Preferential maternal derivation in inv dup(15): analysis of eight new cases

Rare partial trisomy and tetrasomy of 15q11-q13 associated with developmental delay and autism spectrum disorder

Background

Small supernumerary marker chromosomes (sSMCs), are additional abnormal chromosomes, which can’t be detected accurately by banding cytogenetic analysis. Abnormal phenotypes were observed in about 30% of SMC carriers. Duplication of chromosome 15 and related disorders, characterized by hypotonia motor delays, autism spectrum disorder (ASD), intellectual disability, and epilepsy including infantile spasms, might be account for 50% of the total sSMCs.

Case presentation

An 11-month-old infant with an sSMC found by banding cytogenetics was referred to our clinic because of developmental retardation and autism spectrum disorder. After several months of rehabilitation treatment, the progress of motor development was obvious, but the consciousness was still far from satisfied. High-resolution karyotype analysis, multiplex ligation-dependent probe amplification and copy number variation sequencing (CNV-Seq) were conducted to confirm the identity of the sSMC. A bisatellited dicentric sSMC was observed clearly in high-resolution karyotype analysis and a 10.16-Mb duplication of 15q11.1q13.2 (3.96 copies) together with a 1.84-Mb duplication of 15q13.2q13.3 (3 copies) was showed by CNV-Seq in the proband. It suggested that the molecular cytogenetic karyotype was 47,XY,+dic(15;15)(q13.2;q13.3). Furthermore, the clinical symptoms of the proband mostly fit 15q duplication related disorders which are characterized by hypotonia motor delays, autism spectrum disorder (ASD), and intellectual disability.

Conclusion

We reported for the first time using CNV-Seq to detect sSMCs and find a partial trisomy and tetrasomy of 15q11-q13 associated with developmental delay and autism spectrum disorder. Our report indicates that CNV-seq is a useful and economical way for diagnosis of dup15q and related disorders.

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.

De novo unbalanced translocations in Prader-Willi and Angelman syndrome might be the reciprocal product of inv dup(15)s

The 15q11-q13 region is characterized by high instability, caused by the presence of several paralogous segmental duplications. Although most mechanisms dealing with cryptic deletions and amplifications have been at least partly characterized, little is known about the rare translocations involving this region. We characterized at the molecular level five unbalanced translocations, including a jumping one, having most of 15q transposed to the end of another chromosome, whereas the der(15)(pter->q11-q13) was missing. Imbalances were associated either with Prader-Willi or Angelman syndrome. Array-CGH demonstrated the absence of any copy number changes in the recipient chromosome in three cases, while one carried a cryptic terminal deletion and another a large terminal deletion, already diagnosed by classical cytogenetics. We cloned the breakpoint junctions in two cases, whereas cloning was impaired by complex regional genomic architecture and mosaicism in the others. Our results strongly indicate that some of our translocations originated through a prezygotic/postzygotic two-hit mechanism starting with the formation of an acentric 15qter->q1::q1->qter representing the reciprocal product of the inv dup(15) supernumerary marker chromosome. An embryo with such an acentric chromosome plus a normal chromosome 15 inherited from the other parent could survive only if partial trisomy 15 rescue would occur through elimination of part of the acentric chromosome, stabilization of the remaining portion with telomere capture, and formation of a derivative chromosome. All these events likely do not happen concurrently in a single cell but are rather the result of successive stabilization attempts occurring in different cells of which only the fittest will finally survive. Accordingly, jumping translocations might represent successful rescue attempts in different cells rather than transfer of the same 15q portion to different chromosomes. We also hypothesize that neocentromerization of the original acentric chromosome during early embryogenesis may be required to avoid its loss before cell survival is finally assured.

Genetics of epilepsy: an overview

Studies of the genetics of epilepsy have, until recently, involved epidemiologic or segregation analyses of phenotypic characteristics of a number of seizure disorders. Technical advances in molecular biology involving gene mapping and gene identification have made it possible to examine the heritability of various epilepsy syndromes. Using "reverse genetics" or positional cloning, it is possible to identify an abnormal protein through gene isolation and cloning. Genes are localized through analysis of linkage to phenotypic markers (proteins) or DNA markers such as restriction fragment length polymorphisms, variable number of tandem repeats, and dinucleotides. Methods used to obtain DNA of interest involve digestion of genomic DNA with specific restriction endonucleases or amplification of DNA by polymerase chain reaction technology. Gel electrophoresis is the basis for the separation of different sized DNA. Inherited disorders for which a gene has been cloned or localized have highly penetrant, well-defined clinical phenotypes with no remissions and abundant clinical material. Genetic epilepsies, however, are variably penetrant age-dependent disorders with heterogeneous clinical phenotypes. Despite these difficulties, three genetic epilepsies have been mapped to specific chromosomes: benign familial neonatal convulsions to 20q, juvenile myoclonic epilepsy to 6p, and Baltic progressive myoclonus epilepsy to 21q. Further progress in understanding genetic epilepsies will depend on better definition of syndrome phenotypes, isolation of the epilepsy gene(s), and identification of the abnormal protein(s).

Microdeletion/duplication at 15q13.2q13.3 among individuals with features of autism and other neuropsychiatric disorders

BACKGROUND

Segmental duplications at breakpoints (BP4-BP5) of chromosome 15q13.2q13.3 mediate a recurrent genomic imbalance syndrome associated with mental retardation, epilepsy, and/or electroencephalogram (EEG) abnormalities.

PATIENTS

DNA samples from 1445 unrelated patients submitted consecutively for clinical array comparative genomic hybridisation (CGH) testing at Children's Hospital Boston and DNA samples from 1441 individuals with autism from 751 families in the Autism Genetic Resource Exchange (AGRE) repository.

RESULTS

We report the clinical features of five patients with a BP4-BP5 deletion, three with a BP4-BP5 duplication, and two with an overlapping but smaller duplication identified by whole genome high resolution oligonucleotide array CGH. These BP4-BP5 deletion cases exhibit minor dysmorphic features, significant expressive language deficits, and a spectrum of neuropsychiatric impairments that include autism spectrum disorder, attention deficit hyperactivity disorder, anxiety disorder, and mood disorder. Cognitive impairment varied from moderate mental retardation to normal IQ with learning disability. BP4-BP5 covers approximately 1.5 Mb (chr15:28.719-30.298 Mb) and includes six reference genes and 1 miRNA gene, while the smaller duplications cover approximately 500 kb (chr15:28.902-29.404 Mb) and contain three reference genes and one miRNA gene. The BP4-BP5 deletion and duplication events span CHRNA7, a candidate gene for seizures. However, none of these individuals reported here have epilepsy, although two have an abnormal EEG.

CONCLUSIONS

The phenotype of chromosome 15q13.2q13.3 BP4-BP5 microdeletion/duplication syndrome may include features of autism spectrum disorder, a variety of neuropsychiatric disorders, and cognitive impairment. Recognition of this broader phenotype has implications for clinical diagnostic testing and efforts to understand the underlying aetiology of this syndrome.

Duplication of 15q11.2-q14, including the P gene, in a woman with generalized skin hyperpigmentation

We describe a woman with 15q11.2-q14 duplication who had clinical manifestations of proximal 15q trisomy and hyperpigmentation. Within this region, the P gene, located at chromosome segment 15q11.2-q12, is associated with oculocutaneous albinism type II (OCA2) and with hypopigmentation in the Prader-Willi and Angelman chromosome 15q deletion syndromes. We therefore hypothesized that in this woman skin hyperpigmentation might result from a duplication of the P gene. We carried out chromosomal and interphase fluorescence in situ hybridization (I-FISH) analyses, and determined that the P gene is duplicated in this woman. Our findings demonstrate that trisomy of the P gene can be associated with skin hyperpigmentation.

Molecular characterisation of four cases of intrachromosomal triplication of chromosome 15q11-q14

CONTEXT

Chromosomal abnormalities that involve the proximal region of chromosome 15q occur relatively frequently in the human population. However, interstitial triplications involving one 15 homologue are very rare with three cases reported to date.

OBJECTIVE

To provide a detailed molecular characterisation of four additional patients with interstitial triplications of chromosome 15q11-q14.

DESIGN

Molecular analyses were performed using DNA markers and probes specific for the 15q11-q14 region.

SETTING

Molecular cytogenetics laboratory at the University of Chicago.

SUBJECTS

Four patients with mild to severe mental retardation and features of Prader-Willi syndrome (PWS) or Angelman syndrome (AS) were referred for molecular cytogenetic analysis following identification of a suspected duplication/triplication of chromosome 15q11-q14 by routine cytogenetic analysis.

MAIN OUTCOME MEASURES

Fluorescence in situ hybridisation (FISH) was performed to determine the type of chromosomal abnormality present, the extent of the abnormal region, and the orientation of the extra chromosomal segments. Molecular polymorphism analysis was performed to determine the parental origin of the abnormality. Methylation and northern blot analyses of the SNRPN gene were performed to determine the effect of extra copies of the SNRPN gene on its methylation pattern and expression.

RESULTS

Fluorescence in situ hybridisation (FISH) using probes within and flanking the Prader-Willi/Angelman syndrome critical region indicated that all patients carried an intrachromosomal triplication of proximal 15q11-q14 in one of the two chromosome 15 homologues (trip(15)). In all patients the orientation of the triplicated segments was normal-inverted-normal, suggesting that a common mechanism of rearrangement may have been involved. Microsatellite analysis showed the parental origin of the trip(15) to be maternal in three cases and paternal in one case. The paternal triplication patient had features similar to PWS, one maternal triplication patient had features similar to AS, and the other two maternal triplication patients had non-specific findings including hypotonia and mental retardation. Methylation analysis at exon 1 of the SNRPN locus showed increased dosage of either the paternal or maternal bands in the paternal or maternal triplication patients, respectively, suggesting that the methylation pattern shows a dose dependent increase that correlates with the parental origin of the triplication. In addition, the expression of SNRPN was analysed by northern blotting and expression levels were consistent with dosage and parental origin of the triplication.

CONCLUSIONS

These four additional cases of trip(15) will provide additional information towards understanding the phenotypic effects of this abnormality and aid in understanding the mechanism of formation of other chromosome 15 rearrangements.

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.

Cytogenetic characterization of an extra structurally abnormal chromosome associated with severe mental retardation: inv dup (15) (q13)

We have studied an extra structually abnormal chromosome (ESAC) in a 13 years old boy with profound mental, psychomotor and speech retardation, behavioral problems, seizures and abnormal electroencephalogram. The examination of the bisatellited ESAC with chromosome banding demonstrated that the karyotype was: 47, XY, +inv dup (15) (pter-->q13::q13-->pter). The cytogenetic characterization of the inv dup (15) is reported with special emphasis on the usefulness of DA/DAPI staining when G-banding is sequentially performed to discard possible heteromorphisms in DA/DAPI positive chromosomes, and the importance of Ag-NOR heteromorphisms to ascertain the maternal origin of the inv dup (15). A U-type exchange between two non-sister chromatids is proposed as its mechanism of formation. The clinical features of the case were consistent with those previously reported in similar cases.

Molecular cytogenetic evidence for a common breakpoint in the largest inverted duplications of chromosome 15

Chromosomes from 20 patients were used to delineate the breakpoints of inverted duplications of chromosome 15 (inv dup[15]) that include the Prader-Willi syndrome/Angelman syndrome (PWS/AS) chromosomal region (15q11-q13). YAC and cosmid clones from 15q11-q14 were used for FISH analysis, to detect the presence or absence of material on each inv dup(15). We describe two types of inv dup(15): those that break between D15S12 and D15S24, near the distal boundary of the PWS/AS chromosomal region, and those that share a breakpoint immediately proximal to D15S1010. Among the latter group, no breakpoint heterogeneity could be detected with the available probes, and one YAC (810f11) showed a reduced signal on each inv dup(15), compared with that on normal chromosomes 15. The lack of breakpoint heterogeneity may be the result of a U-type exchange involving particular sequences on either homologous chromosomes or sister chromatids. Parent-of-origin studies revealed that, in all the cases analyzed, the inv dup(15) was maternal in origin.

Autism and maternally derived aberrations of chromosome 15q

Of the chronic mental disabilities of childhood, autism is causally least well understood. The former view that autism was rooted in exposure to humorless and perfectionistic parenting has given way to the notion that genetic influences are dominant underlying factors. Still, identification of specific heritable factors has been slow with causes identified in only a few cases in unselected series. A broad search for genetic and environmental influences that cause or predispose to autism is the major thrust of the South Carolina Autism Project. Among the first 100 cases enrolled in the project, abnormalities of chromosome 15 have emerged as the single most common cause. The four abnormalities identified include deletions and duplications of proximal 15q. Other chromosome aberrations seen in single cases include a balanced 13;16 translocation, a pericentric inversion 12, a deletion of 20p, and a ring 7. Candidate genes involved in the 15q region affected by duplication and deletion include the ubiquitin-protein ligase (UBE3A) gene responsible for Angelman syndrome and genes for three GABA(A) receptor subunits. In all cases, the deletions or duplications occurred on the chromosome inherited from the mother.

Maternal origin of inv dup(15) chromosomes in infantile autism

Six male patients with infantile autism and an extra inverted duplicated chromosome 15[inv dup(15)] were reported in a previous study. These patients had four copies of the chromosome region 15pter-q13, or an inv dup(15)(pter-->q13; q13-->pter). In this new study, DNA from the families of four of the patients were analysed using Southern based RFLPs and microsatellite polymorphisms from the region. In all four cases the inv dup(15) chromosome was of maternal origin. Furthermore, the data suggests that it originated in the maternal meiotic process rather than in an early mitosis in the developmental process of the embryo. The extra chromosome contained material from both of the maternally derived 15-chromosomes. Based on the molecular data presented here, a model for the origin of chromosome markers of this type is proposed.

Cytogenetic and molecular analysis of inv dup(15) chromosomes observed in two patients with autistic disorder and mental retardation

A variety of distinct phenotypes has been associated with supernumerary inv dup(15) chromosomes. Although different cytogenetic rearrangements have been associated with distinguishable clinical syndromes, precise genotype-phenotype correlations have not been determined. However, the availability of chromosome 15 DNA markers provides a means to characterize inv dup(15) chromosomes in detail to facilitate the determination of specific genotype-phenotype associations. We describe 2 patients with an autistic disorder, mental retardation, developmental delay, seizures, and supernumerary inv dup(15) chromosomes. Conventional and molecular cytogenetic studies confirmed the chromosomal origin of the supernumerary chromosomes and showed that the duplicated region extended to at least band 15q13. An analysis of chromosome 15 microsatellite CA polymorphisms suggested a maternal origin of the inv dup(15) chromosomes and biparental inheritance of the two intact chromosome 15 homologs. The results of this study add to the existing literature which suggests that the clinical phenotype of patients with a supernumerary inv dup(15) chromosome is determined not only by the extent of the duplicated region, but by the dosage of genes located within band 15q13 and the origin of the normal chromosomes 15.

Prenatal diagnosis of the derivative chromosome 22 associated with cat eye syndrome by fluorescence in situ hybridization

Cytogenetic studies of cultured amniocytes demonstrated a karyotype of 46,XX/47,XX, +mar. A bisatellited, dicentric, distamycin-DAPI negative, NOR-positive marker was present in 76 per cent of the metaphases examined. Similar markers have been associated with cat eye syndrome (CES). We report on the utilization of fluorescence in situ hybridization (FISH) with a 14/22 alpha-satellite probe and a chromosome 22-specific cosmid for locus D22S9 to determine the origin of the prenatally detected supernumerary marker chromosome. FISH studies demonstrated that the marker is a derivative of chromosome 22 and enabled us to provide the family with additional prognostic information.

Inv dup(15): contribution to the clinical definition of phenotype

One of the primary goals in medical genetics is a precise clinical definition of chromosomal diseases. This is now possible because of the increased number of case reports and new techniques. A male patient, without a clear-cut syndrome, was cytogenetically investigated. Chromosomal analysis showed a small unidentified bisatellited supernumerary marker. In situ hybridization with a biotin-labeled DNA probe for the chromosome 15 centromere (D15Z1) demonstrated that the marker was derived from chromosome 15. Hybridization with the Prader-Willi Syndrome Cosmid biotinylated probe (localized to band 15q11-q13) showed a signal on both ends suggesting a marker with a symmetrical inv dup(15) and a breakpoint localized in q13. It was then possible to define the karyotype as: 47,XY,+ inv dup(15) (pter-q13::q13-pter). All cases of inv dup(15) reported in the literature were reviewed, paying particular attention to the different breakpoints involved, in order to provide a better clinical definition of this syndrome.

Autism associated with tetrasomy 15: A further report

Association of autism with tetrasomy of chromosome 15 has recently been described in six males. In this report, we describe the occurrence of autism in a girl with tetrasomy of chromosome 15. The patient showed hyperactivity, hand-flapping, short-stature, eye abnormalities, and hypotonia, which have been reported in males with tetrasomy of chromosome 15. This suggests that autism may be associated in both sexes with a distinct syndrome characterized by tetrasomy of chromosome 15, mental retardation and characteristic physical features.

Molecular cytogenetic and clinical studies of 42 patients with marker chromosomes

The molecular cytogenetic characterization and clinical details of 20 patients with marker chromosomes are presented. These 20 patients, together with another 22 patients previously published, represent a cohort in which the chromosomal origin of the marker chromosomes was successfully determined in all but one case. Examination of the pooled data suggests that the satellited markers derived from chromosomes 14, 15 (when metacentric or submetacentric), those whose origin is either 13 or 21, and those small ring autosomal markers derived from both alphoid and satellite II or III pericentric heterochromatin of chromosomes 1, 9, 15, and 16 are all associated with a low risk of phenotypic abnormality. The markers identified as i(18p), ring chromosomes derived from various autosomes, and satellited markers derived from chromosome 22 are associated with a high risk of phenotypic abnormality. The phenotype of patients with acrocentric markers derived from chromosome 15 was equivocal, perhaps as a result of imprinting. Additional data are required to confirm these trends. The mild mental retardation and abnormal face of a patient with a small ring chromosome derived from chromosome 4 are described. Identification of patients with small rings originating from particular chromosomes may allow the recognition of new syndromes.

Unique karyotypes in two patients with Prader-Willi syndrome

A physical disruption of the Prader-Willi syndrome (PWS) chromosome region is thought to cause PWS. We describe 2 girls with PWS phenotype, who had unique chromosome 15 abnormalities. The first patient showed mosaicism: 45,XX,t(15;15)(qter----p11.1::q11.200----qter)/46,XX,t(15;15)(qter----p1 1.1::q 11.200----qter), +mar. The band 15q11.2 apparently remained intact in the t(15;15) chromosome, and the mar chromosome was considered as r(15) (p11.1q11.1). The second patient had a karyotype of 47,XX,del(15)(q11.200----q11.207), +idic (15)(pter----q11.1::q11.1----pter). The complex breakage and reunion involving the 15q11.2 regions of the father's homologous chromosomes 15 at meiosis appeared to have resulted in the idic(15) and the del(15) chromosomes. These cytogenetic findings suggest that the PWS chromosome region may be localized on the very proximal portion of band 15q11.2.

De novo (15;21) unbalanced translocation of paternal origin in a girl with Prader-Willi syndrome

We describe a 3-month-old girl with Prader-Willi syndrome and a de novo unbalanced karyotype 45,XX,t(15;21)(q13;q22.3). This rearrangement, resulting in monosomy for the pericentromeric region of chromosome 15 and a virtual monosomy for the 21q distal band, had a paternal origin as demonstrated by Q and NOR staining.

De novo inverted interstitial ("mirror") duplication of chromosome 8(q13----q24.1) in a liveborn male

We report on a newborn boy with a de novo inverted interstitial duplication of chromosome 8(q13----q24.1). This form of cytogenetic abnormality, in which a mirror image interstitial duplication has occurred, is exceedingly rare. Review of the literature and mechanisms to explain the origin of this type of chromosome aberration are presented. A review of the findings from individuals with partial dup(8q) demonstrate remarkable similarity to the infant we describe.

Comparison of the 15q deletions in Prader-Willi and Angelman syndromes: specific regions, extent of deletions, parental origin, and clinical consequences

It has recently been shown that apparently similar deletions of chromosome 15q occur commonly in the Prader-Willi and Angelman syndromes. The distinctness of the syndromes suggests that the deletions are not identical. To address this possibility, the specific bands involved and the sizes of the deletions were compared in seven patients with Prader-Willi syndrome and 10 patients with Angelman syndrome using high-resolution G-, Q-, and fluorescent R-banding techniques. The parental origin of the nine cases of Angelman syndrome for which parents were available for study was determined. The same proximal band was deleted (q11.2) in both syndromes. In general, the deletion in patients with Angelman syndrome was larger, though variable, and included bands q12 and part of q13. All of the studied deletions in patients with Angelman syndrome were of maternal origin. This contrasts with the predominant paternal origin of the deletion in patients with Prader-Willi syndrome. Two possible reasons for these observations are postulated: 1) the deleted regions are different at the cytologic and/or molecular level because of different exchange points in meiosis in males and females or to different mechanisms of breakage in males and females, resulting in differing breakpoints; 2) the deleted regions are essentially the same, but differential expression of the genes in the homologous chromosome 15 has occurred (imprinting).

Restriction fragment length polymorphisms within proximal 15q and their use in molecular cytogenetics and the Prader-Willi syndrome

Restriction fragment length polymorphisms (RFLPs) are described in detail for 6 DNA probes (D15S9-13, D15S18) that localize to the proximal long arm of human chromosome 15 (15q11-15q13: this report and Tantravahi et al., Am. J. Med. Genet. 33:78-87. Multiple RFLPs are detected by the probe that identifies locus D15S13, and these RFLPs are shown by genomic mapping to result from a nearby insertion or deletion of 1.8 kilobases (kb) of DNA. This set of RFLPs detected by proximal 15q probes can be used for studies on the Prader-Willi syndrome (PWS) and on mentally retarded individuals with a supernumerary inv dup(15) chromosome. Five of the polymorphic loci (D15S9-13) map to the region implicated in the cause of the PWS (15q11.2-15q12). Each of 4 families tested with these probes, as well as an additional "PWS-like" patient, was informative by RFLP analysis. The two PWS deletions studied, which occurred de novo, were inherited from the chromosome 15 provided by the father. By contrast, the 2 inv dup(15) chromosomes analyzed were of maternal origin. The use of RFLPs can also simplify the molecular determination of copy number in chromosomal aneuploidy, as exemplified by analysis of individuals with the PWS and a deletion, patients with an inv dup(15), and one patient with a more complex rearrangement involving chromosome 15. Our studies demonstrate the application of DNA probes for both molecular cytogenetic studies on this chromosome region and the development of diagnostic molecular markers to aid early clinical diagnosis of the PWS.

Centromeric association of a microchromosome. A new category of non-random arrangement of metaphase chromosomes

A supernumerary microchromosome measuring 0.5-1 microns found in over half of the metaphases of a CREST scleroderma patient and his daughter has been characterized by various cytogenetic techniques. The microchromosome consisted of constitutive heterochromatin and contained nuclear antigens reacting with specific anti-kinetochore antibodies. The most remarkable property of the microchromosome was its non-random position: it was closely associated with the centromere of any of the normal chromosomes in the majority of the metaphases. Furthermore, an inordinately high rate of Y chromosome aneuploidy was found in the CREST scleroderma patient. The origin and structure of the microchromosome, its possible connection with the CREST variant of scleroderma, and the phenomenon of centromeric association are discussed.

Parental origin of the extra chromosome in the cat eye syndrome: evidence from heteromorphism and in situ hybridization analysis

Two individuals, a boy and girl, with a clinical diagnosis of cat eye syndrome had an extra bisatellited chromosome. In the girl, the diagnosis was made on the basis of coloboma of the right iris, right preauricular pit, and imperforate anus; in the boy, bilateral colobomata of the iris, down-slanting palpebral fissures, right preauricular skin tag, and right preauricular pit. Multiple staining techniques were used to characterize the extra chromosomes. With G-banding the extra chromosome usually appeared monocentric with two major G-positive bands, but with satellites on both ends; with C-banding, two C-band positive regions were evident, indicating that the chromosomes were likely dicentric. Silver staining demonstrated the presence of NORs near each end; Q-banding showed satellites on each end, differing in brightness and size. The chromosomes of the parents were normal; comparisons of Q-band heteromorphisms of the acrocentric chromosomes of the parents with those of the extra chromosome showed in each case one short arm/satellite region of the extra chromosome identical in appearance to one chromosome 22 of the mother and the other end of the extra bisatellited chromosome identical to the short arm/satellite of the mother's second chromosome 22. This extra chromosome, then, is the result of a maternal meiotic error in each case. In situ hybridization studies using the chromosome 22-derived probe p22/34, which identifies locus D22S9, showed 16% of the cells from the female patient to have silver grains on the proximal long arm of the normal chromosome 22 and 14% on the extra chromosome, while 10% of cells from the male had grains on the normal chromosomes 22 and an equal number on the extra chromosome, confirming the chromosome 22 origin of the extra chromosome in these patients.

Deletions of proximal 15q without Prader-Willi syndrome

Fifteen patients with deletion of proximal 15q without typical Prader-Willi syndrome (PWS) have been reported previously [Schwartz et al, 1985]. We report on 2 additional patients without typical PWS found to have deletions of 15q11-13 on chromosome analysis done for evaluation of developmental delay. Their manifestations include broad nasal bridge with telecanthus, full nasal tip with flare of nasal alae, long upper lip, posteriorly angulated ears, highly arched palate, hypotonia, seizures and marked developmental delay. It was suggested that there may be a specific phenotype associated with this deletion which differs from PWS. Whether this deletion differs from the deletion associated with PWS awaits delineation on a molecular level.

Unique mosaicism in Prader-Labhart-Willi syndrome--a contiguous gene or aneuploidy syndrome?

A 16-year-old boy with Prader-Labhart-Willi syndrome (PLWS) had hypotonia, feeding difficulties, failure to thrive, strabismus and bilateral inguinal hernias with cryptorchidism during infancy followed by hyperphagia, marked early-onset obesity with insulin-dependent diabetes mellitus and necrobiosis lipoidica diabeticorum, short stature, hypogonadotropic hypogonadism and some of the facial characteristics of the individuals with the PLWS. IQ is estimated around 90. Cytogenetic studies showed mosaicism: 45,X, t(Y;15) with partial deletion 15 (15pter----15q12); 46,X, t(Y;15), dic (15)(15pter----15q12::15q12----15pter) and 47, X, t(Y;15), dic(15), dic(15). The dic(15) was bisatellited, NOR-positive on both arms and represented inv dup(15). Thus, the 2 lines with the dic(15) showed partial trisomy 15 (15pter----15q12) and partial pentasomy 15 (15pter----15q12), respectively. The cell line ratios were different in lymphocyte and fibroblast cultures. The unique cytogenetic findings in this patient, the reports of a variety of chromosome 15 aberrations in PLWS, as well as aberrations of other chromosomes, suggest that the condition is a contiguous gene syndrome rather than an aneuploidy syndrome.

Marker chromosomes in a series of 10,000 prenatal diagnoses. Cytogenetic and follow-up studies

In a series of 10,000 prenatal diagnoses 15 marker chromosomes were detected in our centre. Six of these were familial whilst nine had originated de novo. They were analysed with various staining methods. DA-DAPI staining was positive in nine out of 12 pregnancies. Six pregnancies were continued. Five normal children were born, one ended in intrauterine fetal death of a normal fetus at 37 weeks. Nine pregnancies were terminated, showing six normal fetuses, one familial cat-eye syndrome, one fetus with Down syndrome caused by additional trisomy 21 and one fetus with cystic kidneys resp. It is concluded that it seems safe to continue the pregnancy in cases of familial marker, identical to that of one parent, whilst a de novo DA-DAPI positive marker seems to present a low risk for fetal anomalies.

Inv dup (15): prenatal diagnosis and postnatal follow-up

A de novo inv dup (15) was diagnosed at amniocentesis. No physical abnormalities were detected after birth. The boy developed severe mental and motor retardation, which became obvious at 16 months of age.

Structure, origin and effects of a supernumerary marker chromosome 15

A de novo supernumerary small marker chromosome 15 was observed in a female infant with mental and statomotoric retardation as well as minor facial dysmorphia. The marker chromosome was analyzed by ten different staining techniques and 5-azacytidine treatment of lymphocyte cultures. It is shown that the supernumerary chromosome was derived by a non-sister chromatid exchange between the two homologous maternal chromosomes 15. The cytogenetical properties of the marker chromosome, the different activity of its two nucleolus organizer regions and the somatic pairing configurations revealed by 5-azacyidine are reported.

Indirect immunofluorescence of inactive centromeres as indicator of centromeric function

Two previous single case reports from the literature showed the presence or absence of centromeric antigens at the site of the inactive centromeres in one (X;X) and in one (9;11) dicentric chromosome. We studied nine different dicentric chromosomes using anticentromeric antibodies and immunofluorescence techniques. In the four autosomal dicentrics the inactive centromere was consistently positive while the dicentrics composed of two X chromosomes were either positive or negative; one case of (X;Y) dicentric was negative. The results indicate that the X chromosome mode of replication may be involved in the suppression of immunofluorescence at the site of the inactive centromere and that one centromere of the dicentric chromosome may lose its function but conserve some of its antigenic properties. This indicates that not all these antigens play a rôle in the microtubules-centromere interaction.

High resolution pattern of an inverted duplication (15)

The nature and the origin of a supernumerary marker chromosome in a mentally retarded boy were determined by various staining techniques on metaphase chromosomes and by GBG high resolution technique. The karyotype was found to be 47,XY, + inv dup(15) (pter----q13: :q13----pter) and the supernumerary chromosome was of maternal origin.

Proximal duplications of chromosome 15: clinical dilemmas

The apparently rare cytogenetic abnormality of partial trisomy 15 was diagnosed by the authors in a patient presenting with developmental retardation, macrocephaly with ventricular enlargement and prominent subarachnoid spaces, hypotonia, low-set ears, hyperextensible wrists and hands, high arched palate, tapering fingers, right esotropia, and bilateral metatarsus adductus. Clinical findings in this case are similar to previously reported cases of proximal duplications of chromosome 15 and bear some similarity to the Prader-Willi syndrome. However, our patient did not have the severe hypotonia, early failure to thrive, or genital abnormalities seen in classical Prader-Willi syndrome. This case supports the theory that a variety of cytogenetic aberrations in proximal 15q can cause a "Prader-Willi-like" syndrome. Increased clinical suspicion is needed when patients are seen with hypotonia, retarded development and mild dysmorphism if the variety of phenotypes are to be delineated.

Forty four probands with an additional "marker" chromosome

Information is presented which has been obtained from an exhaustive examination of 44 probands with a supernumerary marker chromosome (mar) and their families. The data include the derivation of the mar, frequency in various populations, inheritance and possible effect on fertility, congenital abnormality, and mental ability. The practical problems in assessing the risk of abnormality in a foetus discovered during prenatal diagnosis to be carrying a mar, are discussed.

Supernumerary microchromosomes identified as inverted duplications of chromosome 15: a report of three cases

Supernumerary bisatellited microchromosomes detected in three unrelated patients were identified as inverted duplications of chromosome 15. Each of these chromosomes contained a small euchromatic interstitial band presumably derived from the proximal portion of region 15q1. The clinical significance of this material was difficult to assess. Two of our cases were ascertained as the result of routine amniotic fluid studies. One of the affected fetuses had an unusual form of mosaicism 46,XY/48,XY, + inv dup(15), + inv dup(15), but no apparent developmental abnormalities. The inv dup (15) of the second fetus was familial in origin; no phenotypic abnormalities or evidence of mosaicism were detected in the carrier parent. The third inv dup(15) was found in a 20.5-month-old boy referred for developmental retardation. The clinical findings in this case were similar to those seen in patients with large inv dup(15)'s and did not suggest Prader-Willi syndrome.

Marker chromosomes: cytogenetic characterization and implications for prenatal diagnosis

Satellited marker chromosomes were identified in four individuals from unrelated families; one was first encountered in cultured amniotic fluid cells obtained for prenatal diagnostic studies. We present cytogenetic characterization of these marker chromosomes and clinical findings in the individuals carrying them. Identification of a marker chromosome in amniotic fluid cell cultures presents problems in genetic counseling, as it is often difficult to determine the clinical significance of such a finding. Chromosome-banding techniques now allow the precise identification of satellited marker chromosomes originating from chromosome 15. Presence of a supernumerary bisatellited der(15) marker chromosome containing the proximal long arm of 15 has been associated with mental and developmental retardation. Application of chromosome-banding techniques was useful in characterization of the marker chromosomes and providing prenatal genetic counseling.

A prenatally detected inv dup(15)

An extra G group-like chromosome was found in an amniotic fluid cell culture. With multiple banding techniques it was identified as inv dup(15). BUdR incorporation was used to determine the lateral asymmetry of the marker, which consisted of 2 distal spots arranged contralaterally, consistent with DNA polarity conservation in chromosome rearrangements.

Reproductive outcomes of paracentric inversion carriers: report of a liveborn dicentric recombinant and literature review

An abnormal infant had a dicentric chromosome 14 with an inverted tandem duplication [46,XY,inv dup(14) (pter----q32.3::q24.2----pter)], thus making him trisomic for the proximal two-thirds of chromosome 14. This abnormality was derived from a maternal paracentric inversion in chromosome 14 [46,XX,inv(14)(q24.2q32.3)]. To our knowledge, this is the first report of a liveborn infant carrying a stable, dicentric product of crossing over within a paracentric inversion loop. A review of the reproductive outcomes of paracentric inversion carriers in the literature suggests that they are at some risk for pregnancy wastage. The risk for liveborn recombinants is small but such births have occurred, at least to female carriers.

Chromosome 15 anomalies and the Prader-Willi syndrome: cytogenetic analysis

The behaviour of chromosome 15 is very different from that of the other acrocentric chromosomes. The cytogenetic characteristics of rearrangements associated with Prader-Willi syndrome (PWS) are analyzed as similar rearrangements irrespective of the associated phenotype (reciprocal translocations of chromosome 15, small bisatellited additional chromosomes, Robertsonian translocations, interstitial deletions, pericentric inversions). This study suggests that: (1) The proximal ( 15q ) region and PWS seem to be indissociable ; (2) chromosome 15 has an indisputable cytogenetic originality which could be related to its histochemical properties. Chromosome 15 constitutive heterochromatin usually contains much 5-methylcytosine-rich DNA and a large amount of each of the four satellite DNAs. Furthermore the existence in the proximal ( 15q ) region of one or several palindromic sequences could be postulated to explain the great lability of this region of chromosome 15.

Mosaic inversion duplication of chromosome 15 without phenotypic effect: occurrence in a father and daughter

A mosaic marker chromosome was observed in 2 generations. Multiple staining techniques identified it as an inverted duplication of chromosome 15 (inv dup 15) derived from the paternal grandmother. Although this inv dup 15 included a central R band, there was no noticeable phenotypic effect.

Deficiency, transposition, and duplication of one 15q region may be alternatively associated with Prader-Willi (or a similar) syndrome. Analysis of seven cases after varying ascertainment

Seven patients are described who have some or all of the symptoms of Prader-Willi syndrome. They were ascertained by varying criteria starting either from the clinical picture or from the identification of a chromosome abnormality involving the proximal portion of the long arm of chromosome 15. The chromosome abnormalities consisted of two balanced translocations (15;18 and 8;15), three unbalanced ones (15;18, 15;19, and 9;15), and one interstitial deletion of bands 15q11 and q12. The seventh case had an unidentified extra chromosome. These data and a review of the literature led to the conclusion that deficiency, transposition, and even duplication of the region(s) 15q11-q13 may all result in a syndrome which is identifiable with or similar to the Prader-Willi syndrome.

A 15 leads to 1 translocation in a patient mosaic for presence or absence of an isodic(15p)(q11)

We report a patient with a 15 leads to 1 translocation who was mosaic for presence or absence of an isodic(15p)(q11). Her phenotype is similar to that of patients with deletions of proximal 15q or isodic(15p). Several phenotypes, including the Prader-Labhart-Willi syndrome, have been described with abnormalities of proximal 15q and have in common severe hypotonia, developmental delay, and lack of major congenital anomalies. Our patient is the first to be described with an isodic (15p)(q11) associated with a nonreciprocal translocation. We think her isodic(15p)(q11) arose as a result of a sister chromatid fusion rather than nonsister chromatid exchange as has been proposed in most other cases. Because of the various phenotypes described with proximal 15q abnormalities, we recommend caution in the assignment of specific phenotypes to small chromosome changes in this area.