Apolipoprotein E is a genetic risk factor for fetal iodine deficiency disorder in China.

Fetal iodine deficiency disorder (FIDD) is the principal form of endemic cretinism, and the most common cause of preventable mental deficiency in the world. However not everyone at risk develops FIDD and familial aggregation is common. This suggests that genetic factors may also be involved. The Apolipoprotein E (APOE) gene encodes for a lipoprotein that possesses a thyroid hormone binding domain, and APOE genotype may affect the efficiency with which thyroid hormone influences neuronal cell growth during the first and second trimesters of fetal development. We have compared ApoE genotypes in 91 FIDD cases with 154 local control subjects, recruited from three iodine deficiency areas in central China. We have also genotyped 42 FIDD family cases and 158 normal individuals from the families of local controls, and 375 population controls from Shanghai. APOE epsilon4 genotypes were significantly enriched in FIDD probands from each of the three iodine deficiency areas; the epsilon4 allele frequency was 16% vs 6% in controls. The same effect was also observed when we compared FIDD family cases with controls and control families. Our data suggest that in iodine-deficient areas, the APOE epsilon4 allele is a genetic risk factor for FIDD. The phenomenon may affect population selection and contribute to the low frequency of the epsilon4 allele in Chinese compared to Caucasian populations.

Thrombopoietin is synthesized by bone marrow stromal cells.

We have previously characterized stromal progenitor cells contained in fetal bone marrow by fluorescence-activated cell sorting (FACS) using the differential expression of CD34, CD38, and HLA-DR, and found that a small number were contained within the CD34(+) cell fraction. In the present study, the frequency of stromal progenitors in both the CD34+ and CD34- subpopulations from samples of fetal and adult bone marrow was approximately one in 5,000 of the mononuclear cell fraction. Using multiparameter single-cell sorting, one in 20 fetal bone marrow cells with the CD34+, CD38-, HLA-DR-, CDw90+ phenotype were clonogenic stromal progenitors, whereas greater than one in five single cells with the CD34-, CD38-, HLA-DR-, CDw90+ phenotype formed stromal cultures. We found that cultures initiated by hematopoietic and stromal progenitors contained within the CD34+ fraction of bone marrow cells formed mixed hematopoietic/stromal cell cultures that maintained the viability of the hematopoietic progenitor cells for 3 weeks in the absence of added hematopoietic cytokines. We characterized some of the hematopoietic cytokines synthesized by stromal cultures derived from either CD34+ or CD34- bone marrow cells using reverse transcriptase-polymerase chain reaction (RT-PCR) amplification of interleukin-3 (IL-3), stem cell factor (SCF), CD34, Flt3/Flk2 ligand (FL), and thrombopoietin (TPO) mRNA sequences. We found ubiquitous expression of TPO mRNA in greater than 90% of stromal cultures initiated by either CD34+ or CD34- cells, and variable expression of SCF, FL, and CD34 mRNA. In particular, SCF and CD34 mRNA were detected only in stromal cultures initiated by CD34+ bone marrow cells, although the differences between CD34+ and CD34- stromal cells were not statistically significant. IL-3 mRNA was not found in any stromal cultures. An enzyme-linked immunosorbent assay (ELISA) of soluble SCF and TPO present in culture supernatants demonstrated that biologically significant amounts of protein were secreted by some cultured stromal cells: eight of 16 samples of conditioned media from stromal cultures initiated by fetal and adult bone marrow contained more than 32 pg/mL SCF (in the linear range of the ELISA), with a median value of 32 pg/mL (range, 9 to 230), while 13 of 24 samples of conditioned media had more than 16 pg/mL TPO (in the linear range of the ELISA), with a median of 37 pg/mL (range, 16 to 106). Our data indicate that stromal cultures initiated by single bone marrow cells can make FL, SCF, and TPO. Local production of early-acting cytokines and TPO by stromal cells may be relevant to the regulation of hematopoietic stem cell self-renewal and megakaryocytopoiesis in the bone marrow microenvironment.

The "common stem cell" hypothesis reevaluated: human fetal bone marrow contains separate populations of hematopoietic and stromal progenitors.

There is a long-standing controversy as to whether a single bone marrow (BM)-derived cell can differentiate along both hematopoietic and stromal lineages. Both primitive hematopoietic and stromal progenitor cells in human BM express the CD34 antigen but lack expression of other surface markers, such as CD38. In this study we examined the CD34+, CD38- fraction of human fetal BM by multiparameter fluorescence-activated cell sorting (FACS) analysis and single-cell sorting. CD34+, C38- cells could be divided into HLA-DR+ and HLA-DR- fractions. After single-cell sorting, 59% of the HLA-DR+ cells formed hematopoietic colonies. In contrast, the CD34+, CD38-, HLA-DR- cells were much more heterogeneous with respect to their light scatter properties, expression of other hematopoietic markers (CD10, CD36, CD43, CD49b, CD49d, CD49e, CD50, CD62E, CD90w, CD105, and CD106), and growth properties. Single CD34+, CD38-, HLA-DR- cells sorted into individual culture wells formed either hematopoietic or stromal colonies. The presence or absence of CD50 (ICAM-3) expression distinguished hematopoietic from stromal progenitors within the CD34+, CD38-, HLA-DR- population. The CD50+ fraction had light scatter characteristics and growth properties of hematopoietic progenitor cells. In contrast, the CD50- fraction lacked hematopoietic progenitor activity but contained clonogenic stromal progenitors at a mean frequency of 5%. We tested the hypothesis that cultures derived from single cells with the CD34+, CD38-, HLA-DR- phenotype could differentiate along both a hematopoietic and stromal lineage. The cultures contained a variety of mesenchymal cell types and mononuclear cells that had the morphologic appearance of histiocytes. Immunophenotyping of cells from these cultures indicated a stromal rather than a hematopoietic origin. In addition, the growth of the histiocytic cells was independent of the presence or the absence of hematopoietic growth factors. Based on sorting more than 30,000 single cells with the CD34+, CD38-, HLA-DR- phenotype into individual culture wells, and an analysis of 864 stromal cultures initiated by single CD34+ BM cells, this study does not support the hypothesis of a single common progenitor for both hematopoietic and stromal lineages within human fetal BM.