To study telomere length dynamics in hematopoietic cells with age, we analyzed the average length of telomere repeat sequences in diverse populations of nucleated blood cells. More than 500 individuals ranging in age from 0 to 90 yr, including 36 pairs of monozygous and dizygotic twins, were analyzed using quantitative fluorescence in situ hybridization and flow cytometry. Granulocytes and naive T cells showed a parallel biphasic decline in telomere length with age that most likely reflected accumulated cell divisions in the common precursors of both cell types: hematopoietic stem cells. Telomere loss was very rapid in the first year, and continued for more than eight decades at a 30-fold lower rate. Memory T cells also showed an initial rapid decline in telomere length with age. However, in contrast to naive T cells, this decline continued for several years, and in older individuals lymphocytes typically had shorter telomeres than did granulocytes. Our findings point to a dramatic decline in stem cell turnover in early childhood and support the notion that cell divisions in hematopoietic stem cells and T cells result in loss of telomeric DNA.

The viral titers produced were high enough to achieve transduction of virtually all of the hematopoietic stem cells in the graft with an average of three integrated proviral copies per genome in all transplanted mice; the transduction was sustained for >7 months in both primary and secondary transplants, at which time Ϸ95% of the red blood cells in all mice contained human ␤ globin contributing to 32 ؎ 4% of all ␤-like globin chains. Hematological parameters approached complete phenotypic correction, as assessed by hemoglobin levels and reticulocyte and red blood cell counts. All circulating red blood cells became and remained normocytic and normochromic, and their density was normalized. Free ␣ globin chains were completely cleared from red blood cell membranes, splenomegaly abated, and iron deposit was almost eliminated in liver sections. These findings indicate that virtually complete transduction of the hematopoietic stem cell compartment can be achieved by high-titer lentiviral vectors and that position effect variegation can be mitigated by multiple events of proviral integration to yield balanced, panerythroid expression. These results provide a solid foundation for the initiation of human clinical trials in ␤ thalassemia patients.

No abstract

IntroductionFanconi anemia (FA) is an autosomal recessive disease of childhood characterized by progressive pancytopenia, various developmental abnormalities, and a predisposition to acute myeloid leukemia. 1 Most individuals with FA, however, succumb to the complications of aplastic anemia. 2 FA cells demonstrate increased sensitivity to DNA cross-linking agents such as mitomycin C (MMC), diepoxybutane, and cisplatin, 2,3 a feature that serves as the basis for an important diagnostic test. FA cells treated with these cross-linking agents show a striking increase in double-strand DNA breaks and inhibited growth with cell cycle arrest in G 2 . 2 To date at least 7 potential FA genes have been indicated by complementation studies, and most of these genes, FANCA, FANCC, FANCD2, FANCE, FANCF, and FANCG, whose mutations account for 6 of the complementation groups, have now been characterized. [4][5][6][7][8][9][10] Despite the variety of genes involved in this disorder, mutations in FANCA and FANCC account for approximately 80% of all patients with FA. 11 Murine Fancc, being highly similar to the human ortholog, is able to complement human cells deficient in FANCC, restoring MMC resistance. 12 Fancc-deficient mouse strains were generated through gene targeting. Both had similar phenotypes, 13,14 demonstrating compromised gametogenesis, and an increase in the number of chromosomal aberrations, both spontaneously and after exposure to MMC. However, the targeted lines recapitulated neither the developmental nor the hematologic defects typical of human FA. 13,14 The reason for this interspecies discordance is unknown, but it has limited the utility of the mutant mice as potential models of FA.A number of hypotheses regarding the nature of the primary defect in FA have been suggested, including the proposal that FA proteins constitute a DNA damage recognition and signaling pathway, whose impairment is manifested by chromosomal instability and increased sensitivity to interstrand DNA cross-linking agents. 15 Although a reduced ability to process DNA cross-links is clearly evident, it has also been proposed that an abnormal reduction of MMC in FA cells leads to the production of reactive species that in turn generate cross-links and other types of oxidative lesions. 16 Thus, FA might also result, at least in part, from an abnormal regulation of cell redox state or of the cellular response to oxidative stress or both. In support of this notion, addition of Cu/Zn superoxide dimutase (SOD) to the culture medium of FA cells was reported to attenuate chromosomal breakage as well as MMC cytotoxicity, 16 an effect also observed in FA cells overexpressing thioredoxin. 17 In keeping with an inability to regulate either production, or the consequences of reactive oxygen species (ROS), some FA cells were shown to be hypersensitive to oxygen. 16,18 Thus, cells grew slowly at elevated oxygen levels (eg, 35%) and For personal use only. on May 9, 2018. by guest www.bloodjournal.org From tended to arrest at G 2 , whereas at low oxygen conc...

Summary. Myelodysplastic syndrome (MDS) is considered to be very rare in children. However, the only two published population-based studies reported widely divergent incidence ®gures. To further explore the epidemiology of childhood MDS and to evaluate the accuracy of cancer registry and treatment trial data, we conducted a population-based study of children aged 0±14 years in British Columbia (BC), Canada, between 1982 and 1996. MDS was diagnosed in 31 cases corresponding to an annual incidence of 3´2 per million children or 6% of all leukaemias, compared with an incidence of 6´0/million for acute myeloid leukaemia (AML), and of 0´5/million for chronic myeloid leukaemia. There was a non-signi®cant (P 0´19) trend toward an increase in MDS incidence with time, the increase was partly explained by an increasing number of patients with Down syndrome. Associated abnormalities were found in 48% of the MDS cases with Down syndrome as the most common (seven cases). Only one third of the MDS cases were correctly registered in the Cancer Registry and less than half of the eligible MDS patients were enrolled on a cooperative group study. Data on MDS from treatment-based studies and cancer registries were inaccurate and seemed to signi®cantly underestimate the incidence of MDS in children.

Most patients achieved therapeutic anticoagulation when dosed according to the published guidelines. Children with cardiac conditions or renal insufficiency or those younger than 2 months were more likely to require dosage adjustments to achieve the therapeutic range. Routine monitoring of anti-Xa levels is still necessary in these patient populations, particularly when the early establishment of therapeutic anticoagulation may be critical. Enoxaparin appears to be well tolerated in the authors' patient population.

Number of infants and, in brackets, the % of infants within a given iron class assignment. Information on age at introduction of iron-fortified infant cereals was not given for two infants; one was classified as uncertain and one had normal iron status.

Age- and sex-specific reference intervals based on the 0.025 and 0.975 fractiles of data derived from a healthy pediatric population are presented for zinc, copper, selenium, iron, ferritin, retinol, alpha-tocopherol, and related analytes in serum. Age was an important covariate for copper, selenium, retinol, and tocopherol, and ferritin in boys. Strong correlations were found between retinol and retinol-binding protein, prealbumin (transthyretin), alpha-tocopherol, and selenium. Tocopherol was highly correlated with both cholesterol and triglycerides. We found no relationship between serum zinc and either retinol or retinol-binding protein. Despite exclusion of children in whom anemia, microcytosis, or variant hemoglobins were found, the 0.025 fractile for iron in several age groups was even less than the concentration considered to indicate poor iron nutritional status.