Failure of PHA-stimulated i(12p) lymphocytes to divide in Pallister-Killian syndrome

Pallister-Killian Syndrome versus Trisomy 12p-A Clinical Study of 5 New Cases and a Literature Review

Pallister–Killian syndrome (PKS) is a rare, sporadic disorder defined by a characteristic dysmorphic face, pigmentary skin anomalies, intellectual disability, hypotonia, and seizures caused by 12p tetrasomy due to an extra isochromosome 12p. We present three cases of PKS and two cases of trisomy 12p to illustrate and discuss features rarely cited in the literature, present certain particularities that not yet been cited, and analyze the differences between entities. Moreover, we present alternative methods of diagnosis that could be easily used in daily practice. Features not yet or rarely reported in PKS literature include marked excess of hair on the forehead and ears in the first months of life, a particular eye disorder (abnormal iris color with pointed pupil), connective tissue defects, repeated episodes of infection and autonomic dysfunction, endocrine malfunction as a possible cause of postnatal growth deficit, more complex sensory impairments, and mild early myoclonic jerks. After performing different combinations of tests, we conclude that MLPA (follow-up kit P230-B1) or array CGH using DNA extracted from a buccal swab is a reliable method of diagnosis in PKS and we recommend either one as a first intention diagnostic test. In cases without major defects associated (suspicion trisomy 12p), subtelomeric MLPA should be performed first.

Mosaic complete tetrasomy 21 in a fetus with complete atrioventricular septal defect and minor morphological variations

Background

Tetrasomy 21 is a very rare aneuploidy which could clinically resemble a Down syndrome. It was most often described in its partial form than complete. We report the prenatal, pathological and genetic characteristics of a fetus with mosaic complete tetrasomy 21. This is the second well‐documented description of a complete tetrasomy 21 in the literature.

Methods

Prenatal and fetal pathological examinations, cytogenetic and molecular analyses were performed to characterize fetal features with tetrasomy 21.

Results

Prenatal ultrasound examination revealed an isolated complete atrioventricular septal defect with normal karyotype on amniotic fluid. After termination of pregnancy, clinical examination of the fetus evoked trisomy 21 or Down syndrome. Chromosomal microarray analysis and FISH on lung tissue showed a mosaicism with four copies of chromosome 21 (tetrasomy 21).

Conclusion

Our observation and the review of the literature reported the possibility of very weak mosaicism and disease‐causing confined tissue‐specific mosaicism in fetus or alive patients with chromosome 21 aneuploidy, mainly Down syndrome. In case of clinical diagnosis suggestive of Down syndrome, attention must be paid to the risk of false‐negative test due to chromosomal mosaicism (very weak percentage, different tissue distribution). To overcome this risk, it is necessary to privilege the diagnostic techniques without culture step and to increase the number of cells and tissues analyzed, if possible. This study highlights the limits of microarray as the unique diagnostic approach in case of weak mosaic and French cytogenetics guidelines recommend to check anomalies seen in microarray by another technique on the same tissue.

Prenatal diagnosis of Pallister-Killian syndrome using cord blood samples

Background

Pallister-Killian syndrome (PKS) (OMIM:#601803) is a rare sporadic genetic disorder characterized by multi-malformations which is caused by the presence of the extra isochromosome 12p. PKS is featured by the tissue-limited mosaicism of the isochromosome 12p [i(12p)]. There were a wide spectrum of prenatal ultrasound findings of PKS, which made it difficult to be found in first or second trimester. Polyhydramnios, diaphragmatic hernia, and rhizomelic limb shortening were the most common prenatal ultrasound abnormalities in PKS. This study retrospectively analyzed the ultrasound findings and molecular cytogenetic results of four PKS fetuses diagnosed by using cord blood samples.

Results

The ultrasound anomalies of four PKS fetuses are described as follows: fetal macrosomia, cerebral ventriculomegaly, increased NT thickness, rhizomelic limbs shortening, polyhydramnios. Biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), femur length (FL) measurements were above the mean in three fetuses,while one fetus showed rhizomelic limbs shortening. Combined with this study and previous literature, polyhydramnios was the most frequent anomaly observed in prenatal ultrasound examination of PKS, which accounted for 48% (94/194). Fetal macrosomia was present in 15% (29/194), cerebral ventriculomegaly in 13% (25/194), thickened nuchal fold in 9% (18/194), rhizomelic limbs shortening in 26% (51/194). I(12p) was found in the karyotype analysis of cultured cord blood lymphocytes and the mosaic ratios ranged from 2 to 5%. Single nucleotide polymorphisms array (SNP-array) results suggested that the whole short arm of chromosome 12 was duplicated with 2~3 copies. Fluorescence in situ hybridization (FISH) was performed to confirm the results of karyotype and SNP-array.

Conclusions

In case non-specific indicators such as fetal macrosomia, polyhydramnios and rhizomelic limbs shortening are observed meanwhile in prenatal ultrasound, targeted detection of PKS should be considered. In the prenatal diagnosis of PKS, the combination of SNP-array and FISH with conventional karyotype are the key to seek i(12p) and for precise diagnosis.

Postnatal clinical phenotype of five patients with Pallister-Killian Syndrome (tetrasomy 12p): Interest of array CGH for diagnosis and review of the literature

Background

Pallister–Killian syndrome (PKS) is a rare sporadic disorder caused by tetrasomy of the short arm of chromosome 12. The main clinical manifestations are global developmental delay, intellectual disability, epilepsy, dysmorphic features, hypopigmented and/or hyperpigmented lesions, and multiple congenital anomalies. PKS is associated with tissue mosaicism, which is difficult to diagnose through peripheral blood sample by conventional cytogenetic methods and fluorescence in situ hybridization.

Methods

Here, we report five patients with PKS. We delineate their clinical phenotypes and we compare them with previously published cases. We used array Comparative Genomic Hybridization (aCGH) with DNA extracted from peripheral blood samples. The five patients have also been tested by conventional cytogenetics techniques.

Results

Four out of five patients showed tetrasomy 12p by aCGH. Three of the four patients have typical i(12p) and one of the four demonstrated atypical tetrasomy 12p. The percentage of mosaicism was as low as 20%. Our cohort exhibited the typical PKS phenotypes.

Conclusion

Our results demonstrate the efficacy of aCGH for the diagnosis of PKS from DNA extracted from lymphocytes. Thus, for patients suspected of PKS, we recommend performing aCGH on lymphocytes at an early age before  proceeding to skin biopsy. aCGH on peripheral blood samples is sensitive in detecting low level of mosaicism and it is less invasive method than skin biopsy. We reviewed also the literature concerning the previously published PKS patients diagnosed by aCGH.

First Report of Low-Rate Mosaicism for 20q11.21q12 Deletion and Delineation of the Associated Disorder

Interstitial deletions of the long arm of chromosome 20 are very rare, with only 12 reported patients harboring the 20q11.2 microdeletion and presenting a disorder characterized by psychomotor and growth delay, dysmorphisms, and brachy-/clinodactyly. We describe the first case of mosaic 20q11.2 deletion in a 5-year-old girl affected by mild psychomotor delay, feeding difficulties, growth retardation, craniofacial dysmorphisms, and finger anomalies. SNP array analysis disclosed 20% of cells with a 20q11.21q12 deletion, encompassing the 20q11.2 minimal critical region and the 3 OMIM disease-causing genes GDF5, EPB41L1, and SAMHD1. We propose a pathogenic role of other genes mapping outside the small region of overlap, in particular GHRH (growth hormone releasing hormone), whose haploinsufficiency could be responsible for the prenatal onset of growth retardation which is shared by half of these patients. Our patient highlights the utility of chromosomal microarray analysis to identify low-level mosaicism.

Using Array-Based Comparative Genomic Hybridization to Diagnose Pallister-Killian Syndrome

Pallister-Killian syndrome (PKS) is a rare multisystem disorder characterized by isochromosome 12p and tissue-limited mosaic tetrasomy 12p. In this study, we diagnosed three pediatric patients who were suspicious of having PKS using array-based comparative genomic hybridization (array CGH) and FISH analyses performed on peripheral lymphocytes. Patients 1 and 2 presented with craniofacial dysmorphic features, hypotonia, and a developmental delay. Array CGH revealed two to three copies of 12p in patient 1 and three copies in patient 2. FISH analysis showed trisomy or tetrasomy 12p. Patient 3, who had clinical features comparable to those of patients 1 and 2, was diagnosed by using FISH analysis alone. Here, we report three patients with mosaic tetrasomy 12p. There have been only reported cases diagnosed by chromosome analysis and FISH analysis on skin fibroblast or amniotic fluid. To our knowledge, patient 1 was the first case diagnosed by using array CGH performed on peripheral lymphocytes in Korea.

Newborn screening for severe combined immunodeficiency in 11 screening programs in the United States

IMPORTANCE

Newborn screening for severe combined immunodeficiency (SCID) using assays to detect T-cell receptor excision circles (TRECs) began in Wisconsin in 2008, and SCID was added to the national recommended uniform panel for newborn screened disorders in 2010. Currently 23 states, the District of Columbia, and the Navajo Nation conduct population-wide newborn screening for SCID. The incidence of SCID is estimated at 1 in 100,000 births.

OBJECTIVES

To present data from a spectrum of SCID newborn screening programs, establish population-based incidence for SCID and other conditions with T-cell lymphopenia, and document early institution of effective treatments.

DESIGN

Epidemiological and retrospective observational study.

SETTING

Representatives in states conducting SCID newborn screening were invited to submit their SCID screening algorithms, test performance data, and deidentified clinical and laboratory information regarding infants screened and cases with nonnormal results. Infants born from the start of each participating program from January 2008 through the most recent evaluable date prior to July 2013 were included. Representatives from 10 states plus the Navajo Area Indian Health Service contributed data from 3,030,083 newborns screened with a TREC test.

MAIN OUTCOMES AND MEASURES

Infants with SCID and other diagnoses of T-cell lymphopenia were classified. Incidence and, where possible, etiologies were determined. Interventions and survival were tracked.

RESULTS

Screening detected 52 cases of typical SCID, leaky SCID, and Omenn syndrome, affecting 1 in 58,000 infants (95% CI, 1/46,000-1/80,000). Survival of SCID-affected infants through their diagnosis and immune reconstitution was 87% (45/52), 92% (45/49) for infants who received transplantation, enzyme replacement, and/or gene therapy. Additional interventions for SCID and non-SCID T-cell lymphopenia included immunoglobulin infusions, preventive antibiotics, and avoidance of live vaccines. Variations in definitions and follow-up practices influenced the rates of detection of non-SCID T-cell lymphopenia.

CONCLUSIONS AND RELEVANCE

Newborn screening in 11 programs in the United States identified SCID in 1 in 58,000 infants, with high survival. The usefulness of detection of non-SCID T-cell lymphopenias by the same screening remains to be determined.

Republished: Non-heritable genetics of human disease: spotlight on post-zygotic genetic variation acquired during lifetime

The heritability of most common, multifactorial diseases is rather modest and known genetic effects account for a small part of it. The remaining portion of disease aetiology has been conventionally ascribed to environmental effects, with an unknown part being stochastic. This review focuses on recent studies highlighting stochastic events of potentially great importance in human disease—the accumulation of post-zygotic structural aberrations with age in phenotypically normal humans. These findings are in agreement with a substantial mutational load predicted to occur during lifetime within the human soma. A major consequence of these results is that the genetic profile of a single tissue collected at one time point should be used with caution as a faithful portrait of other tissues from the same subject or the same tissue throughout life. Thus, the design of studies in human genetics interrogating a single sample per subject or applying lymphoblastoid cell lines may come into question. Sporadic disorders are common in medicine. We wish to stress the non-heritable genetic variation as a potentially important factor behind the development of sporadic diseases. Moreover, associations between post-zygotic mutations, clonal cell expansions and their relation to cancer predisposition are central in this context. Post-zygotic mutations are amenable to robust examination and are likely to explain a sizable part of non-heritable disease causality, which has routinely been thought of as synonymous with environmental factors. In view of the widespread accumulation of genetic aberrations with age and strong predictions of disease risk from such analyses, studies of post-zygotic mutations may be a fruitful approach for delineation of variants that are causative for common human disorders.

Persistent mosaicism for 12p duplication/triplication chromosome structural abnormality in peripheral blood

We present a rare case of mosaicism for a structural abnormality of chromosome 12 in a patient with phenotypic features of Pallister-Killian syndrome. A six-month-old child with dysmorphic features, exotropia, hypotonia, and developmental delay was mosaic for both a normal karyotype and a cell line with 12p duplication/triplication in 25 percent of metaphase cells. Utilization of fluorescence in situ hybridization (FISH) identified three copies of probes from the end of the short arm of chromosome 12 (TEL(12p13) locus and the subtelomere (12p terminal)) on the structurally abnormal chromosome 12. Genome-wide SNP array analysis revealed that the regions of duplication and triplication were of maternal origin. The abnormal cell line in our patient was present at 25 percent at six months and 19 months of age in both metaphase and interphase cells from peripheral blood, where typically the isochromosome 12p is absent in the newborn. This may suggest that the gene(s) resulting in a growth disadvantage of abnormal cells in peripheral blood of patients with tetrasomy 12p may not have the same influence when present in only three copies.

Non-heritable genetics of human disease: spotlight on post-zygotic genetic variation acquired during lifetime

The heritability of most common, multifactorial diseases is rather modest and known genetic effects account for a small part of it. The remaining portion of disease aetiology has been conventionally ascribed to environmental effects, with an unknown part being stochastic. This review focuses on recent studies highlighting stochastic events of potentially great importance in human disease-the accumulation of post-zygotic structural aberrations with age in phenotypically normal humans. These findings are in agreement with a substantial mutational load predicted to occur during lifetime within the human soma. A major consequence of these results is that the genetic profile of a single tissue collected at one time point should be used with caution as a faithful portrait of other tissues from the same subject or the same tissue throughout life. Thus, the design of studies in human genetics interrogating a single sample per subject or applying lymphoblastoid cell lines may come into question. Sporadic disorders are common in medicine. We wish to stress the non-heritable genetic variation as a potentially important factor behind the development of sporadic diseases. Moreover, associations between post-zygotic mutations, clonal cell expansions and their relation to cancer predisposition are central in this context. Post-zygotic mutations are amenable to robust examination and are likely to explain a sizable part of non-heritable disease causality, which has routinely been thought of as synonymous with environmental factors. In view of the widespread accumulation of genetic aberrations with age and strong predictions of disease risk from such analyses, studies of post-zygotic mutations may be a fruitful approach for delineation of variants that are causative for common human disorders.

Structural genetic variation in the context of somatic mosaicism

Somatic mosaicism is the result of postzygotic de novo mutation occurring in a portion of the cells making up an organism. Structural genetic variation is a very heterogeneous group of changes, in terms of numerous types of aberrations that are included in this category, involvement of many mechanisms behind the generation of structural variants, and because structural variation can encompass genomic regions highly variable in size. Structural variation rapidly evolved as the dominating type of changes behind human genetic diversity, and the importance of this variation in biology and medicine is continuously increasing. In this review, we combine the evidence of structural variation in the context of somatic cells. We discuss the normal and disease-related somatic structural variation. We review the recent advances in the field of monozygotic twins and other models that have been studied for somatic mutations, including other vertebrates. We also discuss chromosomal mosaicism in a few prime examples of disease genes that contributed to understanding of the importance of somatic heterogeneity. We further highlight challenges and opportunities related to this field, including methodological and practical aspects of detection of somatic mosaicism. The literature devoted to interindividual variation versus papers reporting on somatic variation suggests that the latter is understudied and underestimated. It is important to increase our awareness about somatic mosaicism, in particular, related to structural variation. We believe that further research of somatic mosaicism will prove beneficial for better understanding of common sporadic disorders.

Tissue-limited mosaicism for monosomy 13

Karyotypic discordance between different tissues in an individual is uncommon. We report on a patient with multiple congenital anomalies and mosaicism for monosomy 13 limited to fibroblasts. Findings include microcephaly, agenesis of the corpus callosum, bilateral posterior colobomas, cataract and optic nerve dysplasia, patent foramen ovale, renal hypoplasia, hypospadias and unilateral inguinal hernia, unilateral hypoplasia of the lower limb, sparse and patchy hair, subtle pigmentary mosaicism, and global developmental delay. The lymphocyte karyotype was normal, whereas the fibroblast karyotype showed mosaicism for a del(13)(q11→ter). Review of the literature identified three previous reports of similar patients with multiple congenital anomalies, normal lymphocyte karyotype, and subsequent, diagnostic fibroblast karyotyping. Comparison of the previously reported patients with the patient reported here defines a common phenotype for tissue-limited mosaicism for monosomy 13 consisting of prenatal-onset growth deficiency; microcephaly; facial abnormalities including prominent nasal bridge, hypertelorism, ptosis, epicanthal folds, microphthalmia, coloboma, retinoblastoma, prominent maxilla, micrognathia, and low-set ears; limb abnormalities including small to absent thumbs, clinodactyly of fifth finger, fused metacarpal bones 4 and 5, talipes equinovarus, and short first toe; cardiac defect; renal anomalies; and genitalia abnormalities including hypospadias and cryptorchidism. In conclusion, this case further emphasizes that fibroblast karyotyping should be employed when the diagnosis remains unclear, especially in the presence of pigmentary mosaicism or segmental hypoplasia.

aCGH detects partial tetrasomy of 12p in blood from Pallister-Killian syndrome cases without invasive skin biopsy

Pallister-Killian syndrome (PKS) is a genetic disorder characterized by mental retardation, seizures, streaks of hypo- or hyperpigmentation and dysmorphic features. PKS is associated with tissue-limited mosaic partial tetrasomy of 12p, usually caused by an isochromosome 12p. The mosaicism is usually detected in cultured skin fibroblasts or amniotic cells and rarely in phytohemagluttinin-stimulated lymphocytes, which suggests stimulation of T-lymphocytes may distort the percentage of abnormal cells. We recently reported on the identification by microarray-based comparative genomic hybridization (aCGH) of a previously unsuspected case of partial tetrasomy of 12p caused by an isochromosome 12p. Here we report on seven additional individuals with partial tetrasomy of 12p characterized by our laboratory. All individuals were referred for mental retardation/developmental delay and/or dysmorphic features. In each case, aCGH using genomic DNA extracted from whole peripheral blood detected copy-number gain for all clones for the short arm of chromosome 12. In all but one case, FISH on metaphases from cultured lymphocytes did not detect the copy-number gain; in the remaining case, metaphase FISH on cultured lymphocytes showed an isochromosome in 10% of cells. However, interphase FISH using probes to 12p on peripheral blood smears showed additional hybridization signals in 18-70% of cells. Microarray and FISH analysis on cultured skin biopsies from four individuals confirmed the presence of an isochromosome 12p. Our results demonstrate the usefulness of aCGH with genomic DNA from whole peripheral blood to detect chromosome abnormalities that are not present in stimulated blood cultures and would otherwise require invasive skin biopsies for identification.

Microarray-based CGH detects chromosomal mosaicism not revealed by conventional cytogenetics

Somatic chromosomal mosaicism is a well-established cause for birth defects, mental retardation, and, in some instances, specific genetic syndromes. We have developed a clinically validated, targeted BAC clone array as a platform for comparative genomic hybridization (aCGH) to enable detection of a wide range of pathologic copy number changes in DNA. It is designed to provide high sensitivity to detect well-characterized submicroscopic micro-deletion and duplication disorders while at the same time minimizing detection of variation of uncertain clinical significance. In the course of studying 2,585 samples submitted to our clinical laboratory, chromosomal mosaicism was detected in 12 patient samples; 10 of these cases were reported to have had a normal blood chromosome analysis. This enhanced ability of aCGH to detect mosaicism missed by routine chromosome analysis may be due to some combination of testing multiple cell lineages and/or failure of cytogenetically abnormal T lymphocytes to respond to mitogens. This suggests that aCGH may detect somatic chromosomal mosaicism that would be missed by conventional cytogenetics.

Trisomy 8 mosaicism: selective growth advantage of normal cells vs. growth disadvantage of trisomy 8 cells

A fetus with trisomy 8 mosaicism was identified prenatally due to an abnormal maternal serum triple screen. Tissue samples were taken at birth to determine the level of trisomy 8 mosaicism found within embryonic and extra-embryonic tissues, rates of cell division for the two cell lines, and the effect of mosaicism on the phenotype. The level of trisomy 8 cells in blood and fibroblasts was higher than in placental tissue. Cell cycle kinetics, by incorporation of bromodeoxyuridine for 48 hr, was not significantly different between the trisomy 8 and normal cells for blood or amnion. Fluorescent in situ hybridization (FISH) using centromeric probe for chromosome 8 showed significantly more trisomy 8 in interphase vs. metaphase in lymphoblasts, umbilical cord fibroblasts, and chorion. The loss of trisomy 8 cells is not due to anaphase lag, as determined by micronuclei analysis. The similarity of cell cycle kinetics between trisomy 8 cells and normal diploid cells suggests some trisomy 8 cells are exiting the cell cycle prematurely. This growth disadvantage of trisomy 8 cells results in the appearance of growth advantage for diploid cells.

Mild phenotype in a 15-year-old boy with Pallister-Killian syndrome

Pallister-Killian syndrome is a rare disorder characterized by multiple congenital anomalies, coarse face, pigmentary skin changes, seizures, severe mental retardation, and the presence of an extra metacentric chromosome i(12p) confined to skin fibroblasts only. Here, we report on an unusual case of i(12p) in a 15-year-old boy presenting with mild mental retardation, minor facial features (long face, prognathism, short neck), normal weight, length, and OFC parameters as well as hyperpigmented streaks. The boy attended normal school until the age of 14 years. Because of hyperpigmented stripes, chromosome analysis was performed on skin fibroblasts. This study showed that 37% of the cells had an additional isochromosome for the short arm of chromosome 12. This observation illustrates the phenotypic variability of i(12p) and emphasizes the importance of skin fibroblasts chromosome analysis in patients with pigmentary skin changes.

Pallister-Killian syndrome: difficulties of prenatal diagnosis

The first prenatal diagnosis of Pallister-Killian syndrome (PKS) was reported by Gilgenkrantz et al. in1985. Since this report, about 60 prenatal cases have been reported but both sonographic and cytogenetic diagnoses remain difficult. Although ultrasound anomalies such as congenital diaphragmatic hernia, polyhydramnios and rhizomelic micromelia in association with fetal overgrowth are very suggestive of the syndrome, they are inconstant and they may even be absent. The mosaic distribution of the supernumerary isochromosome 12p greatly increases these difficulties. No prenatal cytogenetic technique is sensitive enough to ensure prenatal diagnosis and false-negative results have been described on fetal blood, chorionic villi and amniocentesis. We report here two prenatal cases of PKS which illustrate the great variability of the fetal phenotype. In reviewing the 63 reported cases, we attempt to determine ultrasound indicators of the syndrome and to define a cytogenetic strategy. In cases where ultrasound indicators are present, our proposal is first to perform chorionic villus or placental sampling and then amniocentesis when the first cytogenetic result is normal. Fetal blood sampling is the least indicated method because of the low frequency of the isochromosome in lymphocytes. In this cytogenetic strategy, fluorescent in situ hybridization (FISH) and especially interphase FISH on non-cultured cells increases the probability or identifying the isochromosome. A misdiagnosis remains possible when ultrasound is not contributory; the identification of new discriminating ultrasound indicators would be very helpful in this context.

Cell-cycle kinetics of cell lines from patients with chromosomal mosaicism

Lymphocyte cultures from five patients with chromosomal mosaicism (two 47,XY,+21/46,XY, one 47,XX,+21/46,XX, one 45,X/46,XX, and one 47,XXY/46,XY) were studied using sister chromatid differential staining technique for cell kinetic evaluation. Aneuploid and normal cell lines were compared to identify changes in cellular proliferation in vitro that could be related to cellular selective advantage and cell-line-proportion changes occurring with age. Comparison of the percentage of cells in different cell generations in 48, 72, and 96 h-cultures shows no differences between the aneuploid and normal cell lines indicating that cell-cycle kinetics is similar in these cells in vitro.

Report of two new cases of Pallister-Killian syndrome confirmed by FISH: tissue-specific mosaicism and loss of i(12p) by in vitro selection

Tissue-specific mosaic distribution of an additional isochromosome 12p is the characteristic chromosomal aberration in Pallister-Killian syndrome. Often it is confined to fibroblasts, whereas lymphocytes show a normal karyotype. Two cases are reported in which the distribution of the additional i(12p) was analysed in various tissues. The isochromosomes were characterised by conventional banding technics and fluorescence in situ hybridization (FISH). In the first case, diagnosed prenatally, 4 different tissues were analysed. A direct preparation of chorionic villi (21 gestational weeks) showed an extra marker chromosome in 19% and two additional copies in 3% of the examined cells. In two cultures of amniocytes (17 and 21 weeks), the i(12p) was observed in 23% and 12%, respectively. It was absent in cultured lymphocytes of fetal blood (21 weeks). The fibroblast long-term culture of umbilical cord showed the i(12p) in 100% of metaphases. In the second case of a term infant the i(12p) was diagnosed in cultured lymphocytes (4%) and fibroblasts (93%). Secondary loss of the isochromosome was evaluated by in vitro selection in case 2 analysing metaphases and interphases of fibroblasts in the 1st, 4th and 5th subculture using FISH. The proportion of cells with i(12p) decreased from 93% to 40% and to 28%, respectively. DNA analysis in case 1 showed a maternal meiotic origin of the i(12p). The prenatally detected clinical findings in both cases showed characteristic abnormalities of the Pallister-Killian syndrome.

The use of interphase FISH for prenatal diagnosis of Pallister-Killian syndrome

Pallister-Killian syndrome (tetrasomy 12p) is a relatively rare aneuploidy syndrome characterized by the presence of mosaicism for an isochromosome 12p [i(12p)]. We report two new cases diagnosed following chorionic villus sampling and an abnormal ultrasound, respectively. Fluorescent in situ hybridization (FISH) was used to enumerate the number of interphase cells containing the isochromosome. The results of these studies illustrate the importance of the use of interphase FISH to detect the presence of the i(12p) in uncultured, non-dividing cells. A review of the literature identified 23 additional cases of Pallister-Killian syndrome diagnosed prenatally. Approximately 50 per cent of these cases were associated with the presence of a congenital diaphragmatic hernia. We suggest that a perinatal-lethal form of Pallister-Killian syndrome is underdiagnosed and recommend that all cases of prenatally detected diaphragmatic hernia be tested for Pallister-Killian syndrome using interphase FISH on uncultured amniocytes.

Pallister-Killian syndrome: a mild case diagnosed by fluorescence in situ hybridization. Review of the literature and expansion of the phenotype

Pallister-Killian syndrome (PKS) is a rare disorder characterized by a specific combination of anomalies, mental retardation and mosaic presence of a supernumerary isochromosome 12p which is tissue-limited. We report an atypical case of PKS with a mild phenotype. Flourescence in situ hybridization (FISH) was used to demonstrate that the supernumerary marker chromosome identified in the patient's fibroblasts was an isochromosome 12p. This study broadens the spectrum of PKS phenotype. It also illustrates the usefulness of fluorescence in situ hybridization in diagnosis of patients with chromosomal abnormalities and mild or atypical clinical findings.

Incidence and significance of chromosome mosaicism involving an autosomal structural abnormality diagnosed prenatally through amniocentesis: a collaborative study

Among 179,663 prenatal diagnosis cases collected from ten institutions and two publications, 555 (0.3 per cent) were diagnosed as having chromosome mosaicism. Of these, 57 (10.3 per cent) were mosaic for an autosomal structural abnormality, 28 (5 per cent) for a sex chromosome structural abnormality, and 85 (15.3 per cent) were mosaic for a marker chromosome. Ninety-five cases of prenatally diagnosed mosaicism with a structural abnormality in an autosome and a normal cell line, and with a known phenotypic outcome, were collected for karyotype-phenotype correlations through our collaboration (40 cases), a prior survey (26 cases), and published reports (29 cases). They included 13 balanced reciprocal translocations, one unbalanced reciprocal translocation, four balanced Robertsonian translocations, four unbalanced Robertsonian translocations, four inversions, 17 deletions, three ring chromosomes, 19 i(20q), seven +i(12p), six other isochromosomes, and 17 partial trisomies resulting from a duplication or other rearrangement. All cases mosaic for a balanced structural rearrangement resulted in a normal phenotype. All cases of 46/46,i(20q) resulted in normal liveborns. Five of seven cases with 46/47,+i(12p) had an abnormal phenotype compatible with Killian-Pallister syndrome. The overall risk for an abnormal outcome for a mosaic case with an unbalanced structural abnormality, excluding 46/46,i(20q) and 46/47,+i(12p), is 40.4 per cent. In the same category, the study also suggested a correlation between the percentage of abnormal cells and an abnormal phenotype. For mosaicism involving a terminal deletion, the possibility of a familial fragile site should be considered.

Pallister-Killian syndrome: normal karyotype in prenatal chorionic villi, in postnatal lymphocytes, and in slowly growing epidermal cells, but mosaic tetrasomy 12p in skin fibroblasts

We report on two patients with Pallister-Killian syndrome: an 18 month old male infant followed since the neonatal period and a 4 year old boy. Prenatal diagnosis by chorionic villi sampling (CVS) in the first case showed a normal karyotype without mosaicism. Chromosome analysis on peripheral lymphocytes of the newborn also showed a normal karyotype. The clinical diagnosis of Pallister-Killian syndrome was made after the first year of life because of the typical facial dysmorphism and other characteristic clinical features, such as frontotemporal alopecia, depigmented area of the skin, sensorineural hearing loss, and severe psychomotor retardation. Chromosome analysis from skin fibroblasts now showed an isochromosome 12p mosaicism. The origin of the extra chromosome was confirmed by in situ hybridisation using a chromosome 12 specific library. In the second case chromosomal analysis from peripheral lymphocytes at the age of 19 months showed a normal karyotype 46,XY. Following the clinical diagnosis of Pallister-Killian syndrome a superficial skin biopsy was performed which showed very poor and slow growth of cells and a normal karyotype. Because of the typical symptoms a larger and deeper skin biopsy was performed from which there was rapid growth of fibroblasts. Now the diagnosis was established on the basis of the presence of an i(12p) extra chromosome in 69% of the metaphases.

Tetrasomy 12p (Pallister-Killian syndrome): ultrasound indicators and confirmation by interphase fish

Tetrasomy 12p (Pallister-Killian syndrome) is a mosaic aneuploidy syndrome in which the isochromosome is present in amniocytes with a much greater percentage than fetal lymphocytes. Two new cases identified by prenatal diagnosis are reported. Indications for prenatal diagnosis were advanced maternal age and fetal anomalies. The most consistent reported prenatal ultrasound findings for tetrasomy 12p include polyhydramnios with short femurs and a diaphragmatic hernia. Recognition of congenital malformation patterns prenatally may allow appropriate selection of tissue for chromosome analysis. Molecular cytogenetic analysis using fluorescence in situ hybridization was used retrospectively to confirm the presence of the isochromosome 12p in various formalin-fixed fetal tissues. The levels of mosaicism detected in fetal and placental tissues were lower than those detected prenatally.

Discordance between direct and PHA-stimulated chromosome preparations from neonates

Four examples of discordance between karyotypes prepared from direct and PHA-stimulated neonatal blood samples are presented. The specimens were initially studied by direct methods to establish a neonatal karyotype rapidly. The patients are described and the significance of the findings is discussed.

Tissue-specific mosaicism among fetuses with prenatally diagnosed diaphragmatic hernia

OBJECTIVE

Our purpose was to determine if cytogenetic discrepancies between fetal blood and amniotic fluid are present in fetuses with prenatally diagnosed diaphragmatic hernia.

STUDY DESIGN

Chromosome analysis was performed on 15 fetuses with prenatally diagnosed diaphragmatic hernia. Fourteen had both amniotic fluid and fetal blood studies. One fetus had an amniocentesis followed by postnatal skin and peripheral lymphocyte chromosome analysis.

RESULTS

In one fetus with a normal karyotype on fetal blood, amniotic fluid mosaicism for a supernumerary isochromosome 12p was identified. Another fetus had normal amniotic fluid chromosome analysis but was diagnosed with mosaic isochromosome 12p on skin biopsy after birth. Concordant aneuploidy in both fetal blood and amniocytes was found in five pregnancies (three with trisomy 18, one with an unbalanced translocation, and one with mosaic supernumerary isochromosome 12p). Eight fetuses had normal karyotypes.

CONCLUSION

Because diaphragmatic hernia is a common component of mosaic isochromosome 12p syndrome and this chromosome abnormality is predominantly found in fibroblasts but not lymphocytes, an amniocentesis may be more accurate than fetal blood sampling in defining the true fetal chromosome status when diaphragmatic hernia is detected prenatally.

Mosaic tetrasomy 8p: molecular cytogenetic confirmation and measurement of glutathione reductase and tissue plasminogen activator levels

We report the case of a 5-year-old girl with severe developmental disabilities, skeletal anomalies, hypotonia, rectal atresia, malrotation of the intestine, horseshoe kidney, vesicoureteric reflux, and minor facial anomalies. Conventional cytogenetic techniques suggested that she had a mosaic 46,XX/47,XX,+i(8p) constitution, and the identity of the isochromosome was confirmed by in situ hybridization and chromosome painting. Polymorphic DNA markers are consistent with the i(8p) having arisen as the result of a segregation error and centromere misdivision at the second maternal meiotic division. The i(8p) was seen in 17/25 (68%) lymphocytes at the age of one month but had declined to 31/100 (31%) cells by the age of 5 years. At this time the i(8p) was seen in 30/68 (44%) cultured skin fibroblasts. The proposita had an approximately twofold increase in red cell glutathione reductase activity but a normal level of tissue-plasminogen activator. These enzyme results are consistent with the known localisation of the glutathione reductase gene on the short arm of chromosome 8 but suggest that the tissue-plasminogen activator gene may map outside this region.

Pallister-Killian syndrome in older children and adolescents

The Pallister-Killian syndrome is caused by a mosaic tetrasomy of the short arm of chromosome 12. Although analysis of peripheral blood lymphocytes usually reveals a normal karyotype, an isochromosome 12p mosaicism is detectable in fibroblast cultures; therefore, in this rare chromosomal aberration, clinical recognition is crucial for appropriate cytogenetic investigations. The phenotype of younger children has already been well documented. During childhood and adolescence, however, the phenotype changes markedly. The disorder in older children and young adults is characterized by a coarse and flat facies, macroglossia prognathia, everted lower lip, and severe psychomotor retardation with muscular hypertonia and contractures. Two severely mentally retarded patients are reported whose diagnoses were confirmed by fibroblast cultures at ages 16 and 21 years.

Molecular cytogenetic characterization of marker chromosomes found at prenatal diagnosis

The nature and origin of two de novo small marker chromosomes found at prenatal diagnosis were determined by fluorescence in situ hybridization using chromosome centromere-specific probes and chromosome-specific plasmid libraries. One marker was found in a mosaic state and was shown to be an i(18p). The second marker was characterized as an inv dup(22). We conclude that molecular cytogenetic analysis contributes to the identification of marker chromosomes and therefore facilitates genetic counselling and decision-making for the parents.

New diagnostic method for Pallister-Killian syndrome: detection of i(12p) in interphase nuclei of buccal mucosa by fluorescence in situ hybridization

Detection of the supernumerary isochromosome 12p [i(12p)] was performed on buccal smear preparations from 2 patients with Pallister-Killian syndrome, 21 (patient 1) and 15 months (patient 2) old, by interphase fluorescence in situ hybridization (FISH) using a chromosome 12-specific alpha satellite probe. Isochromosome 12p-positive cells were identified by observing 3 signals over the nucleus, while diploid cells had 2 signals. The proportion of i(12p)-positive cells thus identified was high in the epithelial cells of buccal mucosa at 68 and 53% from patients 1 and 2, respectively. Further, the frequencies of i(12p)-positive cells were also studied in PHA-stimulated peripheral lymphocytes, cultured skin fibroblasts (both patients), and directly harvested T and B-cells (patient 1). Of these tissues, buccal mucosa showed the highest proportion of i(12p)-positive cells. These findings indicate that epithelial cells of buccal mucosa are likely to retain i(12p)-positive cells. Detection of i(12p) using direct buccal smear preparations by interphase FISH is a rapid, effective and non-invasive method for confirming the diagnosis of the Pallister-Killian syndrome.