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IBMFS in zebrafish
observed rescue of cell proliferation, which partially sup- pressed the degenerative phenotypes.55,57 In another study, Kishi et al. studied the effect of ablation of terfa; they found multiple malformations mainly in brain, spinal cord, and eye.58
Recently, 2 patients with a phenotype overlapping with DBA and DC (pure red cell aplasia, hypogammaglobu- linemia, growth retardation, and microcephaly) harbored a de novo TP53 germline mutation that caused a C-terminal truncation in the last exon. This resulted in enhanced p53- mediated transcriptional activity. Using an MO that tar- gets the 3’ splice site of intron 10, Toki et al. developed a zebrafish that displayed reduced number of erythrocytes, severe developmental defects, and died at 96 hpf.7
Fanconi anemia
Fanconi anemia is mostly an autosomal recessive condi- tion characterized by congenital abnormalities, progres- sive bone marrow failure, chromosome fragility, and an early onset of cancers such as myelodysplastic syndromes (MDS) /acute myeloid leukemia (AML) and epithelial malignancies. FA is characterized by non-hematologic phenotype, including short stature, microcephaly, microphthalmia, hypogonadism, and infertility. The mechanisms by which FA leads to developmental anom- alies in blood, skeleton, eyes, and gonads are poorly understood; however, genotoxic stress by chemicals, mutagens, and viruses may contribute.59,60
Mutations in at least 20 genes can cause FA. However, since some cases of FA cannot be assigned to any of these genes, additional genes still have to be identified.1,59 Proteins encoded by these genes constitute the FA path- way required for the efficient repair of damaged DNA. The FA core complex consists of at least 8 proteins: FANCA, FANCB, FANCC, FANCE, FANCF, FANCG FANCL, and FANCM. These proteins function as an E3 ligase and mediate the activation of the FANCD2 and FANCI (ID) complex. Once monoubiquitinated, the ID complex interacts with a third group of FANC proteins, including BRCA2 (FANCD1), FANCJ (BRIP1), FANCN (PALB2), FANCO (RAD51C), FANCP (SLX4), BRCA1, FAN1, histone H2AX, and RAD51, thereby contributing to DNA repair via homologous recombination.1,59,61,62 Until now, 20 genes have been associated with causing FA: FANCA, FANCB, FANCC, BRCA2, FANCD2, FANCE, FANCF, FANCG, FANCI, FANCJ, FANCL, FANCN, FANCP, FANCQ, RAD51, BRCA1, FANCT, FANCU, FANCV and FANCW. Information about all these genes is available on the public Fanconi Anemia Mutation Database (http://www.rockefeller.edu/fanconi/).
Although zebrafish contain the full complement of FA family members found in humans,63 loss-of-function mod- els have been described for only a few. Liu et al. analyzed the zebrafish ortholog of the human FANCD2 gene using MO.64 They demonstrated developmental defects that arose during embryogenesis after fancd2 knockdown, phe- nocopying the reduction in body length, and smaller head and eyes, which are frequently observed among FA patients. This suggests that the FA pathway plays a similar role in zebrafish and humans. They showed that the defects in fancd2-deficient embryos were the result of inappropriate and selective activation of Tp53-mediated apoptotic pathways in highly proliferative cells.64
Titus et al. characterized the developmental and tissue- specific expression of FA pathway genes in zebrafish.63
They found maternal deposition of mRNA fanc genes can provide Fanc proteins to repair DNA damage encountered in rapid cleavage divisions. Zebrafish fancl mutants devel- op only as sterile males but without hematopoietic defects. The sex reversal was due to abnormal increase of germ cell apoptosis that compromises survival of develop- ing oocytes and masculinizes the gonads. Interestingly, when the tp53 mutation was introduced, the sex reversal phenotype could be rescued.65 Botthoff et al. created a rad51 knockout zebrafish mutant. In this model, zebrafish lacking rad51 survived to adulthood, but they were all infertile males with fewer HSPCs in the kidney. In earlier stages (2 and 4 dpf), they found that rad51-/- embryos also had a lower number, increased apoptosis, and reduced proliferation of HSPCs compared with their wild-type sib- lings. To study the role of p53 in the rad51 mutants, they generated a zebrafish with mutations in both genes. After four months post fertilization, HSPCs were the same in wild-type and double mutants. The sex reversal was also corrected, but neither females nor male double mutants were fertile.66
Shwachman-Diamond syndrome
Shwachman-Diamond syndrome is an autosomal reces- sive disorder characterized by exocrine pancreatic insuffi- ciency, bone marrow dysfunction, and skeletal abnormali- ties. Hematologic abnormalities are a major cause of mor- bidity and mortality, and include cytopenia(s), MDS, and AML. Neutropenia occurs in approximately 90% of patients and occurs as early as the neonatal period. Skeletal abnormalities, such as metaphyseal chondrodysplasia, tho- racic dystrophy, and short stature are common in SDS. In 2003, mutations in the Shwachman–Bodian-Diamond syn- drome (SBDS) gene were identified.70 In approximately 90% of cases, SDS is caused by two common mutations in exon 2 of SBDS: 183-184TA→CT introduces an in-frame stop codon (K62X) and 258+2T>C (C84Cfs) disrupts the donor splice site of intron 2, allowing a hypomorph to be produced.67 Fifty percent of cases are compound heterozy- gotes with respect to these two mutations. Boocock et al. found that both changes correspond to sequences that occur normally in the pseudogene. Both mutations can also occur in the same allele.67 Studies have identified additional changes in the coding sequence of SBDS that led to frameshift and missense mutations.
In 2007, Menne et al. characterized the function of the yeast SBDS ortholog Sdo1 in 60S maturation and transla- tional activation of ribosomes.68 SBDS is a protein with a well-documented role in the later steps of ribosome bio- genesis. SBDS interacts with the GTPase EFL1 to trigger release of eIF6 from the 60S ribosomal subunit. EIF6 is crit- ical for biogenesis and nuclear export of pre-60S subunits and prevents ribosomal subunit association. Removal of eIF6 is a prerequisite for the association of the 60S with the 40S subunit, and thus for the formation of an actively func- tioning ribosome.4 Recently, mutations in DNAJC2169,70 and EFL171 have been identified in individuals with SDS-like conditions. All of the SDS-associated mutant genes affect ribosome maturation. These important discoveries advance the concept of SDS as a ribosomopathy, and beg the question as to how ribosomopathies like DBA, SDS, or del (5q) can result in different defects in hematopoietic and non-hematopoietic tissues.
There have been no reports of homozygosity for SBDS null alleles, suggesting that human SBDS is essential and
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