Taiwan J Obstet Gynecol • September 2010 • Vol 49 • No 3 381 ■ RESEARCH LETTER ■
Small supernumerary marker chromosomes (sSMCs) are defined as structurally abnormal chromosomes that cannot be identified or characterized by conven-tional banding cytogenetics, and are generally equal in size or smaller than a chromosome 20 [1–3]. sSMCs are present in 0.044% of newborn infants and in 0.075% of prenatal cases [1,3–5]. About 70% of sSMCs arise
de novo [4], about 70% of sSMCs are derived from
acro-centric chromosomes [1,6], and about 70% cases of
de novo sSMCs have no phenotypic effects [5]. Prenatal
diagnosis of sSMCs gives rise to difficulties in genetic counseling, and identification of the nature of the aber-rant chromosome requires molecular cytogenetic tech-nologies [5,7–10]. We present our experience of the prenatal diagnosis and molecular cytogenetic charac-terization of an sSMC derived from chromosome 22 using fluorescence in situ hybridization (FISH) and array comparative genomic hybridization (aCGH).
A 42-year-old woman, gravida 2, para 0, underwent amniocentesis at 18 weeks of gestation because of advanced maternal age. The woman was phenotypically normal. She had previously experienced one sponta-neous abortion. Amniocentesis revealed an sSMC. The sSMC was C-band positive and nucleolar organizing region-stain positive. Cytogenetic analysis of the parents revealed that the mother carried the same sSMC. The
karyotype was 47,XX,+mar mat (Figure 1). FISH using a centromere 14/22-specific α-satellite DNA probe (D14Z1/D22Z1) (cep14/22) (Cytocell, Adderbury, Oxfordshire, UK) and a centromere 22-specific α-satellite DNA probe (p190.22; D22Z4 probe reported by Rocchi et al) [11] revealed that the sSMC was posi-tive for D14Z1/D22Z1 (Figure 2) and posiposi-tive for two D22Z4 signals (Figure 3). The parents decided to con-tinue the pregnancy. A normal female baby weighing 2,828 g was delivered uneventfully at 39 weeks of gesta-tion. She was developing normally at her 4-year follow-up. aCGH analysis using Oligo HD Scan (CMDX, Irvine, CA, USA) showed no genomic imbalance on the peri-centromeric euchromatic region of chromosome 22 (Figure 4). The sSMC was inv dup(22)(q10). The kary-otype was 47,XX, +mar .ish der(22) (D14Z1/D22Z1+, D22Z4++) or 47,XX, +inv dup(22)(q10).
This case shows the limitations of the cep14/22 (D14Z1/D22Z1) probe and the usefulness of the D22Z4
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Chih-Ping Chen1,2,3,4,5,6*, Chyi-Chyang Lin7, Yi-Ning Su8, Fuu-Jen Tsai4,7,9, Schu-Rern Chern2, Chen-Chi Lee1, Wen-Ling Chen1, Li-Feng Chen1, Pei-Chen Wu1, Wayseen Wang2,10
Departments of 1Obstetrics and Gynecology and 2Medical Research, Mackay Memorial Hospital, 5Institute of Clinical and Community Health Nursing and 6Department of Obstetrics and Gynecology,
School of Medicine, National Yang-Ming University, 8Department of Medical Genetics, National Taiwan University
Hospital and 10Department of Bioengineering, Tatung University, Taipei; 3Department of Biotechnology, Asia University, 4School of Chinese Medicine, College of Chinese Medicine, China Medical University and
Departments of7Medical Research and 9Medical Genetics, China Medical University Hospital, Taichung, Taiwan.
*Correspondence to: Dr Chih-Ping Chen, Department
of Obstetrics and Gynecology, Mackay Memorial Hospital, 92, Section 2, Chung-Shan North Road, Taipei, Taiwan. E-mail: [email protected] Accepted: June 11, 2010 1 6 13 14 15 19 20 21 22 mar Y X 16 17 18 7 8 9 10 11 12 2 3 4 5
(p190.22) probe and aCGH in the identification of an sSMC derived from chromosome 22. The cep14/22 probe, along with cep13/21 and cep15, can be used for the rapid identification of an acrocentric chromosome-derived sSMC with positive C-banding and nucleolar organizing region-staining. However, cep14/22 recog-nizes the centromeres of both chromosomes 14 and 22. In contrast, the alphoid p190.22 (D22Z4) probe
specifically recognizes the centromere of chromosome 22 under high stringency hybridization conditions [11], and is therefore useful for differentiating between chromosomes 22 and 14 when the sSMC is hybridized with cep14/22 (D14Z1/D22Z1). Other chromosome 22 centromere-specific probes include p22/1:2.1 (D22Z2) [12] and D22Z3 [13,14]. aCGH has the ability to detect DNA dosage imbalances, including deletions
Taiwan J Obstet Gynecol • September 2010 • Vol 49 • No 3 382 C.P. Chen, et al 22 mar 14 14 22
Figure 3.Fluorescence in situ hybridization using an α-satellite probe D22Z4 showing positive hybridization signals on two chromosomes 22 and the marker chromosome (mar). The marker chromosome contains two positive signals for D22Z4. Genes q11.21 q11.23 q12.1 q12.2 q12.3 q13.1 q13.2 q13.31 22q13 deletion syndrome (pH)H HHH HH H H H H H H H H I H H H H Exons CNVs 180Kv1_CMDX Agilent105KV6 BacHDscanV2 DecipherSyndromes Segmental Dups 0 −2.5 −2.0 −1.5 −1.0 −5 0 0.5 1.0 1.5 2.0 2.5 5 Mb 10 Mb 15 Mb 20 Mb 25 Mb 30 Mb 35 Mb 40 Mb 45 Mb
Figure 4.Oligonucleotide-based array comparative genomic hybridization using Oligo HD Scan showing no genomic imbalance in the pericentromeric euchromatic region of chromosome 22.
14
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22
22
mar
Figure 2.Fluorescence in situ hybridization using an α-satellite probe D14Z1/D22Z1, cep14/22 (spectrum red) showing positive hybridization signals on two chromosomes 14, two chromosomes 22 and the marker chromosome (mar).
and duplications, in the pericentromeric euchromatic regions and is useful for characterizing the genomic imbalance in the sSMC. Multiplex ligation-dependent probe amplification (MLPA) is commercially available for rapid aneuploidy diagnosis. The SALSA MLPA P181 and P182 centromere kits contain one probe for the short arm and one probe for the long arm of chromo-somes other than acrocentric chromochromo-somes, and two probes for the long arm of chromosomes 13, 14, 15, 21 and 22. Each probe is located close to the centromere of a specific chromosome. The designated regions of chromosome 22 are CECR5 and CECR1 at 22q11.1 for the P181 centromere kit, and the designated regions of chromosome 22 are CECR1 at 22q11.1 and SLC25A18 at 22q11.21 for the P182 centromere kit [15]. A high-definition MLPA (MLPA-HD) 22q11 kit has recently been developed to detect copy-number changes at 37 loci encompassing a 3-Mb region on 22q11, including the critical region for DiGeorge syndrome/velocardio-facial syndrome (DGS/VCFS), cat eye syndrome (CES) and commonly deleted distal regions [16].
About 9% of SMCs are derived from chromosome 22 [17]. The 22q11 region is susceptible to chromo-somal rearrangements leading to DGS/VCFS, CES and t(11;22)der(22) syndrome, all three of which have breakpoint regions harboring a similar low-copy repeat (LCR) known as LCR22 [18,19]. Homologous recom-bination events between LCR22s during meiosis have been implicated in DGS/VCFS and CES, and the sites of chromosome breakage on 11q23 and 22q11 in der(22) syndrome occur in the unstable AT-rich palin-dromic sequences leading to nonhomologous recom-bination mechanisms [19–22]. At least 61 patients with an sSMC(22) derived from inv dup(22)(q10)-(q11.21) or min(22)(pter-:p11.2 q11.1:-q11.21:) have been documented to date, with no clinical findings [23]. However, at least 106 patients with an sSMC(22) derived from inv dup(22) or inv dup(22)(q11.21)-(q11.23) have been documented with CES or clinical abnormalities [23]. CES (OMIM 115470) is usually associated with an sSMC(22) presenting as inv dup(22)(q11) and has a highly variable phenotype, including coloboma of the iris, anal atresia with fistula, down-slanting palpe-bral fissures, preauricular tags and/or pits, mild hyper-telorism, cardiac defects, renal malformation, normal or near-normal mental development in 44% of patients, mild or moderate mental retardation in 48% of patients and severe mental retardation in 7% of patients [24–26]. Prenatal diagnosis of sSMC(22) has remained a diag-nostic challenge and should alert clinicians to the pos-sibility of CES with the involvement of trisomy or tetrasomy of the CES chromosome region candidate genes, such as CECR1-CECR9 at 22q11 [19,26–30].
Acknowledgments
We would like to thank Professor Mariano Rocchi, University of Bari, Bari, Italy and Dr Charles Lee, Harvard Medical School, Boston, USA for providing us with the D22Z4 probe. This work was supported by research grants BH92-GC03-1 from Bureau of Health Promo-tion, Department of Health, NHRI EX92-9207SI from National Health Research Institute, NSC-96-2314-B-195-008-MY3 and NSC-97-2314-B-195-006-MY3 from the National Science Council, and MMH-E-99004 from Mackay Memorial Hospital, Taipei, Taiwan.
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