Erythroid pyrimidine 5'-nucleotidase: cloning, developmental expression, and regulation by cAMP and in vivo hypoxia

Novel homozygous nonsense mutation in the P5'N-1 coding gene as an alternative cause for hereditary anemia with basophilic stippling

Hereditary pyrimidine 5-nucleotidase (P5'N-1) deficiency is a very rare disorder. Here, we describe a new mutation in a Turkish family. Although functional tests have not been performed, our findings confirm that the homozygous mutational state leads to clinical manifest P5'N-1 deficiency, while heterozygosity does not lead to hemolysis or anemia.

In embryonic chicken erythrocytes actively transcribed alpha globin genes are not associated with the nuclear matrix

The spatial organization of a 250 Kb region of chicken chromosome 14, which includes the alpha globin gene cluster, was studied using in situ hybridization of a corresponding BAC probe with nuclear halos. It was found that in non-erythroid cells (DT40) and cultured erythroid cells of definite lineage (HD3) the genomic region under study was partially (DT40 cells) or fully (HD3 cells) associated with the nuclear matrix. In contrast, in embryonic red blood cells (10-day RBC) the same area was located in the crown of DNA loops surrounding the nuclear matrix, although both globin genes and surrounding house-keeping genes were actively transcribed in these cells. This spatial organization was associated with the virtual absence of RNA polymerase II in nuclear matrices prepared from 10-day RBC. In contrast, in HD3 cells a significant portion of RNA polymerase II was present in nuclear matrices. Taken together, these observations suggest that in embryonic erythroid cells transcription does not occur in association with the nuclear matrix.

Molecular basis of pyrimidine 5'-nucleotidase deficiency caused by 3 newly identified missense mutations (c.187T>C, c.469G>C and c.740T>C) and a tabulation of known mutations

Hereditary pyrimidine 5'-nucleotidase deficiency is the most frequent enzymopathy of red blood cell nucleotide metabolism that causes hereditary non-spherocytic hemolytic anemia. The disease is usually characterized by mild-to-moderate hemolytic anemia, reticulocytosis and hyperbilirubinemia. To date, diagnosis ultimately depends upon demonstration of a reduced level of pyrimidine 5'-nucleotidase type-I (P5'N-1) activity in red cells and detection of mutations in the P5'N-1 gene. To unravel the causes of the P5'N deficiency and to obtain data for a definitive diagnosis three newly described missense mutations (c.187T>C, c.469G>C and c.740T>C) identified in patients with hemolytic anemia have been characterized at protein level. The mutant enzymes (C63R, G157R and I247T) were obtained as recombinant forms and purified to homogeneity. The enzymes were altered, although to a different extent, in both thermal stability and catalytic efficiency. The catalytic efficiency of all mutants was reduced especially towards UMP (up to more than 200 times), owing to the increased Km values (approximately, 10-25 times higher). The G157R enzyme was severely heat unstable and lost half of its activity after about 23 min of incubation at 37 degrees C. At higher temperature C63R and I247T mutants as well were less stable than the wild-type enzyme. Therefore, although the mutations targeted different regions of the P5'N-1 structure, they produced similar effects on the molecular properties of the enzyme. Thus, all affected amino acids are functionally and structurally important for preserving the enzyme activity during the red cell life span.

Unusual compartmentalization of CTCF and other transcription factors in the course of terminal erythroid differentiation

It is demonstrated that in chicken embryonic and mature erythrocyte nuclei the distribution of a versatile transcription factor CTCF differs drastically from its distribution in nuclei of proliferating erythroid and non-erythroid cells. In the latter case CTCF was distributed throughout the whole nucleus volume, being concentrated in many small compartments (punctuate nuclear staining). In contrast, in embryonic and mature erythrocytes CTCF was concentrated in a limited number of large compartments. These large CTCF-containing compartments were not observed in other cells. Occasionally, but not in all cells, some of these compartments were localized close to nucleoli but did not colocalize with them. In mature erythrocytes a clear exclusion of CTCF-containing compartments from the chromatin domain was observed. This exclusion correlated with a tight association of CTCF with the nuclear matrix. Concentration in relatively large compartments and exclusion from the chromatin domain in nuclei of mature erythrocytes were also observed for RNA polymerase II and several transcription factors. The data are discussed in the context of a hypothesis postulating that relocalization of different components of the transcriptional machinery from the chromatin domain into the interchromatin compartment is an important step of the terminal inactivation of chicken erythrocyte nuclei.

Hereditary pyrimidine 5'-nucleotidase deficiency: from genetics to clinical manifestations

Hereditary pyrimidine 5'-nucleotidase (P5'N) deficiency is the most frequent abnormality of the red cell nucleotide metabolism causing hereditary non-spherocytic haemolytic anaemia. The disorder is usually characterised by mild-to-moderate haemolytic anaemia associated with the accumulation of high concentrations of pyrimidine nucleotides within the erythrocyte. The precise mechanisms leading to the destruction of P5'N deficient red cells are still unclear. The pyrimidine 5'-nucleotidase type-I (P5'N-1) gene is localised on 7p15-p14 and the cDNA has been cloned and sequenced; 20 mutations have been identified so far in 30 unrelated families, most of them at the homozygous level. Recently, the comparison of recombinant mutants of human P5'N-1 with the wild-type enzyme has enabled the effects of amino acid replacements on the enzyme molecular properties to be determined and help to correlate genotype to clinical phenotype.

Functional analysis of pyrimidine 5′-nucleotidase mutants causing nonspherocytic hemolytic anemia

Inherited pyrimidine 5′-nucleotidase type I (P5′N-1) deficiency is the third most common erythrocyte enzymopathy that causes hemolysis. Fourteen different mutations have been identified to date. We have investigated the molecular bases of the disease by studying the biochemical properties of the recombinant wild-type human enzyme and 4 variant proteins (D87V, L131P, N179S, and G230R) bearing missense mutations found in patients affected by nonspherocytic hemolytic anemia. P5′N-1 is a relatively stable protein and has essentially identical catalytic efficiency toward cytidine monophosphate (CMP) and uridine monophosphate (UMP). All investigated mutant proteins display impaired catalytic properties and/or reduced thermostability, providing a rationale for the pathological effects of the mutations. Despite the substantial changes in the kinetic and thermostability parameters, the enzyme activity detected in the red blood cells of patients homozygous for mutations L131P and G230R exhibits moderate alterations. This suggests that P5′N-1 deficiency is compensated, possibly by other nucleotidases or alternative pathways in nucleotide metabolism. Therefore, nucleotidase activity may not be considered a prognostic indicator in patients affected by the enzymopathy. (Blood. 2005;105:3340-3345)

Molecular basis of Japanese variants of pyrimidine 5'-nucleotidase deficiency

The type-I isoform of pyrimidine 5'-nucleotidase (P5N-I) has an important role in the catabolism of pyrimidine mononucleotides during erythroid maturation. Two alternatively spliced forms of P5N-I mRNA have been identified, and we found another alternatively spliced form in reticulocytes, which included an additional 87-bp sequence. The sequence is located 6.2-kb downstream of the exon 2 and 2.7-kb upstream of the exon 3 sequence; consequently, the P5N-I gene encodes 11 exons, which span approximately 48 kb. We identified five novel mutations in nine families with P5N-I deficiency: two missense mutations (425C, 721C), one splice mutation (339C), one 1-bp insertion (251-insA-252) and one 9-bp deletion (del 192-200). All patients were homozygous for each mutation. The mutant P5N-I with 721C (G241R) had lower affinity for cytidine monophosphate, suggesting that Gly241 is important for substrate binding. Haplotype analysis showed that 721C, which had been identified in five unrelated families, was a founder mutation. The mutant P5N was then expressed in Cos-7. The degradation of P5N with 425C (L142P) was significantly faster than a wild-type control, and proteasome inhibitors restored the stability of L142P. These data suggest that L142P increases susceptibility to the degradation by the ubiquitin-proteasome pathway.