Small supernumerary marker chromosomes (sSMC) in patients with a 45,X/46,X,+mar karyotype - 17 new cases and a review of the literature

Identification of a Small Supernumerary Marker Chromosome in a Turner Syndrome Patient with Karyotype mos 46,X,+mar/45,X

Turner Syndrome is characterized by a normal X chromosome and the partial or complete absence of a second sexual chromosome. Small supernumerary marker chromosomes are present in 6.6% of these patients. Because of the wide range of Turner syndrome karyotypes, it is difficult to establish a relationship with the phenotype of the patients. We present the case of a female patient with Turner syndrome, insulin resistance, type 2 diabetes, and intellectual disability. The karyotype revealed the presence of mosaicism with a monosomy X cell line and a second line with a small marker chromosome. FISH of two different tissues was used to identify the marker chromosome with probes for X and Y centromeres. Both tissues presented mosaicism for a two X chromosome signal, differing in the percentage of the monosomy X cell percentage. Comparative genomic hybridization with the CytoScan^TMHD assay was performed in genomic DNA from peripheral blood, allowing us to determine the size and breakage points of the small marker chromosome. The patient presents a phenotype that combines classic Turner syndrome features and unlikely ones as intellectual disability. The size, implicated genes, and degree of inactivation of the X chromosome influence the broad spectrum of phenotypes resulting from these chromosomes.

The Changing Face of Turner Syndrome

Turner syndrome (TS) is a condition in females missing the second sex chromosome (45,X) or parts thereof. It is considered a rare genetic condition and is associated with a wide range of clinical stigmata, such as short stature, ovarian dysgenesis, delayed puberty and infertility, congenital malformations, endocrine disorders, including a range of autoimmune conditions and type 2 diabetes, and neurocognitive deficits. Morbidity and mortality are clearly increased compared with the general population and the average age at diagnosis is quite delayed. During recent years it has become clear that a multidisciplinary approach is necessary toward the patient with TS. A number of clinical advances has been implemented, and these are reviewed. Our understanding of the genomic architecture of TS is advancing rapidly, and these latest developments are reviewed and discussed. Several candidate genes, genomic pathways and mechanisms, including an altered transcriptome and epigenome, are also presented.

Higher CNV Frequencies in Chromosome 14 of Girls With Turner Syndrome Phenotype

CONTEXT

Precise genotype-phenotype correlations in Turner syndrome (TS) have not yet been deciphered. The chromosomal basis of the clinical TS phenotype in the absence of X chromosome aberrations on conventional karyotyping remains more and less unexplored.

OBJECTIVE

To elucidate the high-resolution chromosomal picture and analyze the genotype-phenotype associations in girls with clinical phenotype of TS by chromosomal microarray.

DESIGN AND PATIENTS

Cross sectional observational study conducted between October 2018 and January 2020 on 47 girls presenting the clinical TS phenotype and fulfilling the criteria for chromosomal analysis.

SETTING

Outpatient department at Department of Endocrinology and the Molecular Research Lab at tertiary care teaching institution.

RESULTS

The copy number variation (CNV) polymorphs were more frequent on autosomes than X chromosomes, and they were detected in 89.3%, 61.7%, and 92.8% of patients, respectively, on chromosome 14 or X or both. A total 445 and 64 CNV polymorphs were discovered on chromosome X and 14, respectively. The latter exhibited either gain at 14q32.33, loss at 14q11.2, or both. Karyotype was available for 27 patients; 55.6% of cases displayed X chromosome abnormalities while 44.4% cases had a normal karyotype. Functional interactomes of the genes that were present in chromosome 14 CNVs and those known to be associated with TS showed an overlap of 67% and enriched various development-related cellular pathways underlying TS phenotype.

CONCLUSIONS

On high-resolution karyotype analysis, clinical phenotype of TS can be associated with CNV defects in autosomes, specifically chromosome 14 or X chromosome or both. The syndrome of chromosome 14 CNV defects with and without X-chromosomal defects clinically mimics TS and shares a common genomic network that deserves further investigations.

Clinical characteristics and management of Turner patients with a small supernumerary marker chromosome

OBJECTIVE

To summarize the clinical characteristics of Turner syndrome (TS) with a small supernumerary marker chromosome (sSMC) and discuss the clinical significance and management of TS patients with sSMC.

METHODS

A retrospective analysis was conducted on the clinical data of 244 patients with disorders of sexual development admitted to Peking Union Medical College Hospital from February 1984 to July 2020.

RESULTS

Among the 244 patients with a disorder of sexual development, 69 cases of TS were identified in which 13 patients had sSMC. Their ages ranged from 3 to 28 years old with an average of 14.31 ± 6.40 years. All 13 sSMC-positive patients had typical clinical manifestations of TS except ambiguous genitalia in four cases. SRY gene testing was performed in 11sSMC-positive patients and 10 patients were positive for SRY and one was negative. Among the 10 SRY-positive patients, two cases had hirsutism and clitoral enlargement and two cases had clitoral enlargement only. Nine sSMC and SRY-positive patients underwent gonadectomy and one had left gonadal gonadoblastoma with seminoma in situ and right gonadal seminoma in situ.

CONCLUSIONS

Although the sSMC positive detection rate in DSD patients is uncommon (5.33% in our sample), the positive SRY detection rate in sSMC-positive TS patients was extremely high in our TS patients. And TS patients with sSMC and SRY positive had a significantly increased risk of gonadal germ cell tumors. Routine SRY screening should be performed in TS patients with sSMC, and a gonadectomy should be performed in TS patients with sSMC and SRY positive to prevent the occurrence of tumors.

Turner's syndrome mosaicism in girls with neurodevelopmental disorders: a cohort study and hypothesis

BACKGROUND

Turner's syndrome is associated with either monosomy or a wide spectrum of structural rearrangements of chromosome X. Despite the interest in studying (somatic) chromosomal mosaicism, Turner's syndrome mosaicism (TSM) remains to be fully described. This is especially true for the analysis of TSM in clinical cohorts (e.g. cohorts of individuals with neurodevelopmental disorders). Here, we present the results of studying TSM in a large cohort of girls with neurodevelopmental disorders and a hypothesis highlighting the diagnostic and prognostic value.

RESULTS

Turner's syndrome-associated karyotypes were revealed in 111 (2.8%) of 4021 girls. Regular Turner's syndrome-associated karyotypes were detected in 35 girls (0.9%). TSM was uncovered in 76 girls (1.9%). TSM manifested as mosaic aneuploidy (45,X/46,XX; 45,X/47,XXX/46,XX; 45,X/47,XXX) affected 47 girls (1.2%). Supernumerary marker chromosomes derived from chromosome X have been identified in 11 girls with TSM (0.3%). Isochromosomes iX(q) was found in 12 cases (0.3%); one case was non-mosaic. TSM associated with ring chromosomes was revealed in 5 girls (0.1%).

CONCLUSION

The present cohort study provides data on the involvement of TSM in neurodevelopmental disorders among females. Thus, TSM may be an element of pathogenic cascades in brain diseases (i.e. neurodegenerative and psychiatric disorders). Our data allowed us to propose a hypothesis concerning ontogenetic variability of TSM levels. Accordingly, it appears that molecular cytogenetic monitoring of TSM, which is a likely risk factor/biomarker for adult-onset multifactorial diseases, is required.

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

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

Prenatal genetic analysis and differential pregnancy outcomes of two de novo cases showing mosaic isodicentric Y chromosome

Background

Fetal cells collected from the amniotic fluid of two pregnant women indicated sex chromosome abnormalities. Therefore, we performed G-banded chromosome karyotype analysis, single nucleotide polymorphism array (SNP array), fluorescence in situ hybridization (FISH), and sequence-tagged sites (STS) analysis of the Y chromosome to determine the rare molecular genetics of the two fetuses.

Case presentation

The karyotypes of the fetuses from patients 1 and 2 were mos 45,X[92]/46,X,+idic(Y)(q11.21)[8] and mos 45,X[20]/46,X,+idic(Y)(q11.223)[80], respectively. Fetus 1 had a 7.76 Mb deletion in Yq11.222q11.23 and a 15.68 Mb duplication in Yp11.2q11.21. Fetus 2 had 21 Mb of repetitive segments in Yp11.3q11.223. Azoospermia factor (AZF) detection by STS analysis revealed a missing AZFb+c region in fetus 1 and three functional AZF regions in fetus 2. The isodicentric Y chromosome (idic (Y)) in both fetuses arose de novo. The pregnancy of patient 1 was terminated, whereas the fetus of patient 2 was delivered and is now 10 months old with normal appearance and growth.

Conclusion

A combination of technologies such as chromosome karyotyping, FISH, SNP arrays, and STS analysis of the Y chromosome is important in prenatal diagnosis to reduce birth defect rates and improve the health of the Chinese population.

Clinical, cytogenetic, and molecular findings of isodicentric Y chromosomes

Background

Isodicentric Y chromosomes [idic(Y)] are one of the most common structural abnormalities of the Y chromosome. The prenatal diagnosis of isodicentric Y chromosomes is of vital importance, and the postnatal phenotypes vary widely. Therefore, we present six patients prenatally diagnosed with isodicentric Y chromosomes and review the literature concerning the genotype-phenotype correlations.

Method

The clinical materials of six patients were obtained. Cytogenetic and molecular approaches were carried out for these six patients.

Results

Isodicentric Y chromosomes were found in all sixpatients. Among them, four patients presented with a mosaic 45,X karyotype, one patient had a 46,XY cell line, and one patient was nonmosaic. Five of these six isodicentric Y chromosomes had a breakpoint in Yq11.2, and the other had a breakpoint in Yp11.3. The molecular analysis demonstrated different duplications and deletions of the Y chromosome. Finally, three patients chose to terminate the pregnancy, two patients gave birth to normal-appearing males, and one patient was lost to follow-up.

Conclusion

The incorporation of multiple cytogenetic and molecular techniques would offer a more comprehensive understanding of this structural chromosomal abnormality for genetic counselling.

Impaired Spermatogenesis due to Small Supernumerary Marker Chromosomes: The Reason for Infertility Is Only Reliably Ascertainable by Cytogenetics

Infertile male with small supernumerary marker chromosomes (sSMCs) were studied. Overall, 37 own patients and 166 cases from the literature were included. sSMCs of our own cases were characterized by multicolor-FISH probe sets. Available clinical data of the infertile males were also evaluated, and meta-analysis on suitability of molecular karyotyping for sSMC characterization was done. As a result, sSMCs can be optimally characterized by single-cell directed (molecular) cytogenetics. In infertile males, sSMCs derive predominantly from one of the acrocentric chromosomes, mainly chromosomes 15, 14, and 22. Interestingly, altered spermiograms were found in 62% of the males with an sSMC, while the remainder cases had infertility in connection with recurrent spontaneous abortions. Meta-analysis for detectability of sSMCs by aCGH revealed that 81-87% of the cases would have not been picked up by exclusive use of that approach. Thus, as impaired spermatogenesis is known to be indicative for gross chromosomal anomalies in infertile male patients, it can be concluded from this study that the presence of sSMCs also needs to be considered. However, sSMCs can only be reliably detected by standard karyotyping and not by modern high throughput approaches like aCGH and next-generation sequencing.

Molecular Cytogenetic Diagnostics of Marker Chromosomes: Analysis in Four Prenatal Cases and Long-Term Clinical Evaluation of Carriers

The prenatal finding of a small supernumerary marker chromosome (sSMC) is a challenge for genetic counseling. Our analytic algorithm is based on sSMC frequencies and multicolor FISH to accelerate the procedure. The chromosomal origin, size, and degree of mosaicism of the sSMC then determine the prognosis. We illustrate the effectiveness on 4 prenatally identified de novo mosaic sSMCs derived from chromosomes 13/21, X, 3, and 17. Three sSMC carriers had a good prognosis and apparently healthy children were born, showing no abnormality till the last examination at the age of 4 years. One case had a poor prognosis, and the parents decided to terminate the pregnancy. Our work contributes to the laboratory and clinical management of prenatally detected sSMCs. FISH is a reliable method for fast sSMC evaluation and prognosis assessment; it prevents unnecessary delays and uncertainty, allows informed decision making, and reduces unnecessary pregnancy terminations.

X-derived marker chromosome in patient with mosaic Turner syndrome and Dandy-Walker syndrome: a case report

Background

Small supernumerary marker chromosomes can be derived from autosomes and sex chromosomes and can accompany chromosome pathologies, such as Turner syndrome.

Case presentation

Here, we present a case report of a patient with mosaic Turner syndrome and Dandy-Walker syndrome carrying a marker chromosome. We showed the presence of the marker chromosome in 33.8% of blood cells. FISH of the probe derived from the marker chromosome by microdissection revealed that it originated from the centromeric region of chromosome X. Additionally, we showed no telomeric sequences and no XIST sequence in the marker chromosome. This is the first report of these two syndromes accompanied by the presence of a marker chromosome.

Conclusion

Marker chromosome was X-derived and originated from centromeric region. Patient has mild symptoms but there is no XIST gene in marker chromosome.

Trial registration

CPG137. Registered 03 March 2017.

The clinical analysis of small supernumerary marker chromosomes in 17 children with mos 45,X/46,X,+mar karyotype

Small supernumerary maker chromosome (sSMC) is a type of structurally abnormal chromosome. In order to identify the origin, morphology and other characteristics of sSMCs in children with mos 45,X/46,X,+mar karyotype, 17 patients (16 females and 1 male) were analyzed. All patients underwent general physical examination, gonadal imaging and molecular cytogenetic analyses, including Giemsa banding, dual-color fluorescence in situ hybridization and detection of the sex-determining region Y gene by polymerase chain reaction. Cytogenetic analyses indicated sSMCs in 14/17 cases were derived from the X chromosome, of which 8 individuals presented with ring-shaped sSMCs and 6 with centric minute-shaped sSMCs. The remaining 3 cases were derived from the Y chromosome, and all presented with minute-shaped sSMCs. All female patients exhibited short stature, gonadal dysgenesis and other typical features of Turner syndrome. The male patient exhibited short stature, hypospadias and bilateral cryptorchidism. In conclusion, the majority of the sSMCs in patients with a mos 45,X/46,X,+mar karyotype were derived from the sex chromosomes. The molecular cytogenetic features of sSMCs may provide useful information for genetic counseling, prenatal diagnosis and individualized treatment.

A unique mosaic Turner syndrome patient with androgen receptor gene derived marker chromosome

Patients with Turner syndrome are generally characterized by having short stature with no secondary sexual characteristics. Some abnormalities, such as webbed neck, renal malformations (>50%) and cardiac defects (10%) are less common. The intelligence of these patients is considered normal. Non-mosaic monosomy X is observed in approximately 45% of postnatal patients with Turner syndrome and the rest of the patients have structural abnormalities or mosaicism involving 46,X,i(Xq), 45,X/46,XX, 45,X and other variants. The phenotype of 45,X/46,X,+mar individuals varies by the genetic continent and degree of the mosaicism. The gene content of the marker chromosome is the most important when correlating the phenotype with the genotype. Here we present an 11-year-old female who was referred for evaluation of her short stature and learning disabilities. Conventional cytogenetic investigation showed a mosaic 45,X/46,X,+mar karyotype. Fluorescence in situ hybridization showed that the marker chromosome originated from the X chromosome within the androgen receptor (AR) and X-inactive specific transcript (XIST) genes. Therefore, it is possible that aberrant activation of the marker chromosome, compromising the AR and XIST genes, may modify the Turner syndrome phenotype.

Y chromosome in Turner syndrome: detection of hidden mosaicism and the report of a rare X;Y translocation case

Turner syndrome (TS) is a common genetic disorder in females associated with the absence of complete or parts of a second sex chromosome. In 5-12% of patients, mosaicism for a cell line with a normal or structurally abnormal Y chromosome is identified. The presence of Y-chromosome material is of medical importance because it results in an increased risk of developing gonadal tumours and virilisation. Molecular study and fluorescence in situ hybridisation approaches were used to study 74 Brazilian TS patients in order to determine the frequency of hidden Y-chromosome mosaicism, and to infer the potential risk of developing malignancies. Additionally, we describe one TS girl with a very uncommon karyotype 46,X,der(X)t(X;Y)(p22.3?2;q11.23) comprising a partial monosomy of Xp22.3?2 together with a partial monosomy of Yq11.23. The presence of cryptic Y-chromosome-specific sequences was detected in 2.7% of the cases. All patients with Y-chromosome-positive sequences showed normal female genitalia with no signs of virilisation. Indeed, the clinical data from Y-chromosome-positive patients was very similar to those with Y-negative results. Therefore, we recommend that the search for hidden Y-chromosome mosaicism should be carried out in all TS cases and not be limited to virilised patients or carriers of a specific karyotype.

Entstehungsmechanismen von Zellmosaiken

Zellmosaike bilden sich im Zusammenhang mit „nondisjunction“, Translokationen (balanciert oder unbalanciert), nichthomologem „crossing over“ oder sonstigen chromosomalen oder subchromosomalen „rearrangements“ aus, aber auch durch kompletten oder gewebsspezifischen Chimärismus. Am bekanntesten und häufigsten nachgewiesen sind Zellmosaike, die auf Aneuploidien beruhen, während über die Häufigkeit von submikroskopischen, nur molekulargenetisch oder zytogenetisch nachweisbaren, aber niedriggradigen Zellmosaiken nur wenig bekannt ist. Als Grundlage für die Entstehung von Zellmosaiken gelten „Trisomic“- und/oder „Monosomic-rescue“-Vorgänge. Auch „replikative Fehler“ oder „Endoreduplikation“ einzelner oder mehrere Chromosomen, Isochromosomenbildung oder postzygotisches „non-homologous crossing-over“ werden als Entstehungsmechanismen von Zellmosaiken in der Literatur genannt. Insgesamt ist jedoch festzustellen, dass praktisch alle bekannten Modelle zur Mosaikentstehung bislang auf der deskriptiven Ebene verharren. Ein grundlegendes Verständnis über die tatsächlich z. B. beim Trisomic oder Monosomic rescue ablaufenden Vorgänge ist derzeit mangels Daten nicht vorhanden.

Gonosomale Mosaike

Aberrationen der Geschlechtschromosomen sind mit einer Inzidenz von ca. 0,2 % bei Neugeborenen von großer Relevanz. Häufig liegen sie in Form von numerischen und/oder strukturellen Mosaiken vor. Vor allem aufgrund der variablen Verteilung in verschiedenen Geweben ist die Genotyp-Phänotyp-Korrelation schwierig, was besonders bei pränatalen Befunden eine große Herausforderung darstellt und eine genetische Beratung erforderlich macht. Gonosomale Mosaike führen im weiblichen Geschlecht häufig zu den klinischen Symptomen des Turner-Syndroms (v. a. Kleinwuchs und Infertilität) einem potentiell erhöhten Gonadoblastomrisiko bei Vorhandensein einer XY-Zelllinie. Im männlichen Geschlecht sind Klinefelter-Mosaike ebenfalls häufig (bis ca. 20 %). Mosaike für die Karyotypen 47,XXX und 47,XYY werden seltener beobachtet; dies dürfte auch durch den geringen Krankheitswert und die daraus seltener resultierende Untersuchungsindikation begründet sein.

Human Ring Chromosomes - New Insights for their Clinical Significance

Twenty-nine as yet unreported ring chromosomes were characterized in detail by cytogenetic and molecular techniques. For FISH (fluorescence in situ hybridization) previously published high resolution approaches such as multicolor banding (MCB), subcentromere-specific multi-color-FISH (cenM-FISH) and two to three-color-FISH applying locus-specific probes were used. Overall, ring chromosome derived from chromosomes 4 (one case), 10 (one case), 13 (five cases), 14, (three cases), 18 (two cases), 21 (eight cases), 22 (three cases), X (five cases) and Y (one case) were studied. Eight cases were detected prenatally, eight due developmental delay and dysmorphic signs, and nine in connection with infertility and/or Turner syndrome. In general, this report together with data from the literature, supports the idea that ring chromosome patients fall into two groups: group one with (severe) clinical signs and symptoms due to the ring chromosome and group two with no obvious clinical problems apart from infertility.

Analysis of Turner syndrome patients within the Jordanian population, with a focus on four patients with Y chromosome abnormalities

This study presents findings in Turner syndrome (TS) patients from the Jordanian population, with focus on 4 patients with Y chromosomal abnormalities. From 1989 to 2011, 504 patients with TS stigmata were referred to our institute for karyotyping, resulting in 142 positive TS cases. Of these, 62 (43.7%) had the typical 45,X karyotype and the remaining individuals (56.3%) were found to be mosaics. Fifteen TS patients (10.5%) carried a structural abnormality of the Y chromosome and presented with the mosaic 45,X/46,XY karyotype. From these, 4 TS cases were investigated further. Karyotyping revealed that 1 patient carried a small supernumerary marker chromosome, whereas cytogenetic and molecular analyses showed that 3 patients carried 2 copies of the SRY gene. Further analysis by SRY sequencing revealed no mutations within the gene. The analyzed patients were found to be phenotypically either females or males, depending on the predominance of the cell line carrying the Y chromosome. This study demonstrates the importance of detailed cytogenetic analysis (such as FISH) in TS patients, and it also emphasizes the need for molecular analysis (such as PCR and sequencing) when fragments of the Y chromosome are present.

Neocentric X-chromosome in a girl with Turner-like syndrome

BACKGROUND

Neocentromeres are rare human chromosomal aberrations in which a new centromere has formed in a previously non-centromeric location. We report the finding of a structurally abnormal X chromosome with a neocentromere in a 15-year-old girl with clinical features suggestive of Turner syndrome, including short stature and primary amenorrhea.

RESULT

G-banded chromosome analysis revealed a mosaic female karyotype involving two abnormal cell lines. One cell line (84% of analyzed metaphases) had a structurally abnormal X chromosome (duplication of the long arm and deletion of the short arm) and a normal X chromosome. The other cell line (16% of cells) exhibited monosomy X. C-banding studies were negative for the abnormal X chromosome. FISH analysis revealed lack of hybridization of the abnormal X chromosome with both the X centromere-specific probe and the "all human centromeres" probe, a pattern consistent with lack of the X chromosome endogenous centromere. A FISH study using an XIST gene probe revealed the presence of two XIST genes, one on each long arm of the iso(Xq), required for inactivation of the abnormal X chromosome. R-banding also demonstrated inactivation of the abnormal X chromosome. An assay for centromeric protein C (CENP-C) was positive on both the normal and the abnormal X chromosomes. The position of CENP-C in the abnormal X chromosome defined a neocentromere, which explains its mitotic stability. The karyotype is thus designated as 46,X,neo(X)(qter- > q12::q12- > q21.2- > neo- > q21.2- > qter)[42]/45,X[8], which is consistent with stigmata of Turner syndrome. The mother of this patient has a normal karyotype; however, the father was not available for study.

CONCLUSION

To our knowledge, this is the first case of mosaic Turner syndrome involving an analphoid iso(Xq) chromosome with a proven neocentromere among 90 previously described cases with a proven neocentromere.

Molecular analysis of ring y chromosome in a 10-year-old boy with mixed gonadal dysgenesis and growth hormone deficiency

Ring Y chromosome is a very rare chromosomal aberration. The published mixed gonadal dysgenesis (MGD) patients with a ring Y chromosome are short in stature, but are not growth hormone (GH) deficient. We present the molecular cytogenetic and molecular characterization of ring Y chromosome mosaicism in a 10-year-old boy with MGD whose short stature could be explained by the high percentage of cells monosomic for the X-chromosome, but also by the presence of severe GH deficiency. The ring Y chromosome in our patient is a de novo structural aberration. The father's karyotype was normal.

Characterization of sSMC by FISH and molecular techniques

Small supernumerary marker chromosome (sSMC) is a structurally altered additional chromosome that may not be explicitly clarified by conventional karyotyping alone. About one third of sSMC carriers have abnormal phenotypes and its clinical correlation is difficult, especially in prenatal studies. The present study was aimed at characterizing 19 sSMC identified in 15 patients with dysmorphic features with or without multiple congenital anomalies, conspicuous family history, short stature and/or ambiguous genitalia. All the sSMC were primarily identified by routine cytogenetics studies (performed with banding techniques) from peripheral blood except in one patient, where amniotic fluid was used. All sSMCs were further characterized by array-CGH (using 44 K oligonucleotide probe) and/or fluorescence in situ hybridization (FISH) using multicolor banding (MCB), centromere specific multicolor FISH (cenM-FISH), subcentromere-specific multicolor FISH (subcenM-FISH), micro-dissection and/or reverse FISH. This report demonstrates the worth of advanced molecular (cyto)genetic techniques in characterizing sSMC, their utility in genotype-phenotype correlation and risk of clinical presentation.

Somatic mosaicism in cases with small supernumerary marker chromosomes

Somatic mosaicism is something that is observed in everyday lives of cytogeneticists. Chromosome instability is one of the leading causes of large-scale genome variation analyzable since the correct human chromosome number was established in 1956. Somatic mosaicism is also a well-known fact to be present in cases with small supernumerary marker chromosomes (sSMC), i.e. karyotypes of 47,+mar/46. In this study, the data available in the literature were collected concerning the frequency mosaicism in different subgroups of patients with sSMC. Of 3124 cases with sSMC 1626 (52%) present with somatic mosaicism. Some groups like patients with Emanuel-, cat-eye- or i(18p)- syndrome only tend rarely to develop mosaicism, while in Pallister-Killian syndrome every patient is mosaic. In general, acrocentric and non-acrocentric derived sSMCs are differently susceptible to mosaicism; non-acrocentric derived ones are hereby the less stable ones. Even though, in the overwhelming majority of the cases, somatic mosaicism does not have any detectable clinical effects, there are rare cases with altered clinical outcomes due to mosaicism. This is extremely important for prenatal genetic counseling. Overall, as mosaicism is something to be considered in at least every second sSMC case, array-CGH studies cannot be offered as a screening test to reliably detect this kind of chromosomal aberration, as low level mosaic cases and cryptic mosaics are missed by that.

A small supernumerary marker chromosome present in a Turner syndrome patient not derived from X- or Y-chromosome: a case report

Background

Small supernumerary marker chromosomes (sSMC) can be present in numerically abnormal karyotypes like in a 'Turner-syndrome karyotype' mos 45,X/46,X,+mar.

Results

Here we report the first case of an sSMC found in Turner syndrome karyotypes (sSMC^T) derived from chromosome 14 in a Turner syndrome patient. According to cytogenetic and molecular cytogenetic characterization the karyotype was 46,X,+del(14)(q11.1). The present case is the third Turner syndrome case with an sSMC^T not derived from the X- or the Y-chromosome.

Conclusion

More comprehensive characterization of such sSMC^T might identify them to be more frequent than only ~0.6% in Turner syndrome cases according to available data.

Handling small supernumerary marker chromosomes in prenatal diagnostics

Small supernumerary marker chromosomes (sSMCs) are structurally abnormal chromosomes that cannot be thoroughly characterized by conventional banding cytogenetics and are equal in size or smaller than chromosome 20. They are present in 0.075% of prenatal cases and, overall, approximately 3 million people worldwide are carriers of a sSMC. In prenatal cases with ultrasound abnormalities, sSMCs are found in up to approximately 0.2% of the cases. First described in 1961, it is now known that sSMCs have no phenotypic effects in approximately 70% of de novo cases. Nonetheless, in at least 30-50% of prenatally detected sSMC cases, the pregnancy is terminated; that is, for a certain percentage of potentially healthy children with a sSMC, an abortion is induced. This situation can only be improved by providing increased amounts of and more reliable information on sSMCs. This article provides an overview on current state-of-the-art technologies and how sSMC analysis can be optimized in prenatal diagnostics.

Mosaicism for r(X) and der(X)del(X)(p11.23)dup(X)(p11.21p11.22) provides insight into the possible mechanism of rearrangement

We report a patient with a unique and complex cytogenetic abnormality involving mosaicism for a small ring X and deleted Xp derivative chromosome with tandem duplication at the break point. The patient presented with failure to thrive, muscular hypotonia, and minor facial anatomic anomalies, all concerning for Turner syndrome. Brain MRI revealed mild thinning of the corpus callosum, an apparent decrease in ventricular white matter volume, and an asymmetric myelination pattern. Array comparative genome hybridization analysis revealed mosaicism for the X chromosome, deletion of the short arm of an X chromosome, and a duplication of chromosome region Xp11.21-p11.22. G-banded chromosome and FISH analyses revealed three abnormal cell lines: 46,X,der(X)del(X)(p11.23)dup(X)(p11.21p11.22)/46,X,r(X)(q11.1q13.1)/45,X. The small ring X chromosome was estimated to be 5.2 Mb in size and encompassed the centromere and Xq pericentromeric region. X chromosome inactivation (XCI) studies demonstrated a skewed pattern suggesting that the ring X remained active, likely contributing to the observed clinical features of brain dysmyelination. We hypothesize that a prezygotic asymmetric crossing over within a loop formed during meiosis in an X chromosome with a paracentric inversion resulted in an intermediate dicentric chromosome. An uneven breakage of the dicentric chromosome in the early postzygotic period might have resulted in the formation of one cell line with the X chromosome carrying a terminal deletion and pericentromeric duplication of the short arm and the second cell line with the X chromosome carrying a complete deletion of Xp. The cell line carrying the deletion of Xp could have then stabilized through self-circularization and formation of the ring X chromosome.