Clinical and molecular delineation of the Greig cephalopolysyndactyly contiguous gene deletion syndrome and its distinction from acrocallosal syndrome

Greig cephalopolysyndactyly syndrome (GCPS) is caused by haploinsufficiency of GLI3 on 7p13. Features of GCPS include polydactyly, macrocephaly, and hypertelorism, and may be associated with cognitive deficits and abnormalities of the corpus callosum. GLI3 mutations in GCPS patients include point, frameshift, translocation, and gross deletion mutations. FISH and STRP analyses were applied to 34 patients with characteristics of GCPS. Deletions were identified in 11 patients and the extent of their deletion was determined. Nine patients with deletions had mental retardation (MR) or developmental delay (DD) and were classified as severe GCPS. These severe GCPS patients have manifestations that overlap with the acrocallosal syndrome (ACLS). The deletion breakpoints were analyzed in six patients whose deletions ranged in size from 151 kb to 10.6 Mb. Junction fragments were found to be distinct with no common sequences flanking the breakpoints. We conclude that patients with GCPS caused by large deletions that include GLI3 are likely to have cognitive deficits, and we hypothesize that this severe GCPS phenotype is caused by deletion of contiguous genes.     

Metopic and sagittal synostosis in Greig cephalopolysyndactyly syndrome: five cases with intragenic mutations or complete deletions of GLI3

Greig cephalopolysyndactyly syndrome (GCPS) is a multiple congenital malformation characterised by limb and craniofacial anomalies, caused by heterozygous mutation or deletion of GLI3. We report four boys and a girl who were presented with trigonocephaly due to metopic synostosis, in association with pre- and post-axial polydactyly and cutaneous syndactyly of hands and feet. Two cases had additional sagittal synostosis. None had a family history of similar features. In all five children, the diagnosis of GCPS was confirmed by molecular analysis of GLI3 (two had intragenic mutations and three had complete gene deletions detected on array comparative genomic hybridisation), thus highlighting the importance of trigonocephaly or overt metopic or sagittal synostosis as a distinct presenting feature of GCPS. These observations confirm and extend a recently proposed association of intragenic GLI3 mutations with metopic synostosis; moreover, the three individuals with complete deletion of GLI3 were previously considered to have Carpenter syndrome, highlighting an important source of diagnostic confusion.     

Point mutations in human GLI3 cause Greig syndrome

Greig cephalopolysyndactyly syndrome (GCPS, MIM 175700) is a rare autosomal dominant developmental disorder characterized by craniofacial abnormalities and post-axial and pre-axial polydactyly as well as syndactyly of hands and feet. Human GLI3, located on chromosome 7p13, is a candidate gene for the syndrome because it is interrupted by translocation breakpoints associated with GCPS. Since hemizygosity of 7p13 resulting in complete loss of one copy of GLI3 causes GCPS as well, haploinsufficiency of this gene was implicated as a mechanism to cause this developmental malformation. To determine if point mutations within GLI3 could be responsible for GCPS we describe the genomic sequences at the boundaries of the 15 exons and primer pair sequences for mutation analysis with polymerase chain reaction-based assays of the entire GLI3 coding sequences. In two GCPS cases, both of which did not exhibit obvious cytogenetic rearrangements, point mutations were identified in different domains of the protein, showing for the first time that Greig syndrome can be caused by GLI3 point mutations. In one case a nonsense mutation in exon X generates a stop codon truncating the protein in the C-H link of the first zinc finger. In the second case a missense mutation in exon XIV causes a Pro-->Ser replacement at a position that is conserved among GLI genes from several species altering a potential phosphorylation site.      

MODY type 2 in Greig cephalopolysyndactyly syndrome (GCPS) as part of a contiguous gene deletion syndrome

Maturity-onset diabetes of the young type 2 (MODY2) is a form of monogenic diabetes, characterized by mild fasting hyperglycemia. MODY2 is caused by heterozygous mutations in the GCK gene that encodes the glucokinase enzyme. We describe the clinical features and the underlying genetic defect of MODY2 in a patient with atypical Greig cephalopolysyndactyly syndrome (GCPS). The patient presented with the limb formation and the craniofacial developmental abnormalities typical to GCPS, in addition to mental retardation and epilepsy (assigned as atypical syndrome). Fasting hyperglycemia in the diabetic range, impaired glucose tolerance, and lack of diabetes autoantibodies were compatible with MODY2. In order to delineate the genetic aberrations relevant both to MODY2 and Greig syndrome in this patient, we performed cytogenetic analysis, real-time PCR of the GCK gene, and comparative genomic hybridization (CGH) array. Cytogenetic study has shown a microscopic detectable deletion in the 7p13-15 chromosomal region. Real-time PCR demonstrated a deletion of the GCK gene in the patient but not her parents, and CGH array revealed a deleted region of approximately 12 Mb in the 7p13-15 region. This deleted region included GLI3 and GCK genes (where heterozygous mutations cause GCPS and MODY2, respectively), and many other contiguous genes. Our patient manifests a unique form of MODY2, where GCK gene deletion is part of a large deleted segment in the 7p13-15 chromosomal region.     

Development of a comparative genomic hybridization microarray and demonstration of its utility with 25 well-characterized 1p36 deletions

Chromosomal abnormalities, such as deletions and duplications, are characterized by specific and often complex phenotypes resulting from an imbalance in normal gene dosage. However, routine chromosome banding is not sensitive enough to detect subtle chromosome aberrations (<5-10 Mb). Array-based comparative genomic hybridization (array CGH) is a powerful new technology capable of identifying chromosomal imbalance at a high resolution by co-hybridizing differentially labeled test and control DNAs to a microarray of genomic clones. We used a previously assembled contig of large-insert clones that span 10.5 Mb of the most distal region of 1p36 to design a microarray. The array includes 97 clones from 1p36, 41 clones from the subtelomeric regions of all human chromosomes, and three clones from each of the X and Y chromosomes. We used this microarray to study 25 subjects with well-characterized deletions of 1p36. All array CGH results agree with the deletion sizes and locations of the breakpoints in these subjects as determined previously by FISH and microsatellite analyses. Terminal deletions, interstitial deletions, derivative chromosomes and complex rearrangements were also identified. We anticipate that array CGH will change the diagnostic approach to many congenital and acquired genetic diseases such as mental retardation, birth defects and cancer.     

Spectrum of single- and multiexon NF1 copy number changes in a cohort of 1,100 unselected NF1 patients

Neurofibromatosis type 1 (NF1), the most common tumor-predisposing disorder in humans, is caused by defects in the NF1 tumor-suppressor gene. Comprehensive mutation analysis applying RNA-based techniques complemented with FISH analysis achieves mutation detection rates of approximately 95% in NF1 patients. The majority of mutations are minor lesions, and approximately 5% are total gene deletions. We found 13 single- and/or multiexon deletions/duplications out of 1,050 detected mutations using our RNA-based approach in a cohort of 1,100 NF1 patients and confirmed these changes using multiplex ligation-dependent probe amplification (MLPA). With MLPA, we found another 12 novel multiexon deletion/duplications in 55 NF1 patients for whom analysis with multiple assays had not revealed a NF1 mutation, including 50 previously analyzed comprehensively. The extent of the 22 deletions and 3 duplications varied greatly, and there was no clustering of breakpoints. We also evaluated the sensitivity of MLPA in identifying deletions in a mosaic state. Furthermore, we tested whether the MLPA P122 NF1 area assay could distinguish between type I deletions, with breakpoints in low-copy repeats (NF1-LCRs), and type II deletions, caused by aberrant recombination between the JJAZ gene and its pseudogene. Our study showed that intragenic deletions and/or duplications represent only approximately 2% of all NF1 mutations. Although MLPA did not substantially increase the mutation detection rate in NF1 patients, it was a useful first step in a comprehensive mutation analysis scheme to quickly pinpoint patients with single- or multiexon deletions/duplications as well as patients with a total gene deletion who will not need full sequencing of the complete coding region.     

PTCH1 duplication in a family with microcephaly and mild developmental delay

With the exception of the X chromosome, genomic deletions appear to be more prevalent than duplications. Because of a lack of accurate diagnostic methods, submicroscopic duplications have been under-ascertained for a long period. The development of array CGH has enabled the detection of chromosomal microduplications with nearly the same sensitivity as deletions, leading to the discovery of previously unrecognized syndromes. Using a clinical targeted oligonucleotide array (CMA-V6.3 OLIGO), we identified an approximately 360-kb duplication in 9q22.32 in a 21-month-old boy with developmental delay, failure to thrive, and microcephaly. The same duplication was identified in the patient's mother who is also microcephalic and mildly delayed. We have sequenced the chromosomal breakpoints and determined the duplication as tandem in orientation and 363 599 bp in size. The duplicated segment harbors the entire PTCH1 gene. Deletions or loss-of-function mutations of PTCH1 result in basal cell nevus syndrome (Gorlin syndrome), whereas gain-of-function mutations were proposed to lead to holoprosencephaly 7. We propose that patients with microcephaly or holoprosencephaly of unknown origin should also be screened for PTCH1 duplication.     

Complex balanced translocation t(1;5;7)(p32.1;q14.3;p21.3) and two microdeletions del(1)(p31.1p31.1) and del(7)(p14.1p14.1) in a patient with features of Greig cephalopolysyndactyly and mental retardation

Complex chromosome rearrangements (CCRs) are rare structural abnormalities that involve at least two chromosomes and more than two breakpoints and are often associated with developmental delay, mental retardation, and congenital anomalies. We report on a de novo, apparently balanced translocation t(1;5;7)(p32.1;q14.3;p21.3) involving three chromosomes in a 7-year-old boy with severe psychomotor retardation, neonatal muscular hypertonia, congenital heart defect, polysyndactyly of hands and feet, and dysmorphic features resembling Greig cephalopolysyndactyly syndrome. Analysis of the chromosome breakpoints using fluorescence in situ hybridization (FISH) with locus-specific BAC clones and long-range PCR products did not identify chromosome imbalance at any of the interrogated regions. High-resolution comparative genomic hybridization (HR-CGH) and array CGH (aCGH) revealed two additional cryptic de novo deletions, del(1)(p31.1p31.1) and del(7)(p14.1p14.1), respectively, that are not associated with the translocation breakpoints. FISH and polymorphic marker analyses showed that the deletion on derivative chromosome 1 is between 4.2 and 6.1 Mb, and the deletion on derivative chromosome 7 is approximately 5.1 Mb, and that both are paternal in origin. The deletion on chromosome 7p encompasses the GLI3 gene that is causative for the Greig cephalopolysyndactyly, Pallister-Hall and some cases of Acrocallosal syndromes. We discuss the potential mechanisms of formation of the described CCR.     

The clinical atlas of Greig cephalopolysyndactyly syndrome

Greig cephalopolysyndactyly syndrome (GCPS) is a rare multiple congenital anomaly syndrome that is inherited in an autosomal dominant pattern and is caused by haploinsufficiency of the GLI3 gene. The syndrome typically includes preaxial or mixed pre- and postaxial polydactyly and cutaneous syndactyly, ocular hypertelorism, and macrocephaly in its typical forms, but sometimes includes hydrocephalus, seizures, mental retardation, and developmental delay in more severe cases. Patients with milder forms of GCPS can have subtle craniofacial dysmorphic features that are difficult to distinguish from normal variation. This article presents the spectrum of dysmorphic findings in GCPS highlighting some of its key presenting features to familiarize clinicians with the variable expressivity of the condition.     

Improved structural characterization of chromosomal breakpoints using high resolution custom array‐CGH

Lindstrand A, Schoumans J, Gustavsson P, Hanemaaijer N, Malmgren H, Blennow E. Improved structural characterization of chromosomal breakpoints using high resolution custom array‐CGH. Array‐CGH is a powerful tool for the rapid detection of genomic imbalances. By customizing the array it is possible to increase the resolution in a targeted genomic region of interest and determine the structure of the breakpoints with high accuracy, as well as to detect very small imbalances. We have used targeted custom arrays to zoom in on 38 chromosomal breakpoints from 12 different patients carrying both balanced and unbalanced rearrangements. We show that it is possible to characterize unbalanced breakpoints within 17–20,000 bp, depending on the structure of the genome. All of the deletion and duplication breakpoints were further refined and potential underlying molecular mechanisms of formation are discussed. In one of seven carriers of apparently balanced reciprocal translocations we detected a small deletion of 200 bp within the previously FISH‐defined breakpoint, and in another patient, a large deletion of 11 Mb was identified on a chromosome not involved in the translocation. Targeted custom oligonucleotide arrays make it possible to perform fine mapping of breakpoints with a resolution within the breakpoint region much higher compared to commercially available array platforms. In addition, identification of small deletions or duplications in apparently balanced rearrangements may contribute to the identification of new disease causing genes.     

Applications of comparative genomic hybridisation in constitutional chromosome studies

G band cytogenetic analysis often leads to the discovery of unbalanced karyotypes that require further characterisation by molecular cytogenetic studies. In particular, G band analysis usually does not show the chromosomal origin of small marker chromosomes or of a small amount of extra material detected on otherwise normal chromosomes. Comparative genomic hybridisation (CGH) is one of several molecular approaches that can be applied to ascertain the origin of extra chromosomal material. CGH is also capable of detecting loss of material and thus is also applicable to confirming or further characterising subtle deletions. We have used comparative genomic hybridisation to analyse 19 constitutional chromosome abnormalities detected by G band analysis, including seven deletions, five supernumerary marker chromosomes, two interstitial duplications, and five chromosomes presenting with abnormal terminal banding patterns. CGH was successful in elucidating the origin of extra chromosomal material in 10 out of 11 non-mosaic cases, and permitted further characterisation of all of the deletions that could be detected by GTG banding. CGH appears to be a useful adjunct tool for either confirming deletions or defining their breakpoints and for determining the origin of extra chromosomal material, even in cases where abnormalities are judged to be subtle. We discuss internal quality control measures, such as the mismatching of test and reference DNA in order to assess the quality of the competitive hybridisation effect on the X chromosome.     

Congenital arhinia: molecular-genetic analysis of five patients

Congenital arhinia, complete absence of the nose, is an extremely rare anomaly with unknown cause. To our knowledge, a total of 36 cases have been reported, but there has been no molecular-genetic study on this anomaly. We encountered a sporadic case of congenital arhinia associated with a de novo chromosomal translocation, t(3;12)(q13.2;p11.2). This led us to analyze the patient by BAC-based FISH for translocation breakpoints and whole-genome array CGH for other possible deletions/duplications in the genome. We found in this patient an approximately 19 Mb deletion spanning from 3q11.2 to 3q13.31 but no disruption of any gene(s) at the other breakpoint, 12p11.2. As the deleted segment at 3q was a strong candidate region containing the putative arhinia gene, we also performed the array CGH in four other arhinia patients with normal karyotypes, as well as mutation analysis of two genes, COL8A1 and CPOX, selected among hundreds of genes located to the deleted region, because they are expressed during early stages of human craniofacial development. However, in the four patients, there were no copy number aberrations in the region examined or no mutations in the two genes. Although our study failed to identify the putative arhinia gene, the data may become a clue to unravel the underlying mechanism of arhinia.     

Diverse mutational mechanisms cause pathogenic subtelomeric rearrangements

Chromosome rearrangements are a significant cause of intellectual disability and birth defects. Subtelomeric rearrangements, including deletions, duplications and translocations of chromosome ends, were first discovered over 40 years ago and are now recognized as being responsible for several genetic syndromes. Unlike the deletions and duplications that cause some genomic disorders, subtelomeric rearrangements do not typically have recurrent breakpoints and involve many different chromosome ends. To capture the molecular mechanisms responsible for this heterogeneous class of chromosome abnormality, we coupled high-resolution array CGH with breakpoint junction sequencing of a diverse collection of subtelomeric rearrangements. We analyzed 102 breakpoints corresponding to 78 rearrangements involving 28 chromosome ends. Sequencing 21 breakpoint junctions revealed signatures of non-homologous end-joining, non-allelic homologous recombination between interspersed repeats and DNA replication processes. Thus, subtelomeric rearrangements arise from diverse mutational mechanisms. In addition, we find hotspots of subtelomeric breakage at the end of chromosomes 9q and 22q; these sites may correspond to genomic regions that are particularly susceptible to double-strand breaks. Finally, fine-mapping the smallest subtelomeric rearrangements has narrowed the critical regions for some chromosomal disorders.     

Large duplications at reciprocal translocation breakpoints that might be the counterpart of large deletions and could arise from stalled replication bubbles

Reciprocal chromosome translocations are often not exactly reciprocal. Most familiar are deletions at the breakpoints, up to megabases in extent. We describe here the opposite phenomenon-duplication of tens or hundreds of kilobases at the breakpoint junction, so that the same sequence is present on both products of a translocation. When the products of the translocation are mapped on the genome, they overlap. We report several of these "overlapping-breakpoint" duplications in breast cancer cell lines HCC1187, HCC1806, and DU4475. These lines also had deletions and essentially balanced translocations. In HCC1187 and HCC1806, we identified five cases of duplication ranging between 46 kb and 200 kb, with the partner chromosome showing deletions between 29 bp and 31 Mb. DU4475 had a duplication of at least 200 kb. Breakpoints were mapped using array painting, i.e., hybridization of chromosomes isolated by flow cytometry to custom oligonucleotide microarrays. Duplications were verified by fluorescent in situ hybridization (FISH), PCR on isolated chromosomes, and cloning of breakpoints. We propose that these duplications are the counterpart of deletions and that they are produced at a replication bubble, comprising two replication forks with the duplicated sequence in between. Both copies of the duplicated sequence would go to one daughter cell, on different products of the translocation, while the other daughter cell would show deletion. These duplications may have been overlooked because they may be missed by FISH and array-CGH and may be interpreted as insertions by paired-end sequencing. Such duplications may therefore be quite frequent.     

Identification of novel deletion breakpoints bordered by segmental duplications in the NF1 locus using high resolution array-CGH

Background

Segmental duplications flanking the neurofibromatosis type 1 (NF1) gene locus on 17q11 mediate most gene deletions in NF1 patients. However, the large size of the gene and the complexity of the locus architecture pose difficulties in deletion analysis. We report the construction and application of the first NF1 locus specific microarray, covering 2.24 Mb of 17q11, using a non‐redundant approach for array design. The average resolution of analysis for the array is ∼12 kb per measurement point with an increased average resolution of 6.4 kb for the NF1 gene.

Methods

We performed a comprehensive array‐CGH analysis of 161 NF1 derived samples and identified heterozygous deletions of various sizes in 39 cases. The typical deletion was identified in 26 cases, whereas 13 samples showed atypical deletion profiles.

Results

The size of the atypical deletions, contained within the segment covered by the array, ranged from 6 kb to 1.6 Mb and their breakpoints could be accurately determined. Moreover, 10 atypical deletions were observed to share a common breakpoint either on the proximal or distal end of the deletion. The deletions identified by array‐CGH were independently confirmed using multiplex ligation‐dependent probe amplification. Bioinformatic analysis of the entire locus identified 33 segmental duplications.

Conclusions

We show that at least one of these segmental duplications, which borders the proximal breakpoint located within the NF1 intron 1 in five atypical deletions, might represent a novel hot spot for deletions. Our array constitutes a novel and reliable tool offering significantly improved diagnostics for this common disorder.     

TBX3 and TBX5 duplication: A family with an atypical overlapping Holt-Oram/ulnar-mammary syndrome phenotype

Holt-Oram syndrome (HOS) is a rare, autosomal dominant heart-hand syndrome caused by mutations in the TBX5 gene. A wide spectrum of TBX5 mutations have been reported previously, most resulting in a null allele leading to haploinsufficiency. TBX5 gene duplications have been previously reported in association with typical and atypical HOS phenotypes. Ulnar-Mammary syndrome (UMS) is a distinct rare, autosomal dominant condition caused by mutations in the TBX3 gene. TBX5 and TBX3 are physically linked in cis on human chromosome 12 and contiguous chromosome 12q24 deletions comprising both TBX5 and TBX3 genes have been previously reported but to our knowledge, duplications have never been described. We report on a large German family with at least 17 affected individuals over 6 generations bearing a duplication at 12q24.21 identified on array-CGH comprising both TBX5 and TBX3 genes. Affected patients are presenting with HOS and UMS symptoms, consisting of variable limb anomalies involving the radial and the ulnar rays and cardiac findings such as congenital heart defects, persistent arterial duct or aortic stenosis, and non-classical symptoms, such as supernumerary nipples and cardiomyopathy. Fluorescence in situ hybridisation confirmed a tandem duplication at the 12q24.21 locus. This is the first report of a contiguous TBX3/TBX5 duplication associated with HOS/UMS phenotype.     

Cryptic Xp duplication including the SHOX gene in a woman with 46,X, del(X)(q21.31) and premature ovarian failure

BACKGROUND: Premature ovarian failure (POF) is defined as amenorrhoea for >6 months, occurring before the age of 40, with an FSH serum level in the menopausal range. Although Xq deletions have been known for a long time to be associated with POF, the mechanisms involved in X deletions in order to explain ovarian failure remain unknown. In order to look for potentially cryptic chromosomal imbalance, we used high-resolution genomic analysis to characterize X chromosome deletions associated with POF. METHODS: Three patients with POF presenting terminal Xq deletions detected by conventional cytogenetics were included in the study. Genome wide microarray comparative genomic hybridization (CGH) at a resolution of 1 Mb and fluorescence in situ hybridization (FISH) was performed. RESULTS: Microarray CGH and FISH studies characterized the three deletions as del(X)(q21.2), del(X)(q21.31) and del(X)(q22.33). Microarray CGH showed that the del(X)(q21.31) was also associated with a Xpter duplication including the SHOX gene. In these patients with POF, deletions or duplications of autosomes have been excluded. CONCLUSION: This study is the first one using microarray in patients with POF. It demonstrates that putative X chromosome deletions can be associated with other chromosomal imbalances such as duplications, and therefore illustrates the use of microarray CGH to screen chromosomal abnormalities in patients with POF.