A supernumerary marker chromosome originating from two different regions of chromosome 18

Small supernumerary marker chromosomes: A legacy of trisomy rescue?

We studied by a whole genomic approach and trios genotyping, 12 de novo, nonrecurrent small supernumerary marker chromosomes (sSMC), detected as mosaics during pre- or postnatal diagnosis and associated with increased maternal age. Four sSMCs contained pericentromeric portions only, whereas eight had additional non-contiguous portions of the same chromosome, assembled together in a disordered fashion by repair-based mechanisms in a chromothriptic event. Maternal hetero/isodisomy was detected with a paternal origin of the sSMC in some cases, whereas in others two maternal alleles in the sSMC region and biparental haplotypes of the homologs were detected. In other cases, the homologs were biparental while the sSMC had the same haplotype of the maternally inherited chromosome. These findings strongly suggest that most sSMCs are the result of a multiple-step mechanism, initiated by maternal meiotic nondisjunction followed by postzygotic anaphase lagging of the supernumerary chromosome and its subsequent chromothripsis.

Clinical and molecular delineation of Tetrasomy 9p syndrome: report of 12 new cases and literature review

Tetrasomy 9p is a generic term describing the presence of a supernumerary chromosome incorporating two copies of the 9p arm. Two varieties exist: isodicentric chromosome 9p (i(9p)), where the two 9p arms are linked by a single centromeric region, and pseudodicentric 9p (idic(9p)), where one active and one inactive centromere are linked together by a proximal segment of 9q that may incorporate euchromatic material. In living patients, i(9p) and idic(9p) are usually present in a mosaic state. Fifty-four cases, including fetuses, have been reported, of which only two have been molecularly characterized using array-CGH. Tetrasomy 9p leads to a variable phenotype ranging from multiple congenital anomalies with severe intellectual disability and growth delay to subnormal cognitive and physical developments. Hypertelorism, abnormal ears, microretrognathia and bulbous nose are the most common dysmorphic traits. Microcephaly, growth retardation, joint dislocation, scoliosis, cardiac and renal anomalies were reported in several cases. Those physical anomalies are often, but not universally, accompanied by intellectual disability. The most recurrent breakpoints, defined by conventional cytogenetics, are 9p10, 9q12 and 9q13. We report on 12 new patients with tetrasomy 9p (3 i(9p), 8 idic(9p) and one structurally uncharacterized), including the first case of parental germline mosaicism. All rearrangements have been characterized by DNA microarray. Based on our results and a review of the literature, we further delineate the prenatal and postnatal clinical spectrum of this imbalance. Our results show poor genotype-phenotype correlations and underline the need of precise molecular characterization of the supernumerary marker.

A de novo supernumerary genomic discontinuous ring chromosome 21 in a child with mild intellectual disability

Small supernumerary marker chromosomes (sSMCs) are structurally abnormal extra chromosomes that cannot be unambiguously identified or characterized by conventional banding techniques alone, and they are generally equal in size or smaller than chromosome 20 of the same metaphase spread. Small supernumerary ring chromosomes (sSRCs), a smaller class of marker chromosomes, comprise about 10% of the cases. For various reasons these marker chromosomes have been the most difficult to characterize; although specific syndromes have not yet been defined, 60% of cases are associated with an abnormal phenotype. The chromosomal material involved, the degree and tissutal distribution of mosaicism, and the possible presence of uniparental disomy, are the important factors determining whether or not the ring chromosome will give rise to symptoms. Using conventional and molecular cytogenetics approaches we identified a de novo chromosome 21 sSRC in a child with speech delay and mild intellectual disability. By using aCGH analysis and SNP arrays, we report the presence of two discontinuous regions of chromosome 21 and the paternal origin of the sSRC. A thorough neuropsychiatric evaluation is also provided. Only few other cases of complex discontinuous ring chromosomes have been described in detail.

De novo isochromosome 18p in a female dysmorphic child

Isochromosome 18p results in tetrasomy 18p. Most of the i(18p) cases reported so far in the literature are sporadic due to de novo formation, while familial and mosaic cases are infrequent. It is a rare chromosomal abnormality, occurring once in every 140,000 livebirths, affecting males and females equally. In the present investigation, we report a de novo i(18p) in a female dysmorphic child. The small metacentric marker chromosome was confirmed as i(18p) in the proband by cytogenetic and FISH analysis [47,XX+i(18p)]. Cytogenetic investigations in the family members revealed normal chromosome numbers, indicating the case as a de novo event of i(18p) formation. It could be due to the somewhat advanced maternal age (32 years) and/or expression of recessive genes in the proband, who is the progeny of consanguineous marriage, which could have led to misdivision and nondisjunction of chromosome 18 in meiosis I, followed by failure in the chromatid separation of 18p in meiosis II and by inverted duplication.

Small familial supernumerary ring chromosome 2: FISH characterization and genotype-phenotype correlation

A child and his mother were found to be mosaic for a small supernumerary marker chromosome (SMC) that was identified and characterized by means of fluorescent in situ hybridization. The marker chromosome was derived from the pericentromeric region of chromosome 2; the involvement of proximal 2q was determined by YAC probes. The proband was referred because of psychotic illness and mild mental retardation, whereas his mother presented only minor dysmorphisms. There are only a few published reports concerning SMC(2) or proximal 2q trisomy. We reviewed the previously reported cases in an attempt to establish genotype-phenotype correlations, which are particularly important when SMCs are identified in prenatal diagnosis.

Clinical and molecular-cytogenetic studies in seven patients with ring chromosome 18

We report the results of detailed clinical and molecular-cytogenetic studies in seven patients with ring chromosome 18. Classical cytogenetics and fluorescence in situ hybridization (FISH) analysis with the chromosome 18 painting probe identified five non-mosaic and two complex mosaic 46,XX,dup(18)(p11.2)/47,XX,dup(18)(p11.2),+r(18) and 46,XX,dup(18)(p11.32)/47,XX,dup(18)(p11.32),+r(18) cases. FISH analysis was performed for precise characterization of the chromosome 18 breakpoints using chromosome 18-specific short-arm paint, centromeric, subtelomeric, and a panel of fifteen Alu- and DOP-PCR YAC probes. The breakpoints were assessed with an average resolution of approximately 2.2 Mb. In all r(18) chromosomes, the 18q terminal deletions ranging from 18q21.2 to 18q22.3 ( approximately 35 and 9 Mb, respectively) were found, whereas only in four cases could the loss of 18p material be demonstrated. In two cases the dup(18) chromosomes were identified as inv dup(18)(qter-->p11.32::q21.3-->qter) and inv dup(18)(qter-->p11.32::p11.32-->p11.1: :q21.3-->qter)pat, with no evidence of an 18p deletion. A novel inter-intrachromatid mechanism of formation of duplications and ring chromosomes is proposed. Although the effect of "ring instability syndrome" cannot be excluded, the phenotypes of our patients with characteristic features of 18q- and 18p- syndromes are compared and correlated with the analyzed genotypes. It has been observed that a short neck with absence of cardiac anomalies may be related to the deletion of the 18p material from the r(18) chromosome.

Microdissection and reverse painting in a melanoma cell line: a detailed description of structurally abnormal chromosomes

Seven structurally abnormal chromosomes from a newly established melanoma cell line were microdissected. The DNA from these chromosomes was DOP amplified and labeled with Biotin or Digoxigenin for FISH analysis. The complex nature of these markers is described.