Cryptic duplication of 21q in an individual with a clinical diagnosis of Down syndrome

A rare de novo duplication of chromosome 21q22.12 → q22.3 with other concomitant deletion and duplication of small fragments in 21q associated with Down syndrome: Prenatal diagnosis, molecular cytogenetic characterization

Background

Karyotyping is considered the gold standard for the genome-wide detection of genomic imbalances in prenatal diagnosis, but it has a number of inherent limitations, namely the time required to culture cell and the limited resolution(5 ~ 10 Mb). Although fluorescence in situ hybridization (FISH) can also be used as a rapid prenatal diagnosis for common aneuploidies, it is labor intensive, requires prior knowledge of the regions of interest, and can only be used to diagnose one or a few genomic regions simultaneously. Array comparative genomic hybridization (aCGH) can overcome the resolution, the locus-specific, and the time limitations of the karyotyping and FISH techniques and is currently the most powerful method for detecting chromosomal alterations in pre and postnatal clinical cases. Several investigations have suggested that the aCGH testing should be considered a first-tier test for the diagnosis of cytogenetic aberrations in the fetus.

Results

This study used karyotyping, FISH, sequence-tagged site (STS) analysis and aCGH to diagnose a case of de novo duplication of chromosome 21q22.12 → q22.3 with other concomitant deletion and duplication of small fragments in 21q associated with Down syndrome prenatally.

Conclusions

FISH, aCGH and STS analysis are useful in prenatal investigation of the nature of de novo alterations of small fragments of the chromosome.

Clinical, cytogenetic, and molecular characterization of a girl with some clinical features of Down syndrome resulting from a pure partial trisomy 21q22.11-qter due to a de novo intrachromosomal duplication

We report a girl with a de novo pure partial trisomy 21 with some clinical features of Down syndrome. The girl patient presented a flat broad face, brachycephaly, and a flat nasal bridge. She also had upwardly slanted palpebral fissures, epicanthal folds, blepharitis, brushfield spots, and strabismus. Her mouth was wide with downturned corners, prominent lower lip, narrow and furrowed tongue, and short palate. G-banded chromosomal analysis of metaphases in cells from both skin and blood showed a 46,XX karyotype with additional chromosomal material on the distal short arm of one chromosome 21. Parental chromosomes were normal. Molecular analyses with the short-tandem-repeat (STR) marker D21S2039 (interferon-alpha/beta receptor [IFNAR]) (21q22.1) showed a triallelic pattern. Subtelomeric fluorescent in situ hybridization (FISH) analyses, LSI 13 (retinoblastoma 1 [RB1])/LSI 21(21q22.13-q22.2), and whole chromosome painting probes specific for chromosome 21 showed trisomy for the segment 21q22.13-21q22.2 due to a de novo intrachromosomal duplication. A 500K SNP microarray analysis was then performed and revealed a 13-Mb duplication of 21q22.11-qter. This duplicated material had been translocated onto the end of the "p" arm of one of the chromosome 21s. The karyotype was provisionally defined as 46,XX,add(21)(p12).ish der (21)t(21;21)(p12;q22.11)(WCP21q+,PCP21q++,D215259/D21S341/D21S342++)dn. At the age of 4 years and 10 months, a comprehensive psychological examination was performed and the diagnostic criteria for mental retardation were not fulfilled. In comparison with previously published cases of pure partial trisomy 21, this is a rare finding. Additional studies of such rare patients should aid in the study of the pathogenesis of Down syndrome.

Prenatal diagnosis of trisomy 21 without the Down syndrome phenotype

OBJECTIVE

To report a patient with the prenatal diagnosis of trisomy 21 without the clinical Down syndrome (DS) phenotype secondary to the absence of the Down syndrome chromosomal region (DSCR) in a derivative chromosome 21.

CASE REPORT AND METHODS

A newborn patient with prenatal diagnosis of duodenal atresia. Cytogenetic studies revealed a regular trisomy 21. At birth, she did not present the clinical features of DS. FISH analysis was performed in the patient with the LSI spectrum probe for the DSCR and in the mother with FISH multicolor analysis using painting probes for chromosomes 20 and 21.

RESULTS

FISH analysis in the patient showed two hybridization signals suggesting that the third chromosome 21 did not have the DSCR region explaining the absence of the DS phenotype. FISH multicolor analysis in the mother showed three hybridization signals for chromosomes 20 and 21, concluding a maternal karyotype, 46,XX,t(20;21)(p11.2;q22.1).

CONCLUSIONS

The patient was found to have a derivative chromosome 21 secondary to a nondisjunction error in meiosis II without the DS critical region and the phenotype was mostly secondary to the combination of the two partial trisomies.

Microarray-based comparative genomic hybridization and its applications in human genetics

Through the years, several techniques capable of detecting DNA copy number changes have been developed. A number of those, such as karyotyping and fluorescence in situ hybridization (FISH), have proven to be valuable tools in both research and diagnostics. Recently, a new technique, called microarray-based comparative genomic hybridization (array CGH), has been introduced. Array CGH has proven to be a specific, sensitive, and fast technique, with considerable advantages compared to other methods used for the analysis of DNA copy number changes. Array CGH enables analysis of the whole genome in a single experiment. Until now, its applications have been mainly directed at detecting genomic abnormalities in cancer. However, array CGH is also suitable for the analysis of DNA copy number aberrations that cause human genetic disorders. This review gives an overview of array CGH and its applications in human genetics. Advantages, limitations, and future perspectives of array CGH are discussed.

Refining chromosomal region critical for Down syndrome-related heart defects with a case of cryptic 21q22.2 duplication

We report here a patient with features of Down syndrome and tetralogy of Fallot who had a 21q22 duplication. The extent of the duplication was defined using fluorescent hybridization probes that map to the critical region on chromosome 21. Included within the interval was the cell adhesion molecule DSCAM but not the collagen COL6A1. The present case provides further support to the concept that there exists Down syndrome-associated congenital heart disease gene(s) on chromosome 21q22 and that over-expression of DSCAM may contribute to the cardiac defects of Down syndrome.

Down syndrome with pure partial trisomy 21q22 due to a paternal insertion (4;21) uncovered by uncultured amniotic fluid interphase FISH

OBJECTIVE

To emphasize the usefulness and reliability of fluorescence in situ hybridization (FISH) on uncultured amniotic fluid cells in the prenatal diagnosis of common chromosomal aneuploidies.

METHODS

FISH analyses utilizing centromeric, locus-specific or whole chromosome paint DNA probes specific for chromosomes X, Y, 13, 18, 21, and 4 were performed on uncultured amniotic fluid cells or the peripheral blood specimen from the father. Routine chromosome analysis was carried out as well.

RESULTS

A prenatal case with partial trisomy 21 due to a paternal cryptic insertion (4;21) was ascertained by a rapid overnight FISH on uncultured amniotic fluid cells. The fetus was delivered at term and had classical features of Down syndrome.

CONCLUSION

Our results stress the importance of FISH on uncultured amniotic fluid cells to supplement routine cytogenetics, especially in cases with abnormal ultrasound findings.

Rapid detection of aneuploidy (trisomy 21) by allele quantification combined with melting curves analysis of single-nucleotide polymorphism loci

BACKGROUND

Molecular approaches for the detection of chromosomal abnormalities will allow the development of rapid, cost-effective screening strategies. We present here a molecular alternative for the detection of aneuploidies and, more specifically, trisomy 21.

METHODS

We used the quantitative value of melting curve analysis of heterozygous genetic loci to establish a relative allelic count. The two alleles of a given single-nucleotide polymorphism (SNP) were differentiated by thermodynamic stability with a fluorescently labeled hybridization probe and were quantified by relative areas of derivative melting curves detected after fluorescence resonance energy transfer. Heterozygous SNPs provided internal controls for the assay.

RESULTS

We selected six SNPs, heterozygous in at least 30% of a random population, to form a panel of informative loci in the majority of a random population. After normalization to a heterozygous control, samples segregated into three categories; nontrisomic samples had mean allele ratios of 0.96-1.09, whereas trisomic samples had mean ratios of 1.84-2.09 or 0.46-0.61, depending on which allele was duplicated. Within-run mean CVs of ratios were 6.5-27%, and between-assay mean CVs were 13-24%.

CONCLUSIONS

The use of melting curve analysis of multiple SNPs is an alternative to the use of small tandem repeats for the detection of trisomies. Because of the high density of SNPs, the approach may be specifically useful for very fine mapping of the regions of chromosome 21 that are critical for Down syndrome; it is also applicable to aneuploidies other than trisomy 21 and to specimens that are not amenable to cytogenetic analysis.