conference of the American College of Medical Genetics has recently established guidelines regarding the evaluation of patients with MR [Curry et al., 1997], emphasizing the high diagnostic utility of cytogenetic studies and neuroimaging in certain clinical settings. However, since then there has been substantial progress in molecular cytogenetics and neuroimaging techniques, the use of which has allowed recognition and definition of new disorders, thus increasing the diagnostic yield. This review will focus on the most appropriate investigations shown to be, at present, necessary to define the etiology of DD/MR, in the context of recommendations for the clinical evaluation of the patient with undiagnosed MR.

Double supernumerary isodicentric chromosomes derived from 15 resulting in partial hexasomy

We report two unrelated patients each with two supernumerary marker chromosomes (SMCs) derived from chromosome 15, and thus resulting in partial hexasomy. Hexasomy in the one case (family 1) was diagnosed at prenatal diagnosis and did not include the Prader-Willi/Angelman critical region (PWACR). The double SMCs were also found in the mother, the pregnancy continued to term, and an apparently phenotypically normal child was born. This represents the first report of transmission of double SMCs from mother to child. In the second case (family 2), the hexasomy did include the PWACR and was de novo in origin. This patient manifested severe psychomotor retardation, clefting of the soft palate, hypotonia, seizure-like episodes, and other phenotypic features. The aberrant phenotype is attributable to the hexasomy for the PWACR gene loci. The normal homologs of chromosome 15 proved to be biparental in origin while the two SMCs appeared maternal.

Prenatal diagnosis: molecular genetics and cytogenetics

The technologies developed for the Human Genome Project, the recent surge of available DNA sequences resulting from it and the increasing pace of gene discoveries and characterization have all contributed to new technical platforms that have enhanced the spectrum of disorders that can be diagnosed prenatally. The importance of determining the disease-causing mutation or the informativeness of linked genetic markers before embarking upon a DNA-based prenatal diagnosis is, however, still emphasized. Different fluorescence in situ hybridization (FISH) technologies provide increased resolution for the elucidation of structural chromosome abnormalities that cannot be resolved by more conventional cytogenetic analyses, including microdeletion syndromes, cryptic or subtle duplications and translocations, complex rearrangements involving many chromosomes, and marker chromosomes. Interphase FISH and the quantitative fluorescence polymerase chain reaction are efficient tools for the rapid prenatal diagnosis of selected aneuploidies, the latter being considered to be most cost-effective if analyses are performed on a large scale. There is some debate surrounding whether this approach should be employed as an adjunct to karyotyping or whether it should be used as a stand-alone test in selected groups of women.

Supernumerary marker chromosomes derived from chromosome 15: analysis of 32 new cases

Small supernumerary marker chromosomes (SMC) are a heterogeneous group of chromosomes with an estimated frequency of approximately 0.14-0.72 per 1000 newborns and higher frequencies in particular populations such as the mentally retarded or infertile males. With a frequency of about 50%, derivatives of chromosome 15 represent the most common SMC. Here we present the results of a detailed analysis of 32 SMC(15) carriers who were ascertained in pre- or post-natal routine cytogenetic diagnostics. SMC(15) with euchromatic content led to mental and psychomotor retardation. In contrast, SMC(15) without euchromatin were found to have no influence on the carrier's phenotype but were detected with a high incidence among infertile males. The majority of SMC(15) are pseudodicentric homologous rearrangements. Based on our investigations a further characterization of der(15) was possible.

Prenatal diagnosis of partial tetrasomy 14: a case study

Prenatal specimens were received from a fetus with abnormalities noted on ultrasound. A supernumerary marker chromosome (SMC) was detected: 47,XY,+mar. Fluorescence in situ hybridisation (FISH) further classified this to be partial tetrasomy for chromosome 14. We compare this finding with other cases of SMC (14) and further classify phenotype with karyotype.

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.

Segmental duplications: an 'expanding' role in genomic instability and disease

The knowledge that specific genetic diseases are caused by recurrent chromosomal aberrations has indicated that genomic instability might be directly related to the structure of the regions involved. The sequencing of the human genome has directed significant attention towards understanding the molecular basis of such recombination 'hot spots'. Segmental duplications have emerged as a significant factor in the aetiology of disorders that are caused by abnormal gene dosage. These observations bring us closer to understanding the mechanisms and consequences of genomic rearrangement.

Segregation of a supernumerary del(15) marker chromosome in sperm

Supernumerary marker chromosomes (SMC) can be associated with both normal and abnormal phenotypes. In addition, SMC are found at higher frequency in males with infertility. We identified a SMC, characterized as a del(15)(q11.2) chromosome, in a phenotypically normal male. Using fluorescence in situ hybridization (FISH), we examined the segregation of the del(15) chromosome in sperm from this patient. Only 6.23% of sperm nuclei showed disomy using a chromosome 15 alpha-satellite FISH probe, instead of the expected 50%. In addition, FISH analysis showed no increase for non-disjunction of chromosome 18, excluding an interchromosomal effect for this chromosome. The significant decrease in sperm bearing the del(15) may be due to tissue-specific mosaicism or a result of some form of selection against the del(15) during spermatogenesis. This finding provides a basis for the observation that SMC(15) are less likely to be inherited from a paternal carrier.

Structure of chromosomal duplicons and their role in mediating human genomic disorders

Chromosome-specific low-copy repeats, or duplicons, occur in multiple regions of the human genome. Homologous recombination between different duplicon copies leads to chromosomal rearrangements, such as deletions, duplications, inversions, and inverted duplications, depending on the orientation of the recombining duplicons. When such rearrangements cause dosage imbalance of a developmentally important gene(s), genetic diseases now termed genomic disorders result, at a frequency of 0.7-1/1000 births. Duplicons can have simple or very complex structures, with variation in copy number from 2 to >10 repeats, and each varying in size from a few kilobases in length to hundreds of kilobases. Analysis of the different duplicons involved in human genomic disorders identifies features that may predispose to recombination, including large size and high sequence identity between the recombining copies, putative recombination promoting features, and the presence of multiple genes/pseudogenes that may include genes expressed in germ cells. Most of the chromosome rearrangements involve duplicons near pericentromeric regions, which may relate to the propensity of such regions to accumulate duplicons. Detailed analyses of the structure, polymorphic variation, and mechanisms of recombination in genomic disorders, as well as the evolutionary origin of various duplicons will further our understanding of the structure, function, and fluidity of the human genome.

Symptomatic generalized epilepsy associated with an inverted duplication of chromosome 15

An inverted duplication of chromosome 15 (inv dup[15] chromosome) is the most common supernumerary marker chromosome in humans. Inv dup(15) chromosomes are commonly associated with mental retardation, epilepsy, behavioral problems and structural malformations. Though epilepsies associated with inv dup(15) chromosomes are often intractable, there have been very few reports regarding the seizure manifestations or types. We report a patient with severe mental retardation and intractable epilepsy, associated with an inv dup(15) chromosome. The seizures recorded with EEG-VTR monitoring were axial and generalized tonic seizures, and our case was diagnosed as symptomatic generalized epilepsy. Molecular and cytogenetic analysis showed an inv dup(15) chromosome containing the Prader-Willi syndrome/Angelman syndrome region mapped within bands 15q 11-q13.

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.

Maternal isodisomy of chromosome 9 with no impact on the phenotype in a woman with two isochromosomes: i(9p) and i(9q)

We describe a 34-year-old healthy woman with isochromosomes for the short and long arm of chromosome 9 who was ascertained because of repeated spontaneous abortions. Molecular analysis demonstrated maternal uniparental isodisomy for the whole chromosome 9, thus the origin of the isochromosomes was maternal. Because the patient had a normal phenotype, the maternal isodisomy supports the previous assumption that there are no maternally imprinted genes on chromosome 9.