Segmental haplosufficiency: transmitted deletions of 2p12 include a pancreatic regeneration gene cluster and have no apparent phenotypic consequences

An intragenic deletion within CTNNA2 intron 7 in a boy with short stature and speech delay: A case report

Background/Objectives:

Deletions on the short arm of chromosome 2 at bands p11 and p12 have been detected in association with short stature, mild mental retardation and speech delay.

Results:

We describe a 4 year-old boy with some facial dysmorphic traits, congenital malformations and pre- and post-natal growth failure. He also presented marked expressive language problems. The molecular karyotype revealed a 108 Kb deletion within the seventh intron of the CTNNA2 gene at 2p11.2-p12. We observed that some features (short stature, facial dysmorphisms and speech delay) were present in our patient and in patients carrying much larger overlapping deletions.

Conclusions:

The description of this small intragenic rearrangement might help to elucidate the role of the single genes included in the 2p11.2-p12 critical region.

Rare interstitial deletion of chromosome 2p11.2p12. Report of a new patient with developmental delay and unusual clinical features

De novo interstitial deletions of the short arm of chromosome 2 are rare chromosomal abnormalities. Patients showing these kind of microdeletions have developmental delay/intellectual disability, minor facial anomalies including high forehead, frontal bossing, broad nasal bridge, abnormal ears and congenital defects such as skeletal and genital malformations. We describe the second child of a healthy and non consanguineous couple presenting at birth multiple malformations and minor facial anomalies. Because of the clinical findings, an array CGH analysis was performed using Agilent 60K microarray oligonucleotide. The analysis detected a 9.3 Mb deletion on the short arm of chromosome 2 at band p11.2p12 spanning the bases 77,946,599-87,277,610. The five patients previously described display a minimal common deleted region which explains the clinical features shared by all of them, while their individual characteristics might be explained by the different sizes of the deletion. The common deleted region involves several genes (CTNNA2, LRRTM1, REEP1), highly expressed in the nervous system. The deletion found in this case overlaps with most of those reported in literature but our patient displays extra clinical signs such as bilateral choanal atresia and atrial septal defect. It was impossible to find any direct correlation between the genes involved in the deletion and the choanal atresia and the heart defect. The question remains open as to whether these clinical features are a consequence of the deletion or are due to a second pathogenic event. Our case emphasizes the difficulties to find a close correlation between a large deletion and a well defined clinical picture. As only five patients with 2p11.2p12 deletions, reported in the literature are characterized by array CGH, further reports will be necessary to well define a clinical phenotype related to the 2p11.2p12 microdeletion.

Interstitial microduplication at 2p11.2 in a patient with syndromic intellectual disability: 30-year follow-up

Background

Copy number variations at 2p11.2 have been rare and to our knowledge, no abnormal phenotype with an interstitial 2p11.2 duplication has yet been reported. Here we report the first case with syndromic intellectual disability associated with microduplication at 2p11.2.

Results

We revisited a white female subject with a chromosome translocation, t(8;10)(p23;q23)mat and a 10q telomeric deletion suspected by G-banding 30 years ago. This female with severe intellectual disability, no speech, facial dysmorphism, intractable epilepsy, recurrent infection, and skeletal abnormalities has been observed from the birth until her death. The karyotype analysis reconfirmed the previously reported chromosome translocation with a revision as 46,XX,t(8;10)(p23.3;q23.2)mat by adding more detail in chromosomal sub-bands. The array comparative genomic hybridization, however, did not detect the 10q terminal deletion originally reported, but instead, revealed a 390 kb duplication at 2p11.2; 46,XX,t(8;10)(p23.3;q23.2)mat.arr[hg 19] 2p11.2(85469151x2,85474356-85864257x3,85868355x2). This duplication region was confirmed by real-time quantitative PCR and real-time reverse transcriptase quantitative PCR.

Conclusions

We suggest three positional candidate genes for intellectual disability and recurrent infection based upon gene function and data from real-time reverse transcriptase quantitative PCR—VAMP8 and RNF181 for intellectual disability and CAPG for recurrent infection.

A recurrent deletion syndrome at chromosome bands 2p11.2-2p12 flanked by segmental duplications at the breakpoints and including REEP1

We identified an identical and recurrent 9.4-Mbp deletion at chromosome bands 2p11.2-2p12, which occurred de novo in two unrelated patients. It is flanked at the distal and proximal breakpoints by two homologous segmental duplications consisting of low copy repeat (LCR) blocks in direct orientation, which have >99% sequence identity. Despite the fact that the deletion was almost 10 Mbp in size, the patients showed a relatively mild clinical phenotype, that is, mild-to-moderate intellectual disability, a happy disposition, speech delay and delayed motor development. Their phenotype matches with that of previously described patients. The 2p11.2-2p12 deletion includes the REEP1 gene that is associated with spastic paraplegia and phenotypic features related to this are apparent in most 2p11.2-2p12 deletion patients, but not in all. Other hemizygous genes that may contribute to the clinical phenotype include LRRTM1 and CTNNA2. We propose a recurrent but rare 2p11.2-2p12 deletion syndrome based on (1) the identical, non-random localisation of the de novo deletion breakpoints in two unrelated patients and a patient from literature, (2) the patients' phenotypic similarity and their phenotypic overlap with other 2p deletions and (3) the presence of highly identical LCR blocks flanking both breakpoints, consistent with a non-allelic homologous recombination (NAHR)-mediated rearrangement.

Narrowing critical regions and determining penetrance for selected 18q- phenotypes

One of our primary goals is to help families who have a child with an 18q deletion anticipate medical issues in order to optimize their child's medical care. To this end we have narrowed the critical regions for four phenotypic features and determined the penetrance for each of those phenotypes when the critical region for that feature is hemizygous. We completed molecular analysis using oligo-array CGH and clinical assessments on 151 individuals with deletions of 18q and made genotype-phenotype correlations defining or narrowing critical regions. These nested regions, all within 18q22.3 to q23, were for kidney malformations, dysmyelination of the brain, growth hormone stimulation response failure, and aural atresia. The region for dysmyelination and growth hormone stimulation response failure were identical and was narrowed to 1.62 Mb, a region containing five known genes. The region for aural atresia was 2.3 Mb and includes an additional three genes. The region for kidney malformations was 3.21 Mb and includes an additional four genes. Penetrance rates were calculated by comparing the number of individuals hemizygous for a critical region with the phenotype to those without the phenotype. The kidney malformations region was 25% penetrant, the dysmyelination region was 100% penetrant, the growth hormone stimulant response failure region was 90% penetrant with variable expressivity, and the aural atresia region was 78% penetrant. Identification of these critical regions suggest possible candidate genes, while penetrance calculations begin to create a predictive phenotypic description based on genotype.

Structural analysis of tissues affected by cytochrome C oxidase deficiency due to mutations in the SCO2 gene

Structural and histochemical studies carried out in a series of seven cases (from five families) with isolated cytochrome c oxidase (COX) deficiency caused by mutations in the SCO2 gene (1, 2) disclosed changes concentrated in the nervous system, skeletal muscle and myocardium. In five patients homozygous for the E140K mutation, the phenotype was predominantly neuromuscular and the average life span ranged between 9 and 15 months. In two cases, the course was more rapid (death at 7 and 11 weeks of life) and featured marked cardiac hypertrophy (3- and 4-fold increase in heart weight). This predominantly cardiomyopathic phenotype was associated with compound heterozygosity (E140K with another nonsense mutation) in the SCO2 gene. Polioencephalopathy with neurodegeneration and neuronal drop out was present in all cases with evidence that retinal neurons might be seriously affected too. Involvement of spinal motoneurons together with cytochrome c oxidase deficiency in muscle represents a "double hit" for the skeletal muscle. The mitochondrial population was not found to be significantly increased or structurally altered, with the exception of two compound heterozygotes in which the cardiac mitochondria were increased in number and size. Our report extends knowledge of the pathology of COX deficiency caused by mutations in the SCO2 gene.

Transmitted cytogenetic abnormalities in patients with mental retardation: Pathogenic or normal variants?

Knowing the origin of cytogenetic abnormalities detected in individuals with mental retardation and dysmorphic features is essential to genetic counselling of affected families. To illustrate this, we report on six families with transmitted cytogenetic abnormalities and discuss the genotype-phenotype correlations, including the possibility of the abnormalities being normal genomic variants. The abnormalities were detected using metaphase HR-CGH; their size was estimated to range from 1.6 to 7.5 Mb using tiling path array-CGH and real-time PCR. The abnormalities were transmitted through two to four generations and included interstitial deletions of 1p31.3-p32.1, 2q13, 10q11.21-q11.23, and 13q31.1; a duplication of 1p34.1-p34.2; and in one family both a deletion of 18q21.1 and a duplication of 4q35.1-q35.2. The probands were mentally retarded and had nonspecific dysmorphic features except for one patient with the Bohring-Opitz syndrome. We considered the abnormalities in two families to be clinically significant: In one family, the proband's brain abnormality was comparable to previously reported abnormalities in individuals with a similar duplication of 1p31-p32. Congenital heart disease was previously mapped to the chromosomal region of 18q that was affected in the proband of another family. The carrier parents in both families had mild clinical features. In two families the abnormalities were considered as coincidental findings, and in two further families the abnormalities were insufficient to explain the phenotypes of the probands but possibly were related to a milder phenotype in other family members. These cases illustrate the need for careful assessment of the extended family in order to interpret the phenotypic consequences of abnormalities identified using array-CGH.

Chromosome abnormalities without phenotypic consequences

Some changes in chromosome morphology, detected during cytogenetic analysis, are not associated with clinical defects. Therefore a proper discrimination of harmless variants from true abnormalities, especially during prenatal diagnosis, is crucial to allow precise counseling. In this review we described chromosome variants and examples of chromosome anomalies that are considered to be unrelated to phenotypic consequences. The correlation between the presence of marker chromosomes and a risk of clinical signs is also discussed. Structural rearrangements of heterochromatic material, satellite polymorphism, or fragile sites, are well-known examples of common chromosome variation. However, the absence of clinical effects has also been reported in some cases of chromosome abnormalities concerning euchromatin. Such euchromatic anomalies were divided into 2 categories: unbalanced chromosome abnormalities (UBCAs), such as deletions or duplications, and euchromatic variants (EVs). Recently so-called molecular karyotyping, especially whole-genome screening by the use of high-resolution array-CGH technique, contributed to revealing a high number of previously unknown small genomic variations, which seem to be asymptomatic, as they are present in phenotypically normal individuals.

An innocuous duplication of 11.2 Mb at 13q21 is gene poor: Sub-bands of gene paucity and pervasive CNV characterize thechromosome anomalies

A boy with autistic spectrum disorder without dysmorphisms was found to have a chromosome duplication of part of band 13q21. His mother and grandfather both of normal intellect had the same chromosomal duplication. Comparison was made with the Chromosome anomaly database www.som.soton.ac.uk/research/geneticsdiv/anomaly%20register which revealed similar cases. Mapping on DNA microarray for the proband and mother showed the duplication to be of length 11.2 Mb, encompassing the 13q21.1-13q21.32 region. The duplicated region is profoundly gene poor, with a mean gene density of 0.45 genes/Mb. We estimate, that the mean gene density in the sub-bands of the chromosome anomalies is 2.4-2.5 genes/Mb. In addition the percentages of the sub-bands reported as copy number variants (CNV) was estimated from the Database of Chromosome Genomic Variants (http://projects.tcag.ca/variation/). It was found that for some of these sub-bands, gene paucity was likely to be a major contributor to their innocuous phenotypic effect, for example, the gene densities were for: 1p31.2 (1.25 genes/Mb); 2p12 (1.7); 4p15.31 (1.3); 5p14.1 (0.22); 5p14.3 (0.8); 5q21.2 (0.6); 5q21.3 (1.2); 8p23.2 (0.25); 13q21.1 (0.9); 14q31.1 (1.4); 18q22.1 (1.4); 21q21.1 (1.2); and 21q21.2 (0.7). For other sub-bands the percentage of the band in which CNV have been reported was found to be markedly increased, for example, 8p23.2 (94.7% of the band is defined by reported CNV); 3p26.3 (81.6); 5p14.3 (59.3); 8p22 (48.8); 2p12 (44.0); 5q21.1 (43.6); 6q24.2 (41.4); 9p23 (38.8); 10q21.1 (36.5); 5q21.2 (35.4), and 11q14.3 (33.8). We argue that both gene paucity and pervasive CNV are major indicators of bands conforming to the Chromosome Anomaly phenomenon.

Interstitial deletion of 6q without phenotypic effect

Cytogenetically detectable euchromatic deletions without phenotypic consequences are rarely encountered. We report on a 34-year-old woman with normal intelligence referred for karyotyping because of recurrent abortions. With the exception of a bicuspid aortic valve without hemodynamic consequences, which is a common minor anomaly in the general population, no dysmorphic features were found on physical examination. Conventional chromosome analysis (GTG-banding) revealed an interstitial deletion in the long arm of chromosome 6. With array comparative genomic hybridization (array-CGH) the size of the deletion was estimated to be between 9.9 and 11.6 Mb and the refined karyotype was 46,XX,del(6)(q22.31q23.1).

Syndromes and constitutional chromosomal abnormalities associated with Wilms tumour

Wilms tumour has been reported in association with over 50 different clinical conditions and several abnormal constitutional karyotypes. Conclusive evidence of an increased risk of Wilms tumour exists for only a minority of these conditions, including WT1 associated syndromes, familial Wilms tumour, and certain overgrowth conditions such as Beckwith-Wiedemann syndrome. In many reported conditions the rare co-occurrence of Wilms tumour is probably due to chance. However, for several conditions the available evidence cannot either confirm or exclude an increased risk, usually because of the rarity of the syndrome. In addition, emerging evidence suggests that an increased risk of Wilms tumour occurs only in a subset of individuals for some syndromes. The complex clinical and molecular heterogeneity of disorders associated with Wilms tumour, together with the apparent absence of functional links between most of the known predisposition genes, suggests that abrogation of a variety of pathways can promote Wilms tumorigenesis.

Directly transmitted unbalanced chromosome abnormalities and euchromatic variants

In total, 200 families were reviewed with directly transmitted, cytogenetically visible unbalanced chromosome abnormalities (UBCAs) or euchromatic variants (EVs). Both the 130 UBCA and 70 EV families were divided into three groups depending on the presence or absence of an abnormal phenotype in parents and offspring. No detectable phenotypic effect was evident in 23/130 (18%) UBCA families ascertained mostly through prenatal diagnosis (group 1). In 30/130 (23%) families, the affected proband had the same UBCA as other phenotypically normal family members (group 2). In the remaining 77/130 (59%) families, UBCAs had consistently mild consequences (group 3). In the 70 families with established EVs of 8p23.1, 9p12, 9q12, 15q11.2, and 16p11.2, no phenotypic effect was apparent in 38/70 (54%). The same EV was found in affected probands and phenotypically normal family members in 30/70 families (43%) (group 2), and an EV co-segregated with mild phenotypic anomalies in only 2/70 (3%) families (group 3). Recent evidence indicates that EVs involve copy number variation of common paralogous gene and pseudogene sequences that are polymorphic in the normal population and only become visible at the cytogenetic level when copy number is high. The average size of the deletions and duplications in all three groups of UBCAs was close to 10 Mb, and these UBCAs and EVs form the "Chromosome Anomaly Collection" at http://www.ngrl.org.uk/Wessex/collection. The continuum of severity associated with UBCAs and the variability of the genome at the sub-cytogenetic level make further close collaboration between medical and laboratory staff essential to distinguish clinically silent variation from pathogenic rearrangement.