A molecular genetic service for diagnosing individuals with familial hypercholesterolaemia (FH) in the United Kingdom

A genetic diagnostic service for familial hypercholesterolaemia (FH) has been established over the last 4 years in the Clinical Molecular Genetics Laboratory at Great Ormond Street Hospital for Children NHS Trust (GOSH), London. In total there have been 368 referrals; 227 probands and 141 family members, which have come from a number of lipid clinics and from general practitioners. FH is caused by mutations in the low-density lipoprotein receptor gene (LDLR) and these are analysed by SSCP, DNA sequencing and direct assays. The clinically indistinguishable disorder, familial defective apolipoprotein B100 (FDB) is caused by one of three mutations in the apolipoprotein B100 gene (APOB) which are analysed by direct assays. Mutations predicted to be pathogenic were found in 76 probands, 67 in LDLR (23 previously undescribed) and nine in APOB. The mutation detection rate was 53% in paediatric probands, 32% in adults with a 'definite' FH diagnosis (tendon xanthoma positive) and 14% in adults with a 'possible' FH diagnosis (tendon xanthoma negative). The predicted loss of sensitivity that would result from reducing the number of exons tested has been assessed, and a molecular screening strategy suitable for UK patients is proposed. A similar strategy may be useful for other countries where genetic heterogeneity results in a wide mutation spectrum for FH.

A gene for FG syndrome maps in the Xq12-q21.31 region

FG syndrome is an X-linked recessive condition in which mental retardation is associated with congenital hypotonia, macrocephaly, characteristic face, and constipation. This syndrome was mapped by Zhu et al. [Cytogenet Cell Genet 1991;58:2091A] to Xq21.31-q22 by linkage analysis with a max lod score of 1.2 for the DXYS1X, DXS178, DXS101, and DXS94 loci and crossovers at DXS16 (Xp22.31) and DXS287 (Xq22.3). However, this mapping was only provisional and needed to be refined. In this paper, we report the results of a new linkage analysis performed on 10 families including that studied by Zhu et al. [1991]. Two-point analysis demonstrated linkage with DXS441 (Zmax = 3.39 at theta = 0.12) at Xq13. In addition, separate analysis of the lod scores obtained for the Xq13 markers suggested linkage exclusion for three families. Genetic heterogeneity was confirmed by analysis of the linkage results with the HOMOG program (max logL = 4.07, theta = 0, alpha = 0.65). Localization of one FG gene between DXS135 and DXS1066 was suggested by analysis of crossovers found in those three families which were assumed to be linked to Xq13 with a probability of 0.95 or more. This region could be reduced to the DXS135-DXS72 interval after combining our data with those from deletions previously described in males in the Xq13-q21 region.

Quality Assurance in Molecular Diagnosis : The UK Experience

Molecular genetic diagnosis is a relatively young discipline and also one of the fastest growing among clinical laboratory sciences. The majority of diagnostic laboratories in this field emerged from groups active in research into human genetic disease, and the development of a service-oriented structure from more loosely organized research laboratories was the first priority for the provision of an efficient diagnostic service. In April 1988, the Clinical Molecular Genetics Society (CMGS) was constituted as the professional body representing molecular geneticists working in a diagnostic capacity in the United Kingdom (UK). This organization quickly saw the need for a central system of external quality assurance (EQA), and a subcommittee was set up to establish such a scheme.

Carrier determination for X-linked agammaglobulinemia using X inactivation analysis of purified B cells

We report the development of a relatively quick and simple method for the assessment of X inactivation status for carrier determination in families affected by X-linked agammaglobulinemia (XLA). This method utilises an immunomagnetic separation technique for B cell purification and a polymerase chain reaction (PCR) based assay for the determination of methylation status at the androgen receptor (AR) gene locus to assess whether X inactivation is random or non-random at this locus. We report the results we have obtained using this assay to investigate females known to be carriers of various X-linked immunodeficiency disorders. In addition, we investigated four females from different families affected by XLA, two of whom were of unknown carrier status, and we discuss the results obtained with this and other X-inactivation assays. A similar assay has recently been described by Allen et al. (1992) and applied to members of one family affected by XLA.

A new restriction fragment length polymorphism at the DXS101 locus allows carrier detection in a family with X linked agammaglobulinaemia

The gene responsible for X linked agammaglobulinaemia (XLA) lies in Xq22 and has recently been identified as atk. DXS101 is a polymorphic locus which is closely linked to the disease locus. In this report we describe the identification, by pulsed field gel electrophoresis, of a new polymorphism at the DXS101 locus with a predicted heterozygosity of 4.9%. Despite this low value, we show how this polymorphism has been important in carrier status determination in a family with XLA where assessment was not possible by other means.

Norrie disease gene: characterization of deletions and possible function

Positional cloning experiments have resulted recently in the isolation of a candidate gene for Norrie disease (pseudoglioma; NDP), a severe X-linked neurodevelopmental disorder. Here we report the isolation and analysis of human genomic DNA clones encompassing the NDP gene. The gene spans 28 kb and consists of 3 exons, the first of which is entirely contained within the 5' untranslated region. Detailed analysis of genomic deletions in Norrie patients shows that they are heterogeneous, both in size and in position. By PCR analysis, we found that expression of the NDP gene was not confined to the eye or to the brain. An extensive DNA and protein sequence comparison between the human NDP gene and related genes from the database revealed homology with cysteine-rich protein-binding domains of immediate--early genes implicated in the regulation of cell proliferation. We propose that NDP is a molecule related in function to these genes and may be involved in a pathway that regulates neural cell differentiation and proliferation.

Physical mapping identifies DXS265 as a useful genetic marker for carrier detection and prenatal diagnosis of X-linked agammaglobulinemia

The gene responsible for X-linked agammaglobulinemia (XLA) has not been identified; however, in the course of genetic linkage studies designed to map the locus more precisely, a number of closely linked polymorphic loci have been identified. These have proved to be useful in identifying carriers and in pre-natal diagnosis of this disease. The DXS178 locus was found to be closest to the XLA locus and has been the most usefully employed probe to date. Using physical mapping techniques, we have identified a previously cloned genetic marker, DXS265, as being situated within 5 kb of DXS178. So far, we have found one family that is not informative for DXS178 but that is informative for DXS265; females in this family can now be offered the possibility of carrier determination and pre-natal diagnosis for this life-threatening disease.

Female twin with Hunter disease due to nonrandom inactivation of the X-chromosome: a consequence of twinning

We report the occurrence of Hunter disease (mucopolysaccharidosis type II) in a karyotypically normal girl who was one of identical twins. Molecular studies showed nonrandom X-inactivation in both her fibroblasts and lymphocytes, while her normal twin showed equal usage of both X chromosomes. In view of previous reports of 7 pairs of identical female twins in which one had Duchenne muscular dystrophy, it seems that twinning may be strongly associated with nonrandom X-inactivation, and is not specific to the properties of the disease causing gene.

Carrier detection for X-linked agammaglobulinaemia (Bruton type) in an Irish family using linked DNA probes

Being an X-linked condition, the sisters of men with X-linked agammaglobulinaemia have a 50% risk of being carriers of the disease gene (provided the disease has not developed as a results of a new mutation). We demonstrate how this risk can be modified very significantly by DNA analysis using linked DNA probes. The value of such tests for genetic purposes is discussed.

Close linkage of PUM and SPTA within chromosome band 1q21

Nine families were examined for linkage on chromosome 1 between the hypervariable gene PUM and the alpha-spectrin (SPTA) gene using DNA probes. Close linkage was found with a maximum lod score of 5.98 at theta = 0.05. Hybridization to DNA from somatic cell hybrids made from a parent line carrying a balanced X:1 translocation showed both genes to lie proximal to the breakpoint at 1q23. DNA hybridization to cell lines carrying deletions of 1q21-25 showed the presence of two polymorphic alleles for both loci. These results, combined with existing in situ hybridization data, suggest PUM and SPTA both lie within 1q21. Linkage analysis between PUM and the Duffy blood group (FY) gave a maximum lod of 2.15 at theta = 0.15. These data combined with data from another laboratory give a maximum lod of 5.21 for linkage between PUM and FY at theta = 0.10.

N-ras-like sequences on chromosomes 9, 6 and 22 with a polymorphism at the chromosome 9 locus

Two clones, pCN1 and pCN2, which together form full-length cDNA for N-ras, were used to search for restriction fragment length polymorphisms. pCN2, which entirely consists of 3' non-translated sequences, revealed more bands on DNA transfer hybridizations than could be accounted for using the known restriction map of N-ras. None of the extra cross hybridizing sequences is located on chromosome 1. One of these sequences showed a high-frequency two-allele polymorphism with the restriction enzyme TaqI and maps to the short arm of chromosome 9. Of the remaining two sequences, one maps to chromosome 22 and the other maps to the short arm of chromosome 6. pCN1A, which contains the 5' untranslated regions and all the coding exons of N-ras only hybridized to the chromosome 1 site. No polymorphisms have been found for pCN1 with TaqI, MspI, BclI, BglI, EcoRI, BstXI, XbaI, BamHI, BglII or HindIII.

The ontogeny of GABA receptors and glutamic acid decarboxylase in regions of the rat brain. Effect of prenatal exposure to diazepam

The postnatal development of the binding of [3H]muscimol to both high-affinity and low-affinity GABA receptors and of the activity of glutamic acid decarboxylase has been studied in the frontal cortex, cerebellum, striatum and hypothalamus of the rat, after prenatal exposure to diazepam. The dams were injected subcutaneously with single daily doses of 1 mg diazepam/kg from day 7-20 of gestation. The developmental profiles of the binding of [3H]muscimol to high-affinity GABA receptors were very similar in both control and diazepam-treated offspring. Prenatal administration of drug revealed no significant alteration in the binding of [3H]muscimol. The results in controls supported the concept of a sequential development of this population of GABA receptors in relation to the different maturation of structures in the brain. For the study of low-affinity GABA receptors, the dams were treated in the same way and, additionally, 1 mg diazepam/kg/day was injected subcutaneously in the offspring from postnatal day 5-10. At postnatal days 21 and 90, no qualitative or quantitative differences in the binding of [3H]muscimol to low-affinity GABA receptors in control and diazepam-exposed groups of animals were observed. The developmental profiles of the activity of glutamic acid decarboxylase were qualitatively very similar in the four areas of the brain studied. Prenatal exposure to diazepam revealed a transient elevation of the activity of the enzyme up to 33% in the frontal cortex, the cerebellum and the hypothalamus in the first postnatal week. The possible functional significance of these alterations is discussed.