Current status of the Plasmodium falciparum genome project.

The Plasmodium falciparum Genome Project is a collaborative effort by many laboratories that will provide detailed molecular information about the parasite, which may be used for developing practical control measures. Initial goals are to prepare an electronically indexed clone bank containing partially sequenced clones representing up to 80% of the parasite's genes and to prepare an ordered set of overlapping clones spanning each of the parasite's 14 chromosomes. Currently, clones of genomic DNA, prepared as yeast artificial chromosomes, are arranged into contigs covering approximately 70% of the genome of parasite clone 3D7, gene sequence tags are available from more than contigs covering approximately 70% of the genome of parasite clone 3D7, gene sequence tags are available from more than 20% of the parasite's genes, and approximately 5% of the parasite's genes are tentatively identified from similarity searches of entries in the international sequence databases. A total of > 0.5 Mb of P. falciparum sequence tag data is available. The gene sequence tags are presently being used to complete YAC contig assembly and localize the cloned genes to positions on the physical map in preparation for sequencing the genome. Routes of access to project information and services are described.

Molecular studies of the fragile X syndrome.

We have studied families segregating for the fragile X syndrome for the presence of amplification of the CGG repeat sequence adjacent to the HpaII Tiny Fragment (HTF) island in the FMR-1 gene. We demonstrate that 138/143 fragile X positive, mentally retarded males show a characteristic smear of fragments corresponding to somatic variation in the amplification of the CGG sequence. In 7/8 normal transmitting males (NTM's), we show that there is a small amplification of sequence but no evidence for somatic variation. Defined mutated fragments in the size range found in NTM's are seen in daughters of NTM's. The daughters of these female carriers show either a defined fragment in the NTM size range, a defined larger fragment or a heterogeneous pattern of fragments. In the latter 2 cases the clinical phenotype of the females cannot easily be predicted, presumably because of variable X inactivation. In some families, the observed DNA genotype does not correlate with the phenotype; in others we demonstrate the occurrence of individuals with a mosaic DNA genotype. The implications of these data for diagnosis of the disease are discussed.

Origins of the fragile X syndrome mutation.

The fragile X syndrome is a common cause of mental impairment. In view of the low reproductive fitness of affected males, the high incidence of the syndrome has been suggested to be the result of a high rate of new mutations occurring exclusively in the male germline. Extensive family studies, however, have failed to identify any cases of a new mutation. Alternatively, it has been suggested that a selective advantage of unaffected heterozygotes may, in part, explain the high incidence of the syndrome. Molecular investigations have shown that the syndrome is caused by the amplification of a CGG trinucleotide repeat in the FMR-1 gene which leads to the loss of gene expression. Further to this, genetic studies have suggested that there is evidence of linkage disequilibrium between the fragile X disease locus and flanking polymorphic markers. More recently, this analysis has been extended and has led to the observation that a large number of fragile X chromosomes appear to be lineage descendants of founder mutation events. Here, we present a study of the FRAXAC1 polymorphic marker in our patient cohort. We find that its allele distribution is strikingly different on fragile X chromosomes, confirming the earlier observations and giving further support to the suggestions of a fragile X founder effect.

DNA testing for fragile X syndrome in schools for learning difficulties.

Fragile X syndrome is the most common inherited cause of mental retardation. Early diagnosis is important not only for appropriate management of individuals but also to identify carriers who are unaware of their high risk of having an affected child. The disorder is associated with a cytogenetically visible fragile site (FRAXA) at Xq27.3, caused by amplification of a (CGG)n repeat sequence within the gene at this locus designated FMR1. Clinical and molecular studies have been undertaken to screen for fragile X syndrome in 154 children with moderate and severe learning difficulties of previously unknown origin. Southern blot analysis of peripheral blood showed the characteristic abnormally large (CGG)n repeat sequence associated with fragile X syndrome in four of the 154 children. The findings were confirmed by cytogenetic observation of the fragile site and by further molecular studies. The families of the affected children were offered genetic counselling and DNA tests to determine their carrier status. These findings show that there are still unrecognised cases of fragile X syndrome. Given the difficulty of making a clinical diagnosis and the implications for families when the diagnosis is missed, screening in high risk populations may be justified. The issues involved in screening all children in special schools for fragile X syndrome are discussed.

The human SB1.8 gene (DXS423E) encodes a putative chromosome segregation protein conserved in lower eukaryotes and prokaryotes.

We report that the human gene SB1.8 (DXS423E) encodes a protein of 1233 amino acids that is highly homologous (30% identity) to the essential yeast protein SMC1 which is required for the segregation of chromosomes at mitosis. Both SB1.8 and SMC1 contain an N-terminal NTP binding site, a central coiled-coil region and a C-terminal helix-loop-helix domain, and have structural features in common with the force generating proteins myosin and kinesin. SB1.8 also exhibits regions of homology and overall structural similarity to the prokaryote (Mycoplasma hyorhinis) protein 115p. Thus SB1.8 and SMC1 are members of a highly conserved and ubiquitous family of proteins that appear to have a fundamental role in cell division. In addition we show that SB1.8 (DXS423E) maps to a cosmid contig that lies centromeric to the OATL2 locus at chromosome Xp11.2.

Complex repetitive arrangements of gene sequence in the candidate region of the spinal muscular atrophy gene in 5q13.

Childhood-onset proximal spinal muscular atrophy (SMA) is a heritable neurological disorder, which has been mapped by genetic linkage analysis to chromosome 5q13, in the interval between markers D5S435 and D5S557. Here, we present gene sequences that have been isolated from this interval, several of which show sequence homologies to exons of beta-glucuronidase. These gene sequences are repeated several times across the candidate region and are also present on chromosome 5p. The arrangement of these repetitive gene motifs is polymorphic between individuals. The high degree of variability observed may have some influence on the expression of the genes in the region. Since SMA is not inherited as a classical autosomal recessive disease, novel genomic rearrangements arising from aberrant recombination events between the complex repeats may be associated with the phenotype observed.

Trinucleotide repeat amplification and hypermethylation of a CpG island in FRAXE mental retardation.

We have cloned the fragile site FRAXE and demonstrate that individuals with this fragile site possess amplifications of a GCC repeat adjacent to a CpG island in Xq28 of the human X chromosome. Normal individuals have 6-25 copies of the GCC repeat, whereas mentally retarded, FRAXE-positive individuals have > 200 copies and also have methylation at the CpG island. This situation is similar to that seen at the FRAXA locus and is another example in which a trinucleotide repeat expansion is associated with a human genetic disorder. In contrast with the fragile X syndrome, the GCC repeat can expand or contract and is equally unstable when passed through the male or female line. These results also have implications for the understanding of chromosome fragility.

A contig of non-chimaeric YACs containing the spinal muscular atrophy gene in 5q13.

We have constructed a contig of non-chimaeric yeast artificial chromosomes (YACs) across the candidate region for childhood autosomal recessive spinal muscular atrophy (SMA) in 5q13. A novel microsatellite reduces the candidate region to approximately 400kb of DNA distal to D5S435. The candidate region contains blocks of chromosome 5 specific repeats which have copies on 5p as well as elsewhere on 5q. Restriction mapping of the YACs reveals at least one CpG island in the SMA gene region. The YAC maps indicate that the contig contains minimal rearrangements or deletions. The data show the value of screening several YAC libraries simultaneously in order to construct a set of overlapping sequences suitable for candidate gene searches and direct genomic sequencing.

Mapping of retrotransposon sequences in the unstable region surrounding the spinal muscular atrophy locus in 5q13.

The mutation that underlies the autosomal recessive disorder spinal muscular atrophy (SMA) is located on chromosome 5q13. Recent studies show that SMA patients frequently have deletions and rearrangements in this region compared to normal controls. During the isolation of candidate cDNAs for the disease, we identified a sequence that shows high homology to the THE-1 retrotransposon gene family. Using YAC fragmentation techniques, we have refined the localization of this sequence to the domain known to show instability in SMA patients. The implication of these results for the mechanism of the mutation in SMA is discussed.

Sequence of Plasmodium falciparum chromosomes 1, 3-9 and 13.

Since the sequencing of the first two chromosomes of the malaria parasite, Plasmodium falciparum, there has been a concerted effort to sequence and assemble the entire genome of this organism. Here we report the sequence of chromosomes 1, 3-9 and 13 of P. falciparum clone 3D7--these chromosomes account for approximately 55% of the total genome. We describe the methods used to map, sequence and annotate these chromosomes. By comparing our assemblies with the optical map, we indicate the completeness of the resulting sequence. During annotation, we assign Gene Ontology terms to the predicted gene products, and observe clustering of some malaria-specific terms to specific chromosomes. We identify a highly conserved sequence element found in the intergenic region of internal var genes that is not associated with their telomeric counterparts.