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Kit ligand (Kitl), encoded by the Steel (Sl) locus, plays an essential role in hematopoiesis, gametogenesis, and melanogenesis during both embryonic and adult life. We have characterized a new spontaneous mutant of the Sl locus in mice designated KitlSl-20J that arose in the breeding colony at Jackson Laboratories. Heterozygous KitlSl-20J mice display a white belly spot and intercrossing results in an embryonic lethal phenotype in the homozygous state. Analysis of homozygous embryos demonstrated a significant reduction in fetal liver cellularity, colony forming unit-erythroid (CFU-E) progenitors, and a total absence of germ cells. Although expressed in vivo, recombinant mutant protein demonstrated loss of bioactivity that was correlated with lack of receptor binding. Analysis of the Sl gene transcripts in heterozygous KitlSl-20J mice revealed an in-frame tandem duplication of exon 3. A long-range polymerase chain reaction (PCR) strategy using overlapping primers in exon 3 amplified an approximately 7-kilobase (kb) product from DNA isolated from heterozygous KitlSl-20J mice but not from wild-type DNA that contained sequences from both introns 2 and 3 and an inverted intron 2 sequence, suggesting a complex rearrangement as the mechanism of the mutation. “Complexity analysis” of the sequence of the amplified product strongly suggests that local DNA motifs may have contributed to the generation of this spontaneous KitlSl-20J allele, likely mediated by a 2-step process. The KitlSl-20J mutation is a unique KitlSl allele and represents an unusual mechanism of mutation. (Blood. 2003;102:3548-3555)

Two alternatively spliced stem cell factor (SCF) transcripts encode protein products, which differ in the duration of membrane presentation. One form, soluble SCF (S-SCF) gets rapidly processed to yield predominantly secreted protein. The other form, membrane-associated SCF (MA-SCF) lacks the primary proteolytic cleavage site but is cleaved slowly from an alternate site, and thus represents a more stable membrane form of SCF. Mutants of SCF that lack the expression of MA-SCF (Steel-dickie) or possess a defect in its presentation (Steel17H) manifest deficiencies in erythroid cell development. In this study, we have compared the consequence(s) of activating Kit, the receptor for SCF by MA-SCF with S-SCF, and an obligate membrane-restricted (MR) form of SCF (MR-SCF) on erythroid cell survival, proliferation, cell cycle progression, and the activation of p38 and ERK MAP kinase pathways. Activation of Kit by MR-SCF was associated with a significantly lower incidence of apoptosis and cell death in erythroid cells compared to either other isoform. MR- or MA-SCF–induced stimulation of erythroid cells resulted in similar and significantly greater proliferation and cell cycle progression compared to soluble SCF. The increase in proliferation and cell cycle progression via MA- or MR-SCF stimulation correlated with sustained and enhanced activation of p38 and ERK MAP kinase pathways. In addition, MR- or MA-SCF–induced proliferation was more sensitive to the inhibitory effects of ERK inhibitor compared to S-SCF–induced proliferation. In contrast, soluble SCF-induced proliferation was more sensitive to the inhibitory effects of p38 inhibitor compared with MR- or MA-SCF. These results suggest that different isoforms of SCF may use different biochemical pathways in stimulation of survival and/or proliferation of erythroid cells.

Fanconi anemia (FA) is a rare autosomal recessive disorder that results from mutations in at least eleven different genes (A, B, C, D1, D2, E, F, G, I, J, L). Recent studies have demonstrated that clinical progression of the disease may be influenced by inter- and intra-genic variations, underlining the importance of subtyping complementation groups. Previously we have utilized retroviral mediated gene transfer of FA cDNAs and subsequent analysis for the correction of hypersensitivity to Mitomycin C, to develop a valid method for subtyping of groups (Hanenberg et al., 2002). However, for lymphoblastic cell lines (LCL), G418 selection was required to compensate for low transduction efficiencies achieved. In the present study we have utilized RD114/GALV psuedotyped (high titer stable producer clones) bicistronic retroviral vectors that coexpress EGFP. This results in higher transduction efficiencies in LCLs (20–70% for C, F, G and 10–20 % for A vector) and allows for specific analysis of transduced EGFP-positive cells within the bulk cultures, thus obviating the need for selection. In addition, the assay relies on the correction of G2/M arrest that is analyzed by FLOW, further reducing the assay time. After transduction with cDNA-containing vectors for groups A, C, F and G, cells are exposed for 48 hours to the alkylating agent Melphalan, stained with Hoechst 33342 dye and cell cycle analysis for FA-specific G2/M arrest is performed by flow cytometry. The percentage of EGFP-positive cells in the G2/M phase of cell cycle for each vector is calculated and compared using the MODFIT software. A complementation group is identified when correction in the G2/M arrest occurs. Results obtained with the assay matched the complementation group known for 12 control LCLs tested. We report here the results obtained for 39 FA patients with unknown complementation groups using this new assay. Of the 39 cell lines tested, complementation group was identified for 20 lines [FA-A(17), FA-C(1), FA-F(1), FA-G(1)], while 15 lines were typed as Non A/C/F/G. Cells from 4 patients did not undergo G2/M arrest. Mutation analysis by direct sequencing of genomic DNA after DHPLC completed for 11 patients thus far, has detected a mutation in one or both alleles confirming the results of the rapid complementation assay in every case (Table). For the 15 cell lines typed as Non A/C/F/G, additional studies including vectors for the FA genes FANCD2, FANCE, FANCL are underway. In summary, we have developed a rapid and efficient method for the identification of complementation groups in FA patients that also serves as a guide for quick mutational analysis. This methodology can also be applied to myeloid progenitor cells as well as fibroblasts, an advantage in patients with lymphoid mosaicism. Table 1. Mutational analysis of cell lines identified by cell cycle analysis | Cell Line Ref # | Cell cycle analysis | Mutation 1(paternal) | Mutation 2 (maternal) | |:--------------- | ------------------- | -------------------- | --------------------- | | RA2593 | A | c.1471-?_2151+?del | c.1471-?_2151+?del | | RA2483 | A | | c.1901-?_2014+?del | | RA2625 | A | | c.3239G>T(p.R1080L) | | RA2644 | A | c.2806G>A(p.E936K) | c.2806G>A(p.E936K) | | RA2685 | A | c.2678G>A(p.W893X) | | | RA2698 | A | c.337_338delTC | | | RA2704 | A | c.3514-?_3626+?del | c.3520_3522delTGG | | RA2724 | C | c.322delG | c.711+4A>T(IVS4+4A>T) | | RA2191 | A | c.3520_3522dellTGG | | | RA2709 | F | c.484-485delCT | c.219delG | | RA2727 | G | c.908T>C | c.908T>C