The genetic significance of accessory bisatellited marker chromosomes

Paired CRISPR/Cas9 Nickases Mediate Efficient Site-Specific Integration of F9 into rDNA Locus of Mouse ESCs

Hemophilia B (HB) is an X-linked recessive bleeding disorder, caused by F9 gene deficiency. Gene therapy combined with the CRISPR/Cas9 technology offers a potential cure for hemophilia B. Now the Cas9 nickase (Cas9n) shows a great advantage in reducing off-target effect compared with wild-type Cas9. In this study, we found that in the multicopy ribosomal DNA (rDNA) locus, the homology directed recombination (HDR) efficiency induced by sgRNA-Cas9n was much higher than sgRNA-Cas9, meanwhile without off-target in six predicted sites. After co-transfection into mESCs with sgRNA-Cas9n and a non-viral rDNA targeting vector pMrnF9, harboring the homology donor template and the human F9 expression cassette, a recombination efficiency of 66.7% was achieved and all targeted clones were confirmed to be site-specific integration of F9 in the rDNA locus by PCR and southern blotting. Targeted mESCs retained the main pluripotent properties and were then differentiated into hepatic progenitor like cells (HPLCs) and mature hepatocytes, which were characterized by hepatic markers and functional assays. Importantly, the differentiated cells could transcribe exogenous F9 and secrete coagulation factor IX (FIX) proteins, suggesting active transcription and stable inheritance of transgenes in the rDNA locus. After intrasplenical transplantation in severe combined immune deficiency (SCID) mice, targeted HPLCs could survive and migrate from spleen to liver, resulting in secretion of exogenous FIX into blood. In summary, we demonstrate an efficient and site-specific gene targeting strategy in rDNA locus for stem cell-based gene therapy for hemophilia B.

Ectopic expression of factor VIII in MSCs and hepatocytes derived from rDNA targeted hESCs

Hemophilia A is an X-linked recessive bleeding disorder caused by FVIII gene deficiency, which may result in spontaneous joint hemorrhages or life-threatening bleeding. Currently, cell-based gene therapy via ex vivo transduction of transplantable cells with integrating gene-expressing vectors offers an attractive treatment for HA. In present study, we targeted an expression cassette of B-domain-deleted FVIII into the ribosomal DNA (rDNA) locus of human embryonic stem cells (hESCs) by transfection with a nonviral targeting plasmid pHrn. The targeted hESCs clone could be expanded and retained the main pluripotent properties of differentiation into three germ layers both in vitro and in vivo. Importantly, under defined induction conditions, the targeted hESCs could differentiated into functional mesenchymal stem cells (MSCs) and hepatocytes, as validated by relevant specific cell markers and functional examination. Tumorgenesis assay demonstrated that these cells are relatively safe for future applications. Analysis on gene expression revealed that exogenous FVIII mRNA and FVIII proteins were both present in differentiated MSCs and hepatocytes. These results indicated that through gene targeting at hESCs rDNA locus a persistent cell source of transplantable genetic-modified cells can be accomplished for HA therapy.

Targeting of the human F8 at the multicopy rDNA locus in Hemophilia A patient-derived iPSCs using TALENickases

Hemophilia A (HA) is a monogenic disease due to lack of the clotting factor VIII (FVIII). This deficiency may lead to spontaneous joint hemorrhages or life-threatening bleeding but there is no cure for HA until very recently. In this study, we derived induced pluripotent stem cells (iPSCs) from patients with severe HA and used transcription activator-like effector nickases (TALENickases) to target the factor VIII gene (F8) at the multicopy ribosomal DNA (rDNA) locus in HA-iPSCs, aiming to rescue the shortage of FVIII protein. The results revealed that more than one copy of the exogenous F8 could be integrated into the rDNA locus. Importantly, we detected exogenous F8 mRNA and FVIII protein in targeted HA-iPSCs. After they were differentiated into endothelial cells (ECs), the exogenous FVIII protein was still detectable. Thus, it is showed that the multicopy rDNA locus could be utilized as an effective target site in patient-derived iPSCs for gene therapy. This strategy provides a novel iPSCs-based therapeutic option for HA and other monogenic diseases.

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.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Screening for chromosomal abnormalities in 2650 infertile couples undergoing ICSI

Chromosomal abnormalities are the major contributor to the genetic risks of infertility treatment associated with intracytoplasmic sperm injection (ICSI). The study objective was to assess prospectively the frequency of chromosomal aberrations in couples undergoing ICSI. A total of 2650 infertile couples (5300 patients) underwent chromosome analysis before undergoing ICSI in the Egyptian IVF-ET Centre. Heparinized blood samples were cultured, harvested and banded according to standard methods. Overall, 96.94% of the patients studied (5138/5300) had a normal karyotype, while the remaining 162 patients (3.06%) had an abnormal karyotype. Male patients constituted the majority of abnormalities; 138 males (85.19%) and 24 females (14.81%). These chromosomal aberrations included 117 cases (2.2%) of sex chromosome abnormalities; 113 males and four females. Forty-five patients (0.85%) had autosomal aberrations; 25 of them were males and 20 were females. The current data show that chromosomal abnormalities affect 3.06% of infertile patients, and occur in both sexes, but more predominantly in males undergoing ICSI for male factor infertility. It is recommended that chromosomal analysis be performed before undergoing ICSI, to identify patients who can be offered preimplantation genetic diagnosis.

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.

Partial hexasomy of chromosome 15

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

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.

Two inv dup(15) chromosomes in a woman with repeated abortions

A 40-year-old, phenotypically normal woman, with a history of two repeated abortions and no child, had two additional, small, bisatellited, and apparently metacentric chromosomes. Various banding and microsatellite analyses indicated that the additional chromosomes were inv dup(15)(q11q11) without the Prader-Willi/Angelman syndromes critical region, and therefore without phenotypic effects. Her father had a single, identical additional inv dup(15) chromosome. Her husband was chromosomally normal, but sperm analysis indicated a reduced motility and a reduced frequency of morphologically normal sperm. In view of these findings, it was deduced that the inv dup(15) chromosome in the father was transmitted in duplicate to the woman. Individuals with two additional inv dup(15) chromosomes in the literature were reviewed, and possible correlation of the two additional inv dup(15) chromosomes in the woman and her repeated abortions was discussed.

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.

Chromosomal alterations and male infertility

Reduced male fertility can be caused by genetic factors affecting gamete formation or function; in particular, chromosome abnormalities are a possible cause of male subfertility as shown by their higher frequency in infertile men than in the general male population. Meiotic studies in a number of these males have shown spermatogenesis breakdown, often related to alterations in the process of chromosome synapsis. Indeed, any condition that can interfere with X-Y bivalent formation and X-chromosome inactivation is critical to the meiotic process; furthermore, asynapsed regions may themselves represent a signal for the meiotic checkpoint that eliminates spermatocytes with synaptic errors. We performed cytogenetic, hormonal and seminal studies in 333 infertile patients selected because azoospermic, severely oligozoospermic or normozoospermic with failure to fertilize the partner's oocytes in an in vitro fertilization (IVF) program. Our findings: 1) confirm the high incidence of chromosomal anomalies among infertile males; 2) highlight the relevance in male infertility of quantitative/positional modifications of the constitutive heterochromatin; and 3) underline the relevance of cooperation between andrologists and cytogenetists prior to every kind of assisted reproduction, above all prior to intracytoplasmic sperm injection, in which selective hurdles eliminating abnormal germ cells are bypassed.

Type and frequency of chromosome aberrations in 781 couples undergoing intracytoplasmic sperm injection

Cytogenetic investigations were performed in 781 couples prior to intracytoplasmic sperm injection (ICSI) because of severe male infertility or fertilization failures in previous in-vitro fertilization attempts. Out of these 1562 patients, 1012 had a normal karyotype without any aberrations (64.8%), 204 patients had an abnormal karyotypes (13.1%). These chromosome aberrations included constitutional aberrations (4.4%), fragile sites of autosomes (3.0%), low level mosaicism of sex chromosomes (4.0%) and secondary structural chromosome aberrations (4.2%). Combinations of different types of abnormalities were stated. Another 346 patients (22.1%) showed single cell aberrations; the significance of these is unclear at the moment. Constitutional chromosome aberrations were detected in 69 patients. The following chromosome aberrations were observed: 35 sex chromosomal aberrations (comprising hyperploidies of X or Y chromosomes, mosaicisms and derivative X and Y chromosomes), 34 autosomal aberrations including 14 reciprocal translocations, five Robertsonian translocations, six inversions, one marker chromosome, one trisomy 18 mosaicism and seven other structural aberrations. Three autosomal regions showed fragile sites: 6q13 in 2.9% of the patients, 17p12 and 10q24 in 0.05% each. In conclusion, our data show that a high number of infertile couples in an ICSI programme are affected by chromosome aberrations which occur in both sexes. It is suggested that a chromosomal analysis should be performed on both partners before ICSI treatment is initiated.

Scanning electron microscopy of fragmentary marker chromosomes observed by light microscopy

To study the fine structure of fragmentary marker chromosomes, we performed scanning electron microscopy (SEM) on samples isolated from two carriers (Case 1: 46, XY/47, XY, +mar/48, XY, +mar, +mar; Case 2: 47, XY, +mar). In both cases, light microscopic observation revealed that marker chromosomes lacked a centromere and were fragmented in appearance. However, SEM observation of the metaphasic cells in both cases showed three variations. One variation was a structure that seemed to be metacentric, another was a structure that seemed to be submetacentric, and the remaining one was essentially fragmentary. However, neither the usual chromatid nor centromere formations were observed in the metacentric-like and submetacentric-like structures, even when both cases were observed by SEM. Moreover, the marker chromosomes of the boy of Case I, who suffered from various clinical troubles, included a greater population of metacentric-like or submetacentric-like structures than of essentially fragmentary structures. The marker chromosomes of the fetus of Case 2, who suffered from no clinical problems, included a much greater population of essentially fragmentary structures than metacentric-like or submetacentric like structures. Therefore, SEM observation of fragmentary marker chromosomes that are visible on light microscopy might be used to define specific structures. Moreover, SEM observation might provide clinical criteria relating to the pathogenesis of fragmentary marker chromosomes found on light microscopy.

Complete and precise characterization of marker chromosomes by application of microdissection in prenatal diagnosis

A straightforward and extremely efficient reverse chromosome painting technique is described which allows the rapid and unequivocal identification of any cytogenetically unclassifiable chromosome rearrangement. This procedure is used to determine the origin of unknown marker chromosomes found at prenatal diagnosis. After microdissection of the marker chromosome and amplification of the dissected fragment by a degenerate oligonucleotide-primed polymerase chain reaction (DOP-PCR), fluorescence in situ hybridization (FISH) to aberrant and normal metaphase chromosomes with the marker-derived probe pool is performed. With this strategy, marker chromosomes present in amniotic fluid samples were successfully identified in three cases. The origin of the supernumerary markers was ascertained as deriving from 3p(p12-cen), 18p(pter-cen) and 9p(p12-cen), respectively. Since a specific FISH signal on chromosomes can be obtained within 2 working days using a probe generated without any pretreatment from one chromosomal fragment only and without additional image processing devices, this technique is considered to be highly suitable for routine application in pre- and postnatal cytogenetic analysis.

Identification of supernumerary ring chromosome 1 mosaicism using fluorescence in situ hybridization

We report on a 15-year-old black boy with severe mental retardation, multiple congenital anomalies, and a supernumerary ring chromosome mosaicism. Fluorescence in situ hybridization with a chromosome 1 painting probe (pBS1) identified the ring as derived from chromosome 1. The karyotype was 46,XY/47,XY,+r(1)(p13q23). A review showed 8 reports of ring chromosome 1. In 5 cases, the patients had a non-supernumerary ring chromosome 1 resulting in partial monosomies of the short and/or long arm of chromosome 1. In 3 cases, the presence of a supernumerary ring resulted in partial trisomy of different segments of chromosome 1. In one of these cases the supernumerary ring was composed primarily of the centromere and the heterochromatic region of chromosome 1, resulting in normal phenotype. Our patient represents the third report of a supernumerary ring chromosome 1 resulting in abnormal phenotype.

Molecular cytogenetic study of supernumerary marker chromosomes in an unselected group of children

We report on an unselected group of 24 children with small supernumerary marker chromosomes, found in a large sample of 34,910 consecutive newborns karyotyped at birth. Sixteen of these were available for reexamination. With the use of in situ hybridization with alpha-satellite centromere probes and satellite III, ribosomal and beta-satellite DNA probes, we have characterized these markers. In 14 of the 16 cases we have been able to determine the chromosomal origin of the marker. Twelve of the markers are derived from the acrocentric chromosomes. Of these 12 markers, 4 are derived from chromosome 14, 4 from chromosome 22, 3 from chromosome 15 and one is from either chromosome 13 or 21. Ten of these markers were initially ascertained with the satellite III DNA probe, taking advantage of the fact that satellite III DNA is found in the centromeric region of the following chromosomes: 1, 5, 9, 13, 14, 15, 16, 20, 21, 22, and Y. Two markers were derived from chromosomes 4 and 8. The origin of the last 2 markers could not be determined with the techniques employed. Only one of these children is psychometrically retarded and has a peculiar appearance. Unfortunately we were not able to determine the origin of the marker in her case. All other children developed normally.

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

Extra structurally abnormal chromosomes (ESACs) are small supernumerary chromosomes often associated with developmental abnormalities and malformations. We present 50 probands with ESACs characterized by fluorescence in situ hybridization using centromere-specific probes and chromosome-specific libraries. ESAC-specific libraries were constructed by flow sorting and subsequent amplification by DOP-PCR. Using such ESAC-specific libraries we were able to outline the chromosome regions involved. Twenty-three of the 50 ESACs were inverted duplications of chromosome 15 [inv dup(15)], including patients with normal phenotypes and others with similar clinical symptoms. These 2 groups differed in size and shape of the inv dup(15). Patients with a large inv dup(15), which included the Prader-Willi region, had a high risk of abnormality, whereas patients with a small inv dup(15), not including the Prader-Willi region, were normal. ESACs derived from chromosomes 13 or 21 appeared to have a low risk of abnormality, while one out of 3 patients with an ESAC derived from chromosome 14 had discrete symptoms. One out of 3 patients with an ESAC derived from chromosome 22 had severe anomalies, corresponding to some of the manifestations of the cat eye syndrome. Small extra ring chromosomes of autosomal origin and ESACs identified as i(12p) or i(18p) were all associated with a high risk of abnormality.

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

During 7 years (1985-1992), 39,105 consecutive prenatal diagnoses (34,908 amniocenteses and 4197 chorionic villus samples) were made at the five largest clinical genetic laboratories in Sweden. Thirty-one cases of extra structurally abnormal chromosomes (ESACs) were found, giving a total prevalence of 0.8 per 1000. Twelve ESACs were inherited, 14 were de novo and in five the parental origin was unknown. This gives an estimated prevalence of 0.3-0.4 per 1000 for familial and 0.4-0.5 per 1000 for de novo ESACs. Retrospectively, the ESACs were characterized by fluorescence in situ hybridization (FISH). In nine cases, no material was available for this analysis. In 21 of the remaining 22 cases, the chromosomal origin could be identified by FISH. Seventeen of these (81 per cent) were derived from the acrocentric chromosomes, of which 13 originated from chromosome 15 (62 per cent). The most common ESAC was the inv dup(15) (57 per cent). Two cases were derived from chromosome 22, one from chromosome 14, and one from either chromosome 13 or chromosome 21. The four remaining cases consisted to two i(18p)s and two small ring chromosomes derived from chromosomes 4 and 19, respectively.

Two Prader-Willi/Angelman syndrome loci present in an isodicentric marker chromosome

We found an abnormal 47,XX,+mar karyotype in a patient with developmental delay, hypotonia, microcephaly, failure to thrive, and cognitive delay. When metaphases were hybridized with Prader-Willi and Angelman loci-specific probes by the FISH technique, two sites were noted at opposite positions on the marker chromosome. The alphoid satellite DNA probe documented the isodicentric nature while retention of the p arms on both sides of the marker chromosome was demonstrated by beta satellite probe. The patient does not exhibit manifestations of either syndrome despite the presence of these loci in tetrasomic dose. The present investigation suggests that other marker chromosomes be reevaluated, as their clinical manifestations are quite variable.

Infertility in carriers of two bisatellited marker chromosomes

Two unrelated sterile patients, a male and a female, with karyotypes characterized by the presence of two supernumerary bisatellited marker chromosomes, were studied with the aid of fluorescence in situ hybridization (FISH). Using alphoid centromeric specific probes, the markers were identified as an idic (15) (pter-q11) and a mar (14). In both cases a single accessory bisatellited marker was present in the karyotype of one of the parents. A possible correlation between the excess of constitutive heterochromatin and meiotic disturbances in the two patients is discussed.

Prenatal detection of multiple copies of a familial supernumerary marker chromosome

Direct preparations and long-term cultures from a chorion villus biopsy, taken because of a known maternal additional marker chromosome, showed a 48,XX,+mar,+mar karyotype in all cells examined. The same karyotype was revealed in a subsequent amniotic fluid sample and in cord blood at delivery. Detailed examination of the child at delivery and at 1 year of age showed no evidence of phenotypic abnormality or developmental problems.

A study of ten small supernumerary (marker) chromosomes identified by fluorescence in situ hybridization (FISH)

In seven cases additional minute chromosomes studied by FISH were identified as no. 3, 11, 15, 18, 21 and X. Findings were unexpected except for partial trisomy 21 in an adolescent with minor features of Down's syndrome. Moreover, an i(18p) in a mentally retarded dysmorphic child and an idic(15) in a child with Fallot tetralogy was confirmed. In a child with r(21), a supernumerary marker was shown to be derived from no. 21, while in the mother an additional marker idic(22) was noted.

Prenatal identification of small mosaic markers of different chromosomal origins

In situ hybridization using a series of alphoid DNA probes has demonstrated the origin of two small accessory mosaic marker chromosomes ascertained from 1079 amniocenteses. These markers appeared to be de novo, derived from acrocentric chromosomes, and identical by traditional cytogenetic staining (G, Q, C, AgNOR, Hoechst-distamycin). Molecular characterization showed that one marker had originated from chromosome 14, the other from chromosome 22. Clinical outcome in both cases was normal.

Risk determination in cases with an extra minute chromosome: prenatal diagnosis

The interpretation of an extra minute chromosome (EMC) detected in utero presents both diagnostic and prognostic problems. Two EMC cases are presented and a flow-chart guideline is proposed for the determination of developmental risk. A prenatally detected EMC was familially inherited through a normal phenotype father. The pregnancy was continued and a normal baby boy was delivered. At one year his development was within normal ranges. The second EMC case was a mentally retarded girl examined at 4 years and 7 months of age. The EMC was identified with a combination of various staining techniques to be an inversion duplication (15). The importance of differential diagnosis of EMCs by DA/DAPI staining is emphasized.

Correlation between phenotypic expression of de novo marker chromosomes and genomic organization using replicational banding

It has been suggested that actively expressed genes are primarily located in early replicating bands. This hypothesis is supported by cytogenetic and pregnancy outcome data from four consecutive cases of prenatally detected de novo marker chromosomes. Two fetuses with major anomalies had large early replicating bands, while the marker in a third phenotypically normal fetus was late replicating. In the fourth case, a ring marker chromosome had only a small early replicating region. Pregnancy termination was elected. While no structural malformations were apparent, potential intellectual function in this case remains unresolved. An understanding of the relationship between genomic organization and chromosome banding is critical in counseling for prenatally detected de novo marker chromosomes. Replicational banding is particularly helpful in recognizing genes that may be actively expressed and result in developmental abnormality.

Prenatal diagnosis of a double bisatellited marker with an unusual copy number ratio

Prenatal diagnosis, by amniocentesis, revealed mosaicism with respect to a bisatellited, apparently dicentric, DA/DAPI positive, de novo marker. The following cell lines were observed in decreasing order of frequency: 46,XX greater than 48,XX,+mar,+mar much greater than 47,XX,+mar. The pregnancy was terminated and post-mortem examination revealed an apparently normal fetus. Cytogenetic studies of fetal and placental tissues revealed approximately the same level of mosaicism together with the unusual copy number ratio seen in the amniotic fluid cultures. Non-disjunction at the first post-zygotic mitotic division giving rise to a mosaic: 46,XX/48,XX,+mar,+mar followed by subsequent mitotic instability of the marker could account for the unusual copy number ratio.

Chromosome imbalance, normal phenotype, and imprinting

A duplication of the sub-bands 1q42.11 and 1q42.12 was found in a boy and his mother. The proband has short stature (around the 10th centile) but a normal phenotype and psychomotor development. His mother is also asymptomatic. We found 30 published cases of normal subjects with an imbalance of autosomal euchromatic material. In these cases the imbalance involved either only one G positive band or a G positive and a G negative band. Thus the absence of a phenotypic effect cannot always be ascribed to the deficiency in the G positive bands of coding DNA. Moreover, in some cases, the method of transmission of the chromosome abnormality was such that an imprinting effect could be postulated.

Familial marker chromosome due to 3:1 disjunction of t(9;15) in a grandparent

An extra small chromosome detected in amniotic fluid was identified as the product of a translocation [46,XX,t(9;15)(p24;q11.2)]. This case is unusual in that individuals with the unbalanced karyotype resulting from a 3:1 disjunction are phenotypically normal.

Multiple "marker" chromosomes: a novel cytogenetic finding in a patient with mental retardation and congenital anomalies

A patient with mental retardation and clinical manifestations suggestive of Noonan syndrome was found to have in her peripheral lymphocytes multiple small accessory marker chromosomes, varying in number from one to five per cell and in size from about half the size of the q arm of a G group chromosome to less than a centromere. Occasionally, in the more elongated markers, a G-positive or a C-positive band could be identified, or the marker had the appearance of a ring. The origin and significance of these marker chromosomes are discussed.

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.

Hypogonadotropic hypogonadism and anosmia (Kallmann's syndrome) associated with a marker chromosome

A patient with hypogonadotropic hypogonadism and anosmia (Kallmann's syndrome) had an associated chromosomal abnormality. Evaluation of the hypothalamic pituitary axis showed undetectable basal LH and FSH and slight increases in both gonadotropins in response to GnRH. Augmented gonadotropin response to GnRH after serial subcutaneous injections of GnRH confirmed a hypothalamic defect. Additional endocrine tests failed to reveal other hormone dysfunctions. A supernumerary chromosome was detected by routine chromosome analysis. The extra genetic material was identified by differential cytogenetic banding procedures as an accessory bisatellited marker chromosome originating from either chromosome group D or G. Chromosome analyses of both parents were normal. Our results suggest that, in at least some cases, the Kallmann's phenotype may be associated with a chromosome abnormality.

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.

Infertility associated with two accessory bisatellited chromosomes

Two extra bisatellited chromosomes identified as inv dup (15) (pter----q11.2::q11.2----pter) were found in an oligoasthenospermic male. Analysis of Ag-staining in the proband and in one fertile brother with a normal karyotype revealed that nucleolar organizer region (NOR) activity was significantly increased in the patient. The frequency of satellite associations was also significantly higher in the index case, but no correlation was found between NOR activity and acrocentric associations. These results suggest that extra NOR activity and the elevated frequency of satellite associations could predispose to gametogenic impairment.

The significance of pericentric inversions of chromosome 2

Thirteen new cases of a pericentric inversion 2 collected from different laboratories are reported. In addition 41 cases of a pericentric inversion 2 were reviewed from the literature. The pooled data were analysed using Weinberg's proband method to evaluate the risk of a carrier for either children with congenital anomalies or reproductive wastage. In the "corrected" sample of 166 lifeborn offspring of carriers of a pericentric inversion 2 there were five who showed phenotypic anomalies and two died a few hours after delivery. The reported anomalies are heterogeneous and probably reflect the basic risk of any couple for abnormal lifeborn offspring. There has been no observation of a lifeborn who inherited an unbalanced recombination of a parental pericentric inversion 2. A carrier of a pericentric inversion 2 obviously has an increased risk for reproductive wastage. This is indicated by (1) an increase of the rate of spontaneous abortions and (2) an increase of the rate of index patients ascertained because of previous miscarriages. The risk of a carrier of a pericentric inversion 2 for a spontaneous abortion or a stillbirth may be about twice the basic risk of the general population.

Pseudoisodicentric bisatellited extra marker chromosome (tetrasomy 22pter----q11, trisomy Yqh), derived from a maternal Y/22 translocation. Association between this tetrasomy and "cat eye" phenotypical features

A patient with multiple congenital anomalies suggestive of the "Cat eye" syndrome was found to have an extra marker bisatellited chromosome 22 derived from a maternal Y/22 translocation, identified by multiple banding patterns in cultures treated with DA. The proband's karyotype is 47,XX, + psu idic(22)(Yqter----Yq12::22p13----22q11::++ +22q11----22p13::Yq12----Yqter), t(22;Y)(p13;q12)mat., being tetrasomic for 22pter----q11, and trisomic for Yqh. Similarity between his clinical features and reported "Cat eye" cases, confirms that this region is responsible for the phenotypical expression of the syndrome.

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.

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.

Chromosome anomalies in 136 couples with a history of recurrent abortions

Cytogenetic studies were performed on 136 couples with a history of two or more abortions referred to us after gynaecological causes of the abortions had been excluded. Fifteen (11%) of the couples were found to have a chromosome anomaly, and when the couples were subdivided according to number of abortions, surprisingly 6 (10%) of the 59 couples with a history of only two abortions had a chromosome anomaly. An increased frequency of mosaicism for X-chromosome aneuploidy (2.2%) in the women from the 136 couples was also found. A review of the literature shows that translocations of some chromosomes (e.g. nos. 1, 7 or 22) preferentially lead to fetal wastage, while those involving, for example, chromosome nos. 5, 9, 14 or 21 are more likely to result in the birth of a handicapped child. Couples with a history of two abortions should be investigated cytogenetically. Other causes of miscarriages must, however, be excluded first.

Ineffectivity of accessory bisatellited marker chromosomes in inducing meiotic nondisjunction

It has been postulated that accessory marker chromosomes, which frequently show satellites, are effective in inducing meiotic nondisjunction in the carrier. This hypothesis has been tested by comparing expectations derived from it with actual data. Due to the discrepancies between these expectations and actual observations this hypothesis does not hold true: The frequency of trisomy 21 in the offspring of a carrier of an accessory bisatellited marker chromosome cannot be shown to be actually increased, and there is no increase of either the rate of miscarriages to such carriers or the frequency of an accessory marker chromosome among the individuals with regular trisomy 21. This indicates that the risk of trisomy in the offspring of a carrier of an accessory marker chromosome corresponds to the overall incidence of this trisomy.