[Genome-wide diagnostics; after the results the real work begins]. / Genoombrede diagnostiek.

Introduction of new genetic test technologies in the last decade have accelerated genetic diagnosis in many medical specialties and have increased diagnostic yield considerably. SNP-arrays have been established as first tier diagnostic tools, more and more being replaced by next generation sequencing strategies, like targeted genomic panels and whole exome sequencing. We present the diagnostic work-up of a clinical case, a girl with congenital vertebral and rib anomalies. This case illustrates the complexity of genetic tests and the need for knowledge and experience to interpret the results. Intensive collaboration between pediatrician, clinical geneticist and laboratory specialist is mandatory, as is long-term commitment to involve parents in the diagnostic journey .

[Fragile X syndrome: new therapeutic strategies]. / Fragiele-X-syndroom: nieuwe therapeutische strategieën.

BACKGROUND: Fragile X syndrome (fxs) is the most common hereditary cause of intellectual disability and autism spectrum disorders. Targeted treatment is currently lacking. In the past decades an enormous amount of knowledge has been obtained concerning the involved molecular pathways, introducing potential targets for disease modifying therapy.
AIM: To present an overview of the development of targeted treatment for fxs.
METHOD: Several important publications were collected and indexed.
RESULTS: While preclinical animal model studies with targeted interventions are promising, the translation to the clinic has been disappointing.
CONCLUSION: Targeted treatment for fxs is necessary and could be applied in other causes of autism spectrum disorders and intellectual disability. Factors relating to translation, study design and outcome measures are possibly contributing to the disappointing results. The clustering of patient care in a center of expertise is required to clinically implement future therapeutic strategies and to facilitate research. In addition, this improves patient care, one example being the recent medical guideline for children with fxs.

[Diagnosis image (262). A newborn baby with unilatreral ptosis]. / Diagnose in beeld (262). Een zuigeling met unilaterale ptosis.

A newborn girl was seen with one eye closed, which she opened synchronous with suction; this was due to a congenital ptosis, so-called Marcus Gunn jaw-winking ptosis.

Cerebellar hypoplasia in respiratory chain dysfunction.

Disruption of early or late fetal brain development resulting in structural abnormalities may be associated with inborn errors of mitochondrial metabolism. It is common in patients with deficiency of pyruvate dehydrogenase activity and it has sporadically been described in patients with dysfunction of the tricarboxylic acid cycle. Mitochondrial respiratory chain disorders are not commonly known to interfere with early brain development. We describe here a girl with an encephalomyopathy likely to be due to a novel type of deficiency of cytochrome c oxidase (complex IV) activity that presented with severe hypotonia, myoclonic seizures, optic atrophy and elevated lactate concentration in cerebrospinal fluid shortly after birth. Cranial magnetic resonance imaging revealed hypoplasia of the cerebellum with rudimentary cerebellar hemispheres and relative sparing of the vermis. This case suggests that deficiency of cytochrome c oxidase and possibly respiratory chain disorders in general have to be considered in the differential diagnosis of cerebellar hypoplasia.

Altered kinetics of cytochrome c oxidase in a patient with severe mitochondrial encephalomyopathy.

Deficiency of cytochrome c oxidase activity was established in a girl born to consanguineous parents. She showed symptoms of dysmaturity, generalized hypotonia, myoclonic seizures and progressive respiratory failure, leading to death on the seventh day of life. Structural abnormalities of the central nervous system consisted of severe cerebellar hypoplasia and optic nerve atrophy. Biochemical analysis of a muscle biopsy specimen demonstrated deficiency of cytochrome c oxidase activity. Cultured fibroblasts from this patient also showed a selective decrease in the activity of cytochrome c oxidase, excluding a muscle-specific type of deficiency. Further investigations in cultured fibroblasts revealed that synthesis, assembly and stability of both the mitochondrial and the nuclear subunits of the enzyme were entirely normal. The steady-state concentration of cytochrome c oxidase in the fibroblasts of the patient was also normal, suggesting that the kinetic properties of the enzyme were altered. Analysis of the kinetic parameters of cytochrome c oxidase demonstrated an aberrant interaction between cytochrome c oxidase and its substrate, cytochrome c, most likely because of a mutation in one of the nuclear subunits of the enzyme.

Tissue distribution of the human MDR3 P-glycoprotein.

BACKGROUND: P-glycoproteins (Pgps) belong to a family of well conserved plasma membrane proteins with two members in humans: MDR1 and MDR3. The MDR1 Pgp can transport drugs; the murine homologue of MDR3, mdr2, was recently shown by us to be involved in transport of the phospholipid phosphatidylcholine (lecithin) into bile. EXPERIMENTAL DESIGN: We have determined the MDR3 mRNA levels in a panel of human tissues by RNase protection. We have also generated polyclonal antibodies specific for the MDR3 Pgp. Detection of the MDR3 Pgp in human tissues with these antibodies was by a streptavidin-ABC procedure. RESULTS: The RNase protection results show that expression of the MDR3 gene has a more restricted distribution than that of MDR1. A high level of MDR3 mRNA was detected in the liver and in low levels in the adrenal gland, heart, striated muscle, spleen, and tonsil. In all of these tissues, some of the previously described splice variants of MDR3 were abundantly expressed. No indications were found for a tissue-specific regulation of alternative splicing of the MDR3 pre-mRNA. Two MDR3 Pgp-specific antibodies stained the bile canalicular membrane of hepatocytes across the entire liver lobule. No staining was found in the epithelial cells of the bile ductules and gall bladder, indicating that the staining at these sites with C219, a monoclonal antibody that recognizes both MDR1 and MDR3 Pgp, (mainly) represents the MDR1 Pgp. No MDR3 was detected by specific antibodies in the adrenal gland, spleen, and muscle. Since no staining was reported with MDR1-specific antibodies in muscle either, our results indicate that the C219 staining in some fibers of striated muscle represents a cross-reaction with another protein. One of the human MDR3-specific antibodies cross-reacted with the highly homologous mouse mdr2 Pgp. Staining with this antibody showed that the distribution of this protein in mouse liver and striated muscle is very similar to that of MDR3 Pgp in human tissues. CONCLUSIONS: The highest expression of the MDR3 Pgp was found in liver in the canalicular membranes of hepatocytes. This is in agreement with a role for MDR3 in the transport of phospholipid into bile.

The expression of two P-glycoprotein (pgp) genes in transgenic Caenorhabditis elegans is confined to intestinal cells.

P-glycoproteins can cause multidrug resistance in mammalian tumor cells by active extrusion of cytotoxic drugs. The natural function of these evolutionarily conserved, membrane-bound ATP binding transport proteins is unknown. In mammals, P-glycoproteins are abundantly present in organs associated with the digestive tract. We have studied the tissue-specific expression of Caenorhabditis elegans P-glycoprotein genes pgp-1 and pgp-3 by transformation of nematodes with pgp-lacZ gene fusion constructs in which the promoter area of the pgp genes was fused to the coding region of lacZ. Expression of pgp-1 and pgp-3, as inferred from pgp-lacZ transgenic nematodes, was confined to the intestinal cells. The expression patterns of both genes were virtually indistinguishable. Quantitative analysis of pgp mRNA levels during development showed that pgp-1, -2, and -3 were expressed throughout the life cycle of C.elegans, albeit with some variation indicating developmental regulation. The expression of P-glycoprotein genes in intestinal cells is an evolutionarily conserved feature of these genes, consistent with the hypothesis that P-glycoproteins provide a mechanism of protection against environmental toxins.

The P-glycoprotein gene family of Caenorhabditis elegans. Cloning and characterization of genomic and complementary DNA sequences.

P-glycoproteins, encoded by families of evolutionarily conserved genes, can confer a multidrug-resistant phenotype to mammalian tumor cells. To obtain more information on their functions in normal cells we have cloned genomic and complementary DNA sequences of four P-glycoprotein gene homologs of the genetically well-characterized nematode Caenorhabditis elegans, termed pgp-1, pgp-2, pgp-3 and pgp-4, respectively. The genes were physically mapped on chromosome IV (pgp-1), I (pgp-2) and X (pgp-3 and pgp-4). Phenotypic mutants corresponding to these loci have not yet been described. Two of the genes, pgp-1 and pgp-3, were analyzed in detail. They are predicted to encode ATP-binding membrane-spanning proteins of 1321 and 1254 amino acid residues, respectively, with the characteristic features shared by most P-glycoproteins described thus far. Intra-species divergence of P-glycoprotein genes is more pronounced in C. elegans than in mammals. Only 40% of the amino acids of pgp-1 and pgp-3 are identical, in contrast to 77% identity between human MDR1 and MDR3. pgp-1 consists of 14 exons, pgp-3 of 13. The two genes share only one intron position, whereas they share four (pgp-1) and five (pgp-3) intron positions with mammalian P-glycoprotein genes. pgp-1, pgp-2, and pgp-3 are transcribed into low abundance mRNAs in wild-type nematodes. pgp-1 and pgp-3 mRNAs have the trans-spliced leader SL1 at their 5' ends. Arsenite, emetine and actinomycin D drugs did not increase the steady state levels of pgp mRNA, unlike in some mammalian cell types. Heat shock disturbed trans as well as cis-splicing of pgp-1 and led to the accumulation of partially processed pgp-1 RNA. Thus, in C. elegans these genes are not induced in the context of a general stress response, as has been proposed for mammalian P-glycoprotein genes in certain tissues.

Structure of the human MDR3 gene and physical mapping of the human MDR locus.

Two genes, MDR1 and MDR3, constitute the human P-glycoprotein gene family. To examine the evolutionary relationship between the three known classes of mammalian P-glycoprotein genes, we have cloned the MDR3 gene and compared its structure with that of the human MDR1 and the mouse mdr1 (mdr1b) genes analyzed by other groups. The MDR3 gene contains 28 exons and 27 of these contain coding sequences for the two homologous halves of the protein that correlate with functional domains. This structure is virtually identical to that of the human MDR1 gene and the mouse mdr1 (mdr1b) gene, indicating that the exon/intron structure was fixed before the duplication events that generated different classes of P-glycoproteins, but after the P-glycoproteins diverged from related genes, like the cystic fibrosis transmembrane conductance regulator (CFTR) gene, which has an entirely different exon/intron structure. The four alternatively spliced transcripts of the MDR3 gene arise from alternative splicing of exons 23 and 26. Our analysis of DNA clones covering about 120 kilobases (kb) of the human MDR locus, including the entire MDR3 gene (74 kb) and the intergenic region between both genes (34 kb), combined with pulsed-field gel electrophoresis data shows that the human MDR locus covers about 230 kb. In contrast to the mouse mdr genes, both human genes are transcribed in the same direction (MDR3 located downstream of MDR1). The CpG-rich sequences marking the 5' ends of both genes are hypomethylated to different extents in different cell lines. Hypomethylation roughly correlates with transcriptional activity.

Multidrug resistance phenotype of human BRO melanoma cells transfected with a wild-type human mdr1 complementary DNA.

We have transfected a eukaryotic expression vector containing a mdr1 complementary DNA isolated from normal human liver into human BRO melanoma cells to study the drug-resistant phenotype produced by the exclusive overexpression of normal human mdr1 P-glycoprotein. The drug resistance pattern of mdr1-transfected clones includes relatively high resistance to gramicidin D (about 300-fold), vincristine (about 100-fold), and actinomycin D (about 100-fold) and a lower degree of resistance to doxorubicin (about 10-fold), VP16-213 (about 10-fold), and colchicine (about 6-fold). The transfectants did not exhibit resistance to trimetrexate, cis-platinum, mitomycin C, 1-beta-D-arabinofuranosylcytosine, bleomycin, G418, or magainin-2-amide; they were slightly more sensitive to verapamil (2-fold) but not to Triton X-100. As in other multidrug-resistant cell lines, resistance to vincristine could be reversed by verapamil and, more effectively, by cyclosporin A. Chloroquine only marginally increased drug sensitivity in mdr1-transfected cells. Gramicidin D resistance was also reversed by verapamil, suggesting that the mechanism of resistance to this polypeptide antibiotic is similar to that of other drugs transported by P-glycoprotein. Thus, expression of the wild-type mdr1 complementary DNA induces a drug-resistant phenotype similar to that induced by mdr1 complementary DNAs isolated from drug-resistant cell lines with relatively low colchicine resistance. As other cell lines may display a different pattern of drug resistance, it is clear that other resistance mechanisms or cell type-specific factors may modulate the resistance. mdr1-transfected cell lines provide a convenient tool for the identification of P-glycoprotein-mediated phenomena.

Circular DNA of 3T6R50 double minute chromosomes.

In pulsed field gradient gel electrophoresis (PFGE) the intact deproteinized circular DNA of Mycoplasma (800 kb) and Escherichia coli (4700 kb) remains trapped in the slot. We show here that gamma-irradiation of the DNA in agarose plugs is a convenient method to partially convert these circles into full-length linears, migrating with the expected mobility in PFGE. We have used this method to study the structure of Double Minute chromosomes (DMs) from the methotrexate (MTX)-resistant mouse cell line 3T6R50. Intact deproteinized DM DNA is immobile in these gels, but is converted into a single band of about 2500 kb by either gamma-irradiation, DNaseI in the presence of Mn2+, or restriction enzymes. We conclude that the DM DNA in 3T6R50 cells consists of a homogeneous population of 2500-kb circles.

Growth of chromosome ends in multiplying trypanosomes.

Some of the genes for the variant surface glycoproteins of trypanosomes are located close to a discontinuity in the DNA, presumably a chromosome end. We show here that DNA fragments containing these telomeres increase in length in multiplying trypanosomes at a rate of about 10 base pairs per division. We argue that chromosome growth may not be restricted to trypanosomes and could explain the heterogeneity of telomeric DNA fragments observed in some other organisms.