Nick Dennis

Supernumerary marker 15 chromosomes: a clinical, molecular and FISH approach to diagnosis and prognosis.

Seventeen patients presenting with either de novo or familial supernumerary marker (mar) 15 chromosomes were shown by fluorescent in situ hybridization techniques (FISH) to have markers derived from and composed entirely of chromosome 15 material. Using a combination of conventional cytogenetics, FISH, Southern blotting and multiplex polymerase chain reaction (PCR) methods, it was possible to sub-classify the 17 mar(15)s into six distinct morphological and molecular groups. Analysis of DNA and metaphase spreads from the probands and their parents using probes and primers from the pericentromeric and Prader-Willi/Angelman syndromes critical regions (PWS/AS), clearly differentiated between marker 15 s which included the PWS/AS critical regions and those which did not. A direct correlation between the presence of the PWS/AS region in the mar(15) and severe mental retardation was observed. Based on these results, a system of classification of supernumerary marker 15 chromosomes is proposed.

Importance of microdeletions of chromosomal region 22q11 as a cause of selected malformations of the ventricular outflow tracts and aortic arch: A three-year prospective study

OBJECTIVES To assess the incidence of microdeletions of chromosomal region 22q11 in a population of infants coming to a regional pediatric cardiac center with selected abnormalities of the ventricular outflow tracts and aortic arch and, further, to provide phenotypic/genetic correlations to determine whether patients with 22q11 deletions can be clinically recognized in infancy. BACKGROUND DiGeorge syndrome and velocardiofacial syndrome are frequently associated with malformations of the ventricular outflow tracts and aortic arch. Both are usually caused by microdeletions of chromosomal region 22q11. The overall importance of such deletions as a cause of these cardiac malformations remains to be established. STUDY DESIGN All infants with the candidate cardiac phenotypes during a 34-month period were studied. Dysmorphic features, type of cardiac defect, serum calcium concentration, and thymic status were recorded. Cytogenetic studies, including high-resolution karyotyping and fluorescence in situ hybridization using cosmids (cEO or cH748) from the DiGeorge critical region, were performed after clinical assessment. RESULTS Fifty infants (including 36 with tetralogy of Fallot with or without pulmonary atresia) were seen during the study period. Twenty-six infants (52%) were dysmorphic, including 19 who were considered to have a phenotypic appearance consistent with 22q11 deletion. Genetic analysis confirmed hemizygosity for 22q11 in 8 of these 19 cases. Results of fluorescence in situ hybridization studies were normal in 22 infants without dysmorphic features and in 5 infants with dysmorphic features not suggestive of a 22q11 deletion. CONCLUSIONS Microdeletions of chromosomal region 22q11 are an important cause of selected malformations of the ventricular outflow tracts and aortic arch and account for about 15% to 20% of cases. These deletions may be clinically recognized in early infancy and can be rapidly confirmed by fluorescence in situ hybridization.

Subtelomeric rearrangements: results from a study of selected and unselected probands with idiopathic mental retardation and control individuals by using high-resolution G-banding and FISH.

Abstract. The cause of mental retardation, present in approximately 3% of the population, is unexplained in the majority of cases. Recent reports have suggested that cryptic telomeric rearrangements resulting in segmental aneuploidy and gene-dosage imbalance might represent a significant cause of idiopathic mental retardation (IMR). Two groups of patients with unexplained developmental delay (unselected and selected) and a group of control individuals have been investigated to determine the frequency of submicroscopic telomeric rearrangements associated with IMR and the frequency within the normal population. In contrast to current thinking, our data have shown that true cryptic telomeric rearrangements are not a significant cause of IMR. No fully cryptic abnormalities were detected in our IMR groups, although a semi-cryptic unbalanced telomeric translocation was identified in one selected patient by high-resolution G-band analysis. This abnormality was confirmed and characterised by fluorescence in situ hybridisation (FISH) with telomere-specific probes. A further 13 cytogenetically detected subtle terminal rearrangements were characterised by using multi-telomere FISH. Seven of these had previously been reported as normal, three of which were shown to be interstitial deletions. These cases illustrate the importance of high-resolution analysis to exclude subtle but cytogenetically visible abnormalities prior to subtelomere FISH screening when determining the frequency of cryptic telomeric rearrangements. Unexpectedly, two cryptic telomeric abnormalities were detected among our control individuals, suggesting that submicroscopic telomeric abnormalities may be a not uncommon finding in the general population. Hence, our data have important implications when defining the significance of cryptic telomeric rearrangements detected during screening programmes.

Population studies of the fragile X: a molecular approach.

The fragile X mutation can now be recognised by a variety of molecular techniques. We report a pilot screening survey of a population of children with mental impairment in which we used Southern blotting methods to detect the fragile X mutation, augmented by cytogenetic studies on children whose phenotype suggested a possible chromosome abnormality. There were 873 children with special educational needs in our survey and 310 fulfilled our criteria for testing. A sample was obtained from 254, of whom four were found to have a full fra(X) mutation (delta L) and none to have a premutation. The number of CGG repeats in our population of X chromosomes was measured by PCR analysis and the genotype at the closely linked polymorphic locus FRAXAC1 established. The distribution of CGG repeat numbers was very similar to that of the control population reported by Fu et al and the distribution of FRAXAC1 alleles almost identical to that of the control population reported by Richards et al. Among the non-fragile X chromosomes, we found a very significant correlation between the size of the CGG repeat and the FRAXAC1 genotype. There was a dearth of A and D genotypes in subjects with a small number of CGG repeats and an excess of the A genotype in those with a large number of CGG repeats. These observations are considered in the light of the reported disequilibrium between the A (and possibly also the D) genotype and the fra(X) mutation.

A non-isotopic in situ hybridisation study of the chromosomal origin of 15 supernumerary marker chromosomes in man.

Fifteen patients presenting with mosaic or non-mosaic karyotypes containing a distamycin-DAPI negative de novo or familial supernumerary marker chromosome were studied with non-isotopic in situ hybridisation using a library of alphoid centromere specific and satellite II/III probes. The in situ hybridisation studies showed that seven markers were derived from satellited autosomes (three chromosome 13/21, two chromosome 14, two chromosome 22), six from non-satellited autosomes (two chromosome 4, one chromosome 12, one chromosome 16, two chromosome 19), and one from the Y chromosome. One non-mosaic marker was negative for all the alphoid and satellite II/III probes used.

X-linked Opitz syndrome: Novel mutations in the MID1 gene and redefinition of the clinical spectrum

Opitz (or G/BBB) syndrome is a pleiotropic genetic disorder characterized by hypertelorism, hypospadias, and additional midline defects. This syndrome is heterogeneous with an X‐linked (XLOS) and an autosomal dominant (ADOS) form. The gene implicated in the XLOS form, MID1, encodes a protein containing a RING‐Bbox‐Coiled‐coil motif belonging to the tripartite motif (TRIM) family. To further clarify the molecular basis of XLOS, we have undertaken mutation analysis of the MID1 gene in patients with Opitz syndrome (OS). We found novel mutations in 11 of 63 male individuals referred to us as sporadic or familial X‐linked OS cases. The mutations are scattered throughout the gene, although more are represented in the 3′ region. By reviewing all the MID1‐mutated OS patients so far described, we confirmed that hypertelorism and hypospadias are the most frequent manifestations, being present in almost every XLOS individual. However, it is clear that laryngo‐tracheo‐esophageal (LTE) defects are also common anomalies, being manifested by all MID1‐mutated male patients. Congenital heart and anal abnormalities are less frequent than reported in literature. In addition, we can include limb defects in the OS clinical synopsis as we found a MID1‐mutated patient showing syndactyly. The low frequency of mutations in MID1 and the high variability of the phenotype suggest the involvement of other genes in the OS phenotype.

Molecular analysis of 20 patients with 2q37.3 monosomy: definition of minimum deletion intervals for key phenotypes

Terminal deletions of the long arm of chromosome 2 (2q37) have been recorded in the literature for more than a decade and an associated syndrome first became apparent when nine patients were reported with an Albright hereditary osteodystrophy (AHO)-like metacarpal/metatarsal shortening (brachymetaphalangism).1,2 This is also known as brachydactyly-mental retardation syndrome (BDMR, MIM 600430). To date, some 60 or so cases of 2q37 deletion or monosomy resulting from unbalanced translocations have been reported. Significant variability in clinical presentation is apparent, but almost all patients have some degree of mental retardation and facial dysmorphism. Brachymetaphalangism has been reported in approximately 50% of cases.1–15 Congenital heart anomalies are present in around 20% of patients with 2q37 monosomy,16–22 compared to 1% of the general population. These are predominantly atrial or ventricular septal defects, but more complex problems have been reported.17,22 Additionally, there are two reports of tetralogy of Fallot with monosomy 2q37 resulting from unbalanced translocations,23,24 but both cases were also trisomic for 17q25 and it is not clear which imbalance was causative. Other phenotypes repeatedly associated with 2q37 deletions include Wilms tumor and urogenital anomalies,6,17,19 epilepsy,1,2,7–9,14,16,20,25–27 eczema,2,5–7,16,28,29 and autism or repetitive, hyperkinetic behaviour.1,2,5,7,10,11,15,16,19,25,26,28–32 Situs abnormalities have been reported in two cases9 and holoprosencephaly in one infant.33 Most 2q37 rearrangements reported to date have been only minimally characterised by conventional cytogenetics or subtelomeric fluorescent in situ hybridisation (FISH). A small number have been subjected to more detailed analysis using multiple FISH clones or microsatellite markers,2,10,15,26 but the ability to assign breakpoints and make genotype–phenotype correlations has been …

A study of females with deletions of the short arm of the X chromosome

We have undertaken a clinical and molecular study of 25 females with deletions of the short arm of the X chromosome. We have determined the deletion breakpoints, the parental origin and the activation status of the deleted X chromosomes. Genotype–phenotype correlations suggest that the presence of a single copy of the DFFRX gene, previously postulated as a gene involved in the ovarian failure seen in Turner syndrome, may be compatible with normal ovarian function, and that there may be a gene for Turner-like features located in distal Xp22.3.

Three patients with ring (X) chromosomes and a severe phenotype.

Three patients with mosaicism and a cell line containing a small ring (X) chromosome are described. Their phenotype is similar to several previously reported patients with a 45,X/46,X,r(X) karyotype and a phenotype far more severely affected than expected in Turners syndrome. The clinical picture includes mental retardation, a facial appearance reminiscent of the Kabuki make up syndrome, and limb anomalies. Some of the patients also had streaky hyperpigmentation of the skin in a pattern suggesting dermal mosaicism. It has been hypothesised that the severe phenotype might be the result of the small r(X) chromosome remaining active. However, there is little critical evidence to support this suggestion, while there is considerable evidence against it, including (1) a similar phenotype in 45,X/46,X,r(Y) patients, (2) the late replication of some of the small r(X) chromosomes associated with this phenotype, and (3) the expression of XIST in some of the affected patients.

Microdeletions in FMR2 may be a significant cause of premature ovarian failure

Genetic causes of premature ovarian failure (POF) include X chromosome deletions and fragile X (FRAXA) premutations. While screening a cohort of women with POF for FRAXA premutations, a more distal trinucleotide repeat, FRAXE, was also tested. We found an unexpected excess of FRAXE alleles with apparently fewer than 11 repeats in the POF group. However, sequence analysis of these alleles showed that the excess was caused by three females who carry cryptic deletions in FMR2, the gene associated with FRAXE. We propose that microdeletions withinFMR2 may be a significant cause of premature ovarian failure, being found in 1.5% of women with the condition, and in only 0.04% of the general female population. The deletions may affect transcription of either FMR2 or an adjacent gene.