Resisting Drugs New Light on Night Blindness

HIGHLIGHTS

HUMAN GENETICS New light on night blindness

Courtesy of Robert Gwadz. X-linked congenital stationary night rich repeats (LRRs), an amino-terminal MALARIA blindness (CSNB) is a non-progressive signal peptide, a possible carboxy-termi- retinal disorder that is characterized by nal cleavage site, and a glycosylation site, impaired night vision, reduced visual acu- among other motifs. Both teams propose Resisting drugs ity, and frequently, although not always, that nyctalopin is cleaved at the carboxyl by nystagmus (uncontrollable eye move- terminus and is GPI-anchored at the Chloroquine is a safe and cheap antimalarial drug — ment) and myopia. There are two forms extracellular cell membrane as a glycosy- and it used to be effective. But the malaria parasite, of the condition, called complete and lated proteoglycan. It is likely that the Plasmodium falciparum, has fought back by incomplete X-linked CSNB, which are LRRs confer on nyctalopin the ability to developing drug resistance. The rising prevalence of distinguishable by measuring the electro- interact with other proteins. LRRs can chloroquine resistance is increasing still further the physiological responses of the retina to mediate diverse biological functions, health threat posed by malaria. There is an urgent light. The gene for the incomplete form including cell adhesion and migration — need to understand the basis of chloroquine was identified in 1998, and now, two years in Drosophila melanogaster, for example, resistance (CQR), and genetic studies have just on, Nature Genetics reports the cloning of two LRR proteins, chaoptin and capri- opened the door. the gene for complete CSNB. cious, are cell adhesion molecules that Ten years ago, it was shown that CQR segregates as Two research teams tracked down this are crucial for fly neuronal development. a single Mendelian trait in the P.falciparum genome. gene by a mapping and positional-cloning So, nyctalopin might function as an The region was narrowed down to 35 kb, but, approach. In 22 complete X-linked CSNB adhesion molecule that specifies the for- frustratingly, none of the eight genes identified had families, Bech-Hansen et al. identified 14 mation of synapses between rod pho- mutations that were perfectly associated with CQR. different mutations in a novel gene (NYX) toreceptors (which respond to light) and Fidock et al. therefore decided to have a closer look at that encodes a small leucine-rich proteo- postsynaptic neurons. No doubt, the two the region, by searching for small open reading frames glycan called nyctalopin. Meanwhile, teams will now investigate the function (fewer than 100 codons), and used them to screen Pusch et al. identified NYX deletions in of this protein in establishing the com- cDNA libraries. They found a novel gene, pfcrt, three families with the condition, and plex circuitry of the retina to elucidate comprising 13 exons that covered only 3.1 kb, which went on to identify 14 other mutations in how it becomes miswired in patients was predicted to encode a ten-transmembrane a further 21 families. Both research teams with complete CSNB. channel or transporter protein. The small size of the found a spectrum of NYX mutations, Jane Alfred exons probably explains why the gene was missed in including truncations, deletions, point the earlier studies. mutations and insertions — Bech- References and links Hansen et al. also found one founder ORIGINAL RESEARCH PAPERS Bech-Hansen N. T. Analysis of pfcrt in natural strains of P.falciparum et al. Mutations in NYX, encoding the leucine-rich revealed several coding variants, and there was perfect mutation in seven American families with proteoglycan nyctalopin, cause X-linked complete association between one amino-acid change in the disease. congenital stationary night blindness. Nature Genet. 26, 319–323 (2000) | Pusch, C. M. et al. The complete form particular — K76T — and resistance. Transfection Expression studies by the two teams, of X-linked congenital stationary night blindness is caused studies supported the view that pfcrt from resistant however, produced different results. Bech- by mutations in a gene encoding a leucine-rich protein. parasites could confer resistance on a sensitive strain. Hansen et al.reported NYX to be Nature Genet. 26, 324–327 (2000) WEB SITE Congenital stationary night blindness What does pfcrt do? Chloroquine is thought to expressed robustly in the kidney and reti- affect the way the parasite disposes of toxic products na, whereas Pusch et al. found evidence from haemoglobin digestion, within a specialized for a low expression of NYX in several tis- organelle called the digestive vacuole. Fidock et al. sues, including the retina, kidney, brain have shown that this is where pfcrt is expressed, and and spinal cord. Pusch et al. suggested that that resistant parasites have a significantly reduced these differences might be due to the use pH within the vacuole. As a membrane protein, the of different PCR templates — this team resistant version of PfCRT might antagonize the had used human and fetal tissue, whereas effects of chloroquine indirectly by altering pH, or Bech-Hansen et al. had analysed human directly by affecting the transport of chloroquine adult, tissue-specific cDNA libraries. In across the vacuole membrane. After ten years, the situ data from Bech-Hansen et al. also identification of pfcrt is a well-earned prize, which showed that NYX transcripts localize to will accelerate progress towards an understanding of several cell types, including photoreceptor drug resistance in malaria. cells, interneurons and ganglion cells, in Mark Patterson the different layers of the human retina. References and links Nyctalopin is a 481 amino-acid pro- ORIGINAL RESEARCH PAPER Fidock, D. A. et al. Mutations in the P. tein with many motifs and domains that falciparum digestive vacuole transmembrane protein PfCRT and evidence give clues to its function. It contains a for their role in chloroquine resistance. Mol. Cell 6, 861–871 (2000) WEB SITES Thomas Wellems’s laboratory | The Plasmodium falciparum glycosylphosphatidylinositol (GPI)- genome database anchor sequence, 11 consecutive leucine-