Tetrasomy 9p: an emerging syndrome

An infant with non-mosaic 9p tetrasomy is described. The tetrasomy apparently results from a translocation involving the 9qh region. All the cells analyzed from multiple banding techniques from lymphocyte culture as well as skin fibroblast culture were 9p tetrasomic. The infant, who had the characteristic dysmorphic features of 9p tetrasomy, survived for 2 months. Prominent features included: low birth weight, severe retardation, brachycephaly with large anterior fontanelle, hypertelorism with short bilateral palpebral fissures, beaked nose, bilateral cleft lip and palate, and low-set, malformed ears. Skeletal anomalies, ambiguous genitalia and heart defect were also observed. These features are highly characteristic of the 9p tetrasomy syndrome based on six pure tetrasomy and four cases of tetrasomy that included part of the 9qh region.     

Three cases of tetrasomy 9p

We report three cases of tetrasomy 9p, two of which were confirmed prenatally. All three had characteristic findings on ultrasound and at birth. We also present a review of the literature, which suggests that a recognizable phenotype for this condition is emerging. Common findings on prenatal ultrasound include intrauterine growth restriction, ventriculomegaly, cleft lip or palate, and renal anomalies. These findings can provide a clue toward the prenatal diagnosis of this condition. There is also a clearly recognizable phenotype at birth. Facial characteristics include hypertelorism, broad nasal bridge/bulbous or beaked nose, cleft lip/palate, ear anomalies, and micrognathia. The exact extent of the isochromosome does not seem to predict severity, but mosaic cases are less severe, or at least have a greater probability of survival. © 2002 Wiley‐Liss, Inc.     

Tetrasomy 9p: Tissue-limited idic(9p) in a child with mild manifestations and a normal CVS result. Report and review

Supernumerary isochromosomes resulting in autosomal tetrasomy are rare and have been described only for 12p, 18p, and 9p. Nineteen previous cases of tetrasomy 9p have been reported, and in 6 cases, tissue-specific mosaicism was implied with the i(9p) cell line present exclusively or predominantly in blood. We report on an infant who had apparently normal chromosomes (46,XY) on CVS. He was referred for genetic evaluation because of mild developmental delay and minor anomalies. In 75% of blood cells he had an extra isodicentric 9p chromosome (pter→q12→pter). The interpretation of tetrasomy 9p was confirmed by elevated GALT activity. No tetrasomy 9p cells were seen in 100 skin fibroblasts. This case demonstrates the tissue specific mosaicism in tetrasomy 9p which rendered the anomaly undetectable by CVS. It also demonstrates the mild end of the clinical spectrum associated with tetrasomy 9p. © 1993 Wiley-Liss, Inc.     

Tissue-specific mosaicism for tetrasomy 9p uncovered by array CGH

We report on a patient with a mild clinical phenotype, including genital anomalies, with mosaic tetrasomy 9p. Karyotype analysis of peripheral blood lymphocytes detected a supernumerary isochromosome 9p present in every cell, with the initial result being reported as tetrasomy 9p in non-mosaic form. However,array Comparative Genomic Hybridization (aCGH) studies on DNA extracted from peripheral blood lymphocytes and saliva showed that the patient had tissue-specific mosaicism, with a lower level of abnormal cells in the saliva. These results correlate with the patient's clinical features as non-mosaic cases of tetrasomy 9p have a more severe, often lethal, clinical phenotype. If non-mosaic tetrasomy 9p is identified in a peripheral blood culture then examination of a different tissue type should be undertaken. Array CGH may be used as an alternative to karyotype analysis to estimate the level of mosaicism, and may eliminate the need for invasive skin biopsy as samples such as buccal smear and saliva can be used. Array CGH is able to detect mosaicism, establish the euchromatic content of supernumerary marker chromosomes, and identify imbalances elsewhere in the genome allowing more accurate counselling and prognosis for patients.     

Tetrasomy 5p mosaicism due to an extra i(5p) in a severely affected girl

We present a case of mosaic 5p tetrasomy. The mosaicism 46,XX/47,XX, + i(5p) was found at different ratios in blood lymphocytes, skin fibroblasts, and chondrocytes. The origin of the extra isochromosome was confirmed by FISH. The clinical picture corresponds to that described in trisomy 5p patients, although it was more severe than the two previously reported cases of mosaic 5p tetrasomy. No correlation between clinical severity and proportion of tetrasomic cells in blood or fibroblasts was found in these cases. Am. J. Med. Genet. 68:481–484, 1997. © 1997 Wiley-Liss, Inc.     

Tissue specificity and stability of mosaicism in Pallister–Killian +i(12p) syndrome: Relevance for prenatal diagnosis

We ascertained +i(12p) mosaicism during third trimester in a case of polyhydramnios and diaphragmatic hernia. Primary cultures of amniocytes had colonies with +i(12p), colonies without +i(12p), and mixed colonies with 46/47, +i(12p). The likely explanation was instability and loss of i(12p) during somatic divisions of amniocytes. Fetal blood in third trimester retained +i(12p) in 13% of cells. A review of mosaicism in published cases indicates that factors influencing the presence of +i(12p) include tissue type and in vitro and in vivo age. In blood, amniocyte, and probably bone marrow cultures, +i(12p) is less stable than in fibroblast-like cultures derived from skin and other tissues. Young cultures at early passage are more likely to have +i(12p) than old cultures. Cultures from young (especially fetal) donors are more likely to retain +i(12p) than cultures from adult donors. These rules will be important in determining appropriate tissues for diagnosis and interpretation of mosaicism in this disorder.     

Isochromosome 18p in a mother and her child

We present a familial case of isochromosome 18p [i(18p)] as a supernumerary chromosome. The mother, who is a mosaic for i(18p) with partial tetrasomy 18p syndrome, transmitted the isochromosome to her only child. The child has the full syndrome of tetrasomy 18p. This is the first case of mosaicism i(18p) in an adult patient with clinical manifestations. © 1993 Wiley-Liss, Inc.     

Tetrasomy 9p due to an intrachromosomal triplication of 9p13-p22

To date, approximately 30 patients have been described with a tetrasomy 9p, all being caused by the presence of an isochromosome 9p. We now report on a 3-year-old boy with a de novo intrachromosomal triplication of 9p13-p22, resulting in partial tetrasomy 9p. We compared his phenotype with cases of tetrasomy 9p caused by the presence of an extra isochromosome 9p. He has facial anomalies similar to those of cases of tetrasomy 9p, central nervous system abnormalities, and severe psychomotor retardation but no other major congenital anomalies. Fluorescence in situ hybridization with region-specific probes showed that the middle repeat of the triplicated part is inverted. Microsatellite analysis demonstrated an involvement of both paternal chromosome 9 homologues in the triplication. This is compatible with either unequal crossing over of three of the four chromatids in paternal meiosis I or with a double crossing over in meiosis I and II (or an early mitotic division). Am. J. Med. Genet. 86:168–173, 1999. © 1999 Wiley-Liss, Inc.     

Moving towards a syndrome: a review of 20 cases and a new case of non-mosaic tetrasomy 9p with long-term survival

Tetrasomy 9p is a rare syndrome that has now been described in nearly a score of cases. We present a new case of i(9p) that presented to us early in infancy with significant dysmorphological features, including growth retardation, psycho-motor delay, hemifacial microsomia, auditory canal atresia, high-arched palate, bulbous nose, strabismus, epicanthic folds, congenital heart disease, dislocated hips, hypoplastic external genitalia, simian palmar creases, dysplastic nails and small digits. Chromosomal analysis revealed a 47,XX,idic(9)(q12) karyotype on GTG- and C-banding studies on peripheral blood lymphocytes. Fluorescent in situ hybridization (FISH) studies confirmed the origin of the extra chromosome. A review of the literature and a comparative analysis of the several well-documented cases of i(9p) revealed a pattern of recurring features, including ear malformations, skeletal and joint problems (especially dislocations), hypoplasia of nails and digits, palatal abnormalities, hypertelorism, urogenital anomalies and developmental retardation. In the light of this analysis, we feel that tetrasomy 9p will soon be considered a clinically recognizable syndrome.     

Pallister-Killian syndrome: Review of fetal phenotype

Pallister-Killian syndrome is a multi-system sporadic disorder with developmental delay. It is a rare chromosomal abnormality involving supernumerary isochormosome 12p. The disorder exhibits tissue specific mosaicism. The first prenatal diagnosis of PKS was reported in 1985 after ultrasound detection of fetal anomalies. Since this observation, there have been about 62 reports of fetuses with PKS. In this review, we cover the prenatal aspects of PKS.     

Prenatal diagnosis and clinical findings in a case of hexasomy 12p

We report the first case of hexasomy 12p mosaicism due to 2 copies of an apparent i(12p) [46,XX/48,XX, +i(12p),+i(12p)] In every cell that contained the i(12p), 2 copies of the marker were found. The hexasomy was found in amniocytes (16%) and skin fibroblasts (95%) but not in peripheral blood lymphocytes. The chromosomal origin of the marker was confirmed with the use of in situ hybridization of alpha-satellite specific for the centromere of chromosome 12. The present case was diagnosed following chromosome analysis for anomalies on ultrasound. The hexasomy 12p patient showed striking phenotypic similarities with severely affected tetrasomy 12p cases and died shortly after birth. We propose that the more severe presentation of this case is due to the 4 extra copies of 12p. © 1993 Wiley-Liss, Inc.     

Parental origin of the isochromosome 12p in Pallister-Killian syndrome: Molecular analysis of one patient and review of the reported cases

Pallister-Killian syndrome (PKS) is characterized by multiple congenital anomalies including pigmentary skin changes, mental retardation, and the mosaic presence of a tissue-limited isochromosome 12p [i(12p)]. Mechanism(s) of formation and parental origin of the isochromosome are not well understood. In this study, microsatellite DNA markers of chromosome 12p were used to identify the parental origin of the extra chromosome in an 8-year-old previously reported patient with PKS. The i(12p) was found to be maternally inherited. Reported cases of PKS where the parental origin of the i(12p) was determined were also reviewed. In all the cases, with one exception, the errors were found to be maternal in origin. Premeiotic mitotic error may be the most likely mechanism for i(12p) formation in this syndrome. Am. J. Med. Genet. 84:111–115, 1999. © 1999 Wiley-Liss, Inc.     

Mosaic "tetrasomy" 8p: case report and review of the literature

A male infant presenting with multiple anomalies including a midline cleft palate, anasarca, hepatomegaly, pulmonary edema, agenesis of the corpus callosum, and complex congenital cardiac anomalies was found to have mosaicism for an additional chromosome that appeared (following GTG-banding and FISH) to be a monocentric isochromosome of the short arm of chromosome 8 (46,XY/47,XY,+i(8p)). Nine other cases of mosaicism for an additional i(8p) were reviewed. Considerable phenotypic variation was noted. Consistent features were identified including agenesis of the corpus callosum, cardiac malformations, and minor facial dysmorphology. The phenotype of these patients partially overlaps those of trisomy 8 and trisomy 8p. By studying additional individuals with this condition, mosaic tetrasomy 8p may emerge as a recognizable clinical phenotype.     

Pallister-Killian syndrome: cytogenetic and molecular studies

Pallister-Killian syndrome is a dysmorphic syndrome characterized by a tissue-limited mosaicism: a majority of fibroblasts have 47 chromosomes with an extra small metacentric chromosome, whereas the karyotype of lymphocytes is normal. In this study, the interpretation of the extra chromosome as composed of two short arms of chromosome 12 is confirmed, using molecular methods. Furthermore, restriction fragment length polymorphisms indicate that the two arms are identical, which is compatible with the hypothesis of an isochromosome 12p. A new feature which may be important in understanding the mechanism of origin of the abnormality is described: the proportion of abnormal mitoses falls dramatically during long-term culture of fibroblasts.     

New case of mosaic tetrasomy 9p with additional neurometabolic findings

Tetrasomy 9p is a rare chromosomal aberration that was described in 28 previous patients. Here we report on a newborn girl who was referred for genetic evaluation because of developmental delay, hypertonicity, microcephaly, minor anomalies, and neurometabolic findings. She had an isochromosome 9p (pter → p10 → pter) in 32% of blood cells. The extra chromosome was not found in amniocytes. Examination of fibroblasts from different skin biopsies also showed mosaicism in this tissue. In a first biopsy from the abdominal wall, the cells (n = 50) had a normal chromosomal complement. Further analysis of fibroblasts from the left forearm showed the isochromosome 9p in 5 out of 8 mitoses. Fluorescence in situ hybridization (FISH), using a whole chromosome 9 probe, confirmed that the extra marker was 9 in origin. Molecular studies showed that the isochromosome was of maternal origin. Meiotic nondisjunction was followed by centromeric misdivision and postzygotic loss of the marker. Am. J. Med. Genet. 75:530–533, 1998. © 1998 Wiley-Liss, Inc.     

Mosaic tetrasomy 8p in two patients: Clinical data and review of the literature

We report on 2 girls with mosaic tetrasomy 8p. Patient 1 showed the extra iso 8p chromosome in 20% of cultured lymphocytes and 18% of cultured fibroblasts [46,XX/47,XX, + i(8p)]. She presented with growth retardation, mild facial alterations, and motor developmental delay. Patient 2 presented with developmental delay, hypotonia, and slight facial alterations; she had the extra iso 8p chromosome in 94% of cultured peripheral lymphocytes. The patients are compared to the 6 previously reported cases. In our experience, the presently reported patients clinically resemble children with inv dup(8)(p21-p22) and patients with mosaic trisomy 8. © 1994 Wiley-Liss, Inc.     

Risk effect of maternal age in Pallister i(12p) syndrome

Pallister syndrome is consequent to mosaicism for i(12p). The isochromosome is found mainly in skin fibroblast cultures, but rarely also in cultured peripheral blood lymphocytes. Maternal age for reported cases of Pallister syndrome was significantly older (p less than 0.005) compared to maternal age for the general population, and similar to maternal age in cases of Down syndrome (p less than 0.5). Paternal age in cases of Pallister syndrome was that expected in the general population from the age of their spouses (p less than 0.9). These data complement the maternal age effect seen in other aneuploidy conditions and suggest, as is found in mosaic cases of Down syndrome, that the chromosomally normal cell line in Pallister syndrome arises post-conception from a zygote already aneuploid consequent to meiotic nondisjunction.     

Maternal origin of a unique extra chromosome,der(9)(pter→q13::q13→q12:) in a girl with typical trisomy 9p syndrome

We report on a girl with the typical trisomy 9p syndrome who had an additional E‐sized metacentric chromosome. On the basis of GTG‐ and CBG‐banding, her karyotype was considered to be 47,XX,+der(9)(pter→q13::q13→q12:) de novo. Results of a fluorescence in situ hybridization study using a chromosome 9‐specific painting probe were compatible with this cytogenetic interpretation. Molecular analyses of six highly polymorphic dinucleotide repeat loci on the short arm and the proximal long arm of chromosome 9 demonstrated that the girl inherited one allele from her father and two identical or different alleles from the mother. We speculated that the extra chromosome may have resulted from either nondisjunction of chromosome 9 followed by a U‐type exchange and a crossing‐over between different sister chromatids during maternal meiosis I and subsequent breakage and malsegregation during meiosis II, or nondisjunction during meiosis II followed by isochromosome formation in one of the two maternal chromosomes 9 and subsequent breakage. © 2001 Wiley‐Liss, Inc.