Identification of supernumerary der(20) chromosomes by FISH in three patients

A systematic analysis of small supernumerary marker chromosomes using array CGH exposes unexpected complexity

PURPOSE

A small supernumerary marker chromosome is often seen in patients with developmental disorders. Prior to array-based comparative genomic hybridization markers were rarely genotyped end to end. In this study, a valid genotype-to-phenotype correlation was possible because the supernumerary marker chromosomes were fully characterized by array-based comparative genomic hybridization in a genome-wide analysis.

METHODS

Ten consecutive de novo small supernumerary marker chromosome cases were systematically genotyped using G-banding, C-banding, AgNOR staining, whole-genome array-based comparative genomic hybridization, and fluorescence in situ hybridization.

RESULTS

Among 10 small supernumerary marker chromosome cases studied, 4 (40%) were not identified by array-based comparative genomic hybridization because of low-level mosaicism or because they lacked euchromatin. One case (10%) was a simple pericentromeric marker extending from 5p13.3 to 5q11.2. Five (50%) markers showed unexpected complexity. Two cases had markers that were derivative acrocentric (AgNOR+) chromosomes with the euchromatin from chromosomes 18p or 19p. Each of the other three cases with complex markers had unusual characteristics including a marker from noncontiguous segments of chromosome 19q, a highly complex rearrangement involving a chromosome 20 homolog as well as the small supernumerary marker chromosome, and a mosaic duplication of a proximal 8p marker.

CONCLUSION

Small supernumerary marker chromosomes are frequently complex on the basis of our small sample. Whole-genome array-based comparative genomic hybridization characterization of the small supernumerary marker chromosome provided informed genetic counseling.

A case of partial trisomy 2p23-pter syndrome with trisomy 18p due to a de novo supernumerary marker chromosome

Partial trisomy 2p is a rare but relatively well-defined syndrome with distinctive clinical features, including marked psychomotor delay, dysmorphic face, and congenital heart disease. The phenotype of trisomy 18p is variable, from normal appearance to moderate mental retardation. Most cases of trisomy 2p and trisomy 18p result from the inheritance of an unbalanced segregant from a balanced parental translocation or due to de novo duplication. Here, we present the first report of a combined partial trisomy 2p and trisomy 18p due to a supernumerary marker chromosome (SMC). The final karyotype of the patient was 47,XX,+der(18)t(2;18)(p23.1;q11.1)[22]/46,XX[8]. The patient had typical dysmorphic features of partial trisomy 2p23-pter syndrome and congenital heart disease. SMCs are remarkably variable in euchromatic DNA content and mosaicism level. The precise identification of the origin and composition of SMCs is essential for genotype-phenotype correlation and genetic counseling.

Characterization of a prenatally assessed de novo supernumerary minute ring chromosome 20 in a phenotypically normal male

Background

The heterogeneous group of small supernumerary marker chromosomes (sSMCs) presents serious counseling problems, especially if they are present de novo and diagnosed prenatally. The incidence has been estimated at 1 in 1000 prenatal samples. We present a case of mosaic sSMC diagnosed prenatally after amniocentesis. The sSMC was characterized by various molecular cytogenetic techniques and determined to be a r(20) chromosome. After genetic counseling, the parents decided to continue the pregnancy, and a boy with minor phenotypic variants was born after 39 weeks of pregnancy. The case is compared with four other cases of prenatally detected r(20) mosaicism.

Results

Here we describe a 3 months old male child with normal pre- and postnatal development and with a de novo ring supernumerary marker chromosome in amniocytes cultures. Using new fluorescence in situ hybridization (FISH) techniques, three distinguishable sSMCs (cryptic mosaicism), all derived from chromosome 20, were observed, including ring and minute chromosomes. This heterogeneity was impossible to detect by the conventional G-banding technique or conventional FISH technique that were used before the application of new FISH techniques (subcentromere-specific multicolor-FISH [subcenM-FISH]) and a probe, specific for the 20p12.2 band. The sSMC present in 25% of the cells was present as r(20)(::p12.2~12.3->q11.1::)[5]/r(20;20)(::p12.1->q11.1::q11.1 >p12.1::)[2]/min(20;20)(:p12.1->q11.1::q11.1->p12.1:)[1]. The final karyotype was 47,XY,+r(20)[25%]/46,XY[75%].

Conclusion

We emphasize the importance of application of molecular cytogenetics in a prenatally diagnostic laboratory and description of more cases to enable a better genetic counseling and risk evaluation.

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

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

Supernumerary ring chromosome 20 in a mother and her child

The authors report a familial case of mosaicism for an extra ring 20, identified by fluorescence in situ hybridization (FISH), in a mother and her child. In spite of the fact that both patients had clinical abnormalities, the more severe phenotype present in the child was probably due to the higher percentage of abnormal cells. To our knowledge, this is the first report of a familial extra ring 20 mosaicism.

Prenatal diagnosis of occipital encephalocele, mega-cisterna magna, mesomelic shortening, and clubfeet associated with pure tetrasomy 20p

We present the first case of a fetus with pure tetrasomy 20p proven by cord-blood sampling at 24 weeks of gestation. This case was diagnosed in utero with multiple congenital anomalies including occipital encephalocele, mega-cisterna magna, mesomelic shortening, and clubfeet. An analysis of GTG-banded chromosomes of 20 metaphase cells was performed. Female karyotype [47,XX, +i(20)(p10)] was revealed in all cells. Pure tetrasomy 20p was confirmed using fluorescent in situ hybridization (FISH) with a telomere probe for chromosome 20p in all seven metaphase cells. The pregnancy was terminated because of associated multiple anomalies and severe oligohydramnios. The postmortem examination confirmed the prenatal diagnosis.

A non-sex chromosome marker in a patient with an atypical Ullrich-Turner phenotype and mosaicism of 46,X,mar/46,XX

The absence of a sex chromosome in conjunction with the presence of a marker chromosome generally implicates a sex chromosome origin for such marker chromosomes. These types of findings are frequently associated with Ullrich-Turner syndrome. We report a patient that presented with an atypical Ullrich-Turner phenotype and a cytogenetic mosaicism of 46,X,mar/46,XX. The marker chromosome was derived from chromosome 20, not from the X or Y chromosome. The patient's clinical features are described and discussed relative to the cytogenetic findings. This case further demonstrates the necessity of marker chromosome identification for accurate phenotype-karyotype correlation.

Clinical findings in a patient mosaic for a supernumerary ring chromosome 20

Marker chromosomes present a problem in genetic counseling because there are often no clear phenotype-karyotype correlations. We present the clinical findings in a patient who is mosaic for a supernumerary marker chromosome 20 determined by fluorescence in situ hybridization (FISH) and compare these findings to others reported in the literature.

Delineation of supernumerary marker chromosomes in 38 patients

We present cytogenetic and clinical data on 38 patients with supernumerary marker chromosomes (SMCs). SMCs were characterized using a strategy combining classical banding techniques and molecular cytogenetic studies. Cases were ascertained prenatally, postnatally, and after fetal death. In 26 patients (68%), the SMC originated entirely from acrocentric chromosomes. Among these, most patients carried a der(15). In 11 patients (29%), they were of nonacrocentric origin, including 9 autosomal and 2 gonosomal marker chromosomes. In 1 patient the SMC was of partially acrocentric origin. Patients with small derivatives of chromosome 15 [der(15)] had a normal phenotype. Those with a larger der(15) showed phenotypical abnormalities. Patients with supernumerary marker chromosomes derived from chromosomes 13 or 21, and 14 appeared to have a low risk of abnormalities. Out of this group only 1 patient who carried an additional r(21) had physical anomalies. Patients with an SMC originating from chromosome 22 showed physical abnormalities in 2 out of 6 cases. Supernumerary marker chromosomes identified as i(9p), i(12p), and der(18) were all associated with an abnormal phenotype. Two of the derivatives of chromosome 20 analyzed were correlated with a normal phenotype, while the carrier of the third one showed physical anomalies and motor retardation. Of 2 patients with an extra der(X), 1 was normal and 1 showed an abnormal phenotype.