Angela F. Davies

The gene involved in X-linked agammaglobulinaemia is a member of the src family of protein-tyrosine kinases.

X-linked agammaglobulinaemia (XLA) is a human immunodeficiency caused by failure of pre-B cells in the bone marrow to develop into circulating mature B cells. A novel gene has been isolated which maps to the XLA locus, is expressed in B cells, and shows mutations in families with the disorder. The gene is a member of the src family of proto-oncogenes which encode protein-tyrosine kinases. This is, to our knowledge, the first evidence that mutations in a src-related gene are involved in human genetic disease.

Delineation of two distinct 6p deletion syndromes.

Deletions of the short arm of chromosome 6 are relatively rare, the main features being developmental delay, craniofacial malformations, hypotonia, and defects of the heart and kidney, with hydrocephalus and eye abnormalities occurring in some instances. We present the molecular cytogenetic investigation of six cases with 6p deletions and two cases with unbalanced translocations resulting in monosomy of the distal part of 6p. The breakpoints of the deletions have been determined accurately by using 55 well-mapped probes and fluorescence in situ hybridization (FISH). The cases can be grouped into two distinct categories: interstitial deletions within the 6p22–p24 segment and terminal deletions within the 6p24–pter segment. Characteristics correlating with specific regions are: short neck, clinodactyly or syndactyly, brain, heart and kidney defects with deletions within 6p23–p24; and corneal opacities/iris coloboma/Rieger anomaly, hypertelorism and deafness with deletions of 6p25. The two cases with unbalanced translocations presented with a Larsen-like syndrome including some characteristics of the 6p deletion syndrome, which can be explained by the deletion of 6p25. Such investigation of cytogenetic abnormalities of 6p using FISH techniques and a defined set of probes will allow a direct comparison of reported cases and enable more accurate diagnosis as well as prognosis in patients with 6p deletions.

An interstitial deletion of 6p24-p25 proximal to the FKHL7 locus and including AP-2α that affects anterior eye chamber development

The FKHL7 gene has been implicated in the pathogenesis of glaucoma/autosomal dominant iridogoniodysgenesis (IGDA) (IRID1). This has been supported by mutations in some glaucoma and IGDA patients and the development of anterior eye chamber anomalies in patients with 6p deletions affecting the 6p25 region. We report a case with anterior eye chamber anomalies and an interstitial deletion of 6p24-p25 that does not include theFKHL7 gene, suggesting the possible additional involvement of another locus, within 6p24-6p25, in anterior eye chamber development. A candidate gene isAP-2α, which is contained within the deleted segment and plays a role in anterior eye chamber development.

Deletion of the distal long arm of chromosome 10; is there a characteristic phenotype? A report of 15 de novo and familial cases.

It has been suggested previously that patients with terminal deletions of chromosome 10q have a recognizable phenotype including a characteristic facial appearance combined with other abnormalities including mental retardation, cardiac and anogenital anomalies. We report the largest published series of new cases of terminal 10q deletion, including eight familial and four de novo cases and three cases with interstitial deletions involving chromosome bands 10q25.2‐26.3. The deleted regions were defined by FISH using YAC probes, as well as standard karyotyping. The most consistent clinical features in our cases are cranial anomalies including facial asymmetry, prominent nose and nasal bridge, prominent ears, thin upper lip, along with growth retardation, developmental delay, and digital abnormalities. Visceral abnormalities were only identified in a small number of the patients, with renal involvement in three cases and structural cardiac malformations in two others. Learning difficulties of varying severity were found in 11 cases and behavioral problems described in four. Candidate genes for behavioral and learning difficulties within the deleted region include Calcyon. Other genes in the region that might have a role in causing the phenotype include the genes coding for fibroblast growth factor receptor type 2 (FGFR2) and C‐terminal binding protein 2 (CTBP2).

A detailed investigation of two cases exhibiting characteristics of the 6p deletion syndrome.

Abstract Deletions of the short arm of chromosome 6 are relatively rare, only 16 cases having been described in the literature so far. Here we present a detailed investigation by fluorescence in situ hybridisation of two further cases with different but overlapping interstitial deletions involving 6p22, 6p23 and 6p24. The main features involved are craniofacial malformations, heart and kidney defects, mental retardation/developmental delay, hypotonia and hydrocephalus. By using 36 yeast artificial chromosome and cosmid clones from a contig covering 6p22.3–6p25 and other probes with defined cytogenetic locations within 6p21– 6p22 we have precisely localised the breakpoints involved in each of the cases, estimated the sizes of the deleted regions and defined the region that is hemizygously deleted in both cases.

Delineation of an estimated 6.7 MB candidate interval for an anophthalmia gene at 3q26.33-q28 and description of the syndrome associated with visible chromosome deletions of this region

Anophthalmia or microphthalmia occur in approximately one in 10 children who have severe visual impairment. These eye malformations are often of unknown aetiology, but can be inherited in autosomal dominant, recessive or X-linked forms, and can also occur in association with specific chromosome abnormalities. Four children are described in the medical literature with microphthalmia or anophthalmia in association with chromosome rearrangements involving distal 3q, suggesting the presence of a micro/anophthalmia gene in this region. We have identified two further patients with micro/anophthalmia and chromosome rearrangements involving 3q26→3q27 and identified a 6.7 MB common deleted region. Patient 1 had multiple abnormalities including bilateral anophthalmia, abnormalities of the first and second cranial nerves and partial absence of the corpus callosum. His karyotype was 46,XY,del(3)(q26.33q28). Patient 2 had right anophthalmia and left extreme microphthalmia. Her karyotype was 46,XX,del(3)(q26.33q28)t(3;7)(q28;q21.1). Both patients had intrauterine growth retardation (IUGR) and strikingly similar dysmorphic facies consisting of bossed forehead, downward-slanting palpebral fissures, grooved bridge of the nose, prominent low-set ears, small down-turned mouth and small mandible. We identified BAC clones mapping to distal 3q from the ENSEMBL and NCBI Entrez databases. These BAC clones were used as fluorescence in situ hybridisation (FISH) probes to identify the minimum deleted region common to both patients. This interval, between clones RPC11-134F2 and RPC11-132N15, was estimated to be 6.7 MB. We conclude that there is an anophthalmia locus within this interval. Candidate genes mapping to this region include Chordin and DVL3, a homologue of the Drosophila Dishevelled gene.

A pericentric inversion of chromosome six in a patient with Peutz-Jeghers' syndrome and the use of FISH to localise the breakpoints on a genetic map

Abstract Karyotypic analysis in a patient with Peutz-Jeghers’ syndrome demonstrated a pericentric inversion on chromosome 6. Further investigation was undertaken using fluorescence in situ hybridisation (FISH) with yeast artificial chromosome clones selected to contain genetic markers from chromosome 6, and a probe for the centromeric alphoid repeat array. This analysis located one inversion breakpoint within the alphoid array, in a 1-cM interval between D6S257 and D6S402, and the other in a 4-cM interval between D6S403 and D6S311. The oestrogen receptor gene locus (ESR) is excluded from the latter interval.

Further evidence for the involvement of human chromosome 6p24 in the aetiology of orofacial clefting.

Chromosomal translocations affecting the 6p24 region have been associated with orofacial clefting. Here we present a female patient with cleft palate, severe growth retardation, developmental delay, frontal bossing, hypertelorism, antimongoloid slant, bilateral ptosis, flat nasal bridge, hypoplastic nasal alae, protruding upper lip, microretrognathia, bilateral, low set, and posteriorly rotated ears, bilateral microtia, narrow ear canals, short neck, and a karyotype of 46,XX,t(6;9)(p24;p23). The translocation chromosomes were analysed in detail by FISH and the 6p24 breakpoint was mapped within 50-500 kb of other breakpoints associated with orofacial clefting, in agreement with the assignment of such a locus in 6p24. The chromosome 9 translocation breakpoint was identified to be between D9S156 and D9S157 in 9p23-p22, a region implicated in the 9p deletion syndrome.

Characterization of terminal chromosome anomalies using multisubtelomere FISH

Telomeric repeat sequences (TTAGGG) are known to cap the termini of every human chromosome. Proximal to these repeat sequences are chromosome‐specific repeat sequences, which in turn are distal to gene‐rich regions. Submicroscopic, subtle, or cryptic abnormalities in these regions can now be investigated using commercial probe sets for all of the chromosome‐specific subtelomeric regions of the human genome. Using this technology, previously unidentified genomic imbalance has been found in a proportion of patients with idiopathic developmental delay and learning difficulties. We have used these probe sets to investigate cases with apparently terminal anomalies detected on G‐banded chromosome analysis. As a result of such investigations, we have found that 3 (19%) of 16 apparently terminal deletion cases were the result of more complex rearrangements involving other chromosome subtelomeres. The remaining 13 cases contained no chromosome‐specific subtelomere repeats on the deleted arm, but in all 16 cases, the TTAGGG telomere repeat cap was present. A further case was investigated where extra material was found in the terminal region of the chromosome 12 short arm, found to represent a complex inversion/duplication/deletion rearrangement. Investigation of all cases with terminal anomalies, including apparently terminal deletions, is likely to uncover further cases involving complex rearrangements and should lead to a greater understanding of the mechanisms by which these abnormalities arise.

Separation of the PROX1 gene from upstream conserved elements in a complex inversion/translocation patient with hypoplastic left heart

Hypoplastic left heart (HLH) occurs in at least 1 in 10 000 live births but may be more common in utero. Its causes are poorly understood but a number of affected cases are associated with chromosomal abnormalities. We set out to localize the breakpoints in a patient with sporadic HLH and a de novo translocation. Initial studies showed that the apparently simple 1q41;3q27.1 translocation was actually combined with a 4-Mb inversion, also de novo, of material within 1q41. We therefore localized all four breakpoints and found that no known transcription units were disrupted. However we present a case, based on functional considerations, synteny and position of highly conserved non-coding sequence elements, and the heterozygous Prox1+/− mouse phenotype (ventricular hypoplasia), for the involvement of dysregulation of the PROX1 gene in the aetiology of HLH in this case. Accordingly, we show that the spatial expression pattern of PROX1 in the developing human heart is consistent with a role in cardiac development. We suggest that dysregulation of PROX1 gene expression due to separation from its conserved upstream elements is likely to have caused the heart defects observed in this patient, and that PROX1 should be considered as a potential candidate gene for other cases of HLH. The relevance of another breakpoint separating the cardiac gene ESRRG from a conserved downstream element is also discussed.