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360 Part IV Disorders of Hematopoietic Cell Development

Because genomic instability and a marked predisposition to leu- syndrome (SBDS), has been identified and has been confirmed in 90%
kemia and cancer are features of FA, the wisdom of using granulo- of patients with the classic presentation. SBDS seems to be multi-
poietic growth-promoting cytokines on a long-term basis for FA is functional and promotes cell survival, ribosome biogenesis, mitotic
an issue. There may be a heightened risk of inducing or promoting spindle stability, and chemotaxis. To date, though, no unifying
expansion of a leukemic clone, especially one with monosomy 7. pathogenesis has been able to account for all of the multisystem
Therefore, before starting cytokine therapy, a baseline BM aspirate features of SDS.
and biopsy is recommended and then repeated every 6 months to
document changes in morphology and cytogenetics.
Epidemiology

Genetic Counseling SDS has been reported among all ethnic groups. Older studies sug-
gested a higher incidence in males. However, recent data suggest an
Genetic counseling should be offered to all patients and families with equal distribution between genders as expected from an autosomal
FA. The discussion should include mode of transmission, risk of recessive disorder. Based on data from the CIMFR, SDS is the third
having the disease in family members, risk of recurrence in future most common IBMFS with an incidence of 8.5 cases per million live
pregnancies, available diagnostic tests during pregnancy, and PGD/ births.
IVF and selection of HLA-matched embryos who do not have FA as
potential donors for HSCT. Screening of all first-degree relatives
should be offered. Pathobiology
During pregnancy, the abnormal chromosome breakage pattern
characteristic of FA can be used to make a prenatal diagnosis of FA The identification of SBDS on chromosome 7q11 was the entry point
as well as gene testing. Diagnostic testing can be performed on fetal for studies on the molecular basis for SDS. The gene encodes a 250
amniotic fluid cells obtained at week 16 of gestation or on chorionic amino acid protein product, which is a member of a highly conserved
villus biopsy specimens at 9 to 12 weeks of gestation. A very high protein family of previously unknown function with putative ortho-
degree of prenatal diagnostic accuracy has been obtained by looking logs in diverse species, including Archaea and eukaryocytes. Based on
at both spontaneous and DEB-induced breaks in fetal tissue. DEB structural studies of the ortholog in Archaea and the human protein,
testing of heterozygote carriers is unreliable for diagnosis because the SBDS protein has three main domains (N-terminal, middle, and
there is overlap of results with normal individuals. C-terminal) with predicted protein–protein, protein–DNA, and
An updated manual for the management of patients with FA has protein–RNA binding motifs.
been published by the Fanconi Anemia Research Fund (see There is an adjoining pseudogene (SBDSP) with 97% homology
www.fanconi.org for Fanconi Anemia Guidelines for Diagnosis and in its coding regions to SBDS. The common SBDS mutations are
Management, 4th edition, 2014). composed of sequences that are homologous to SBDSP. Hence these
mutations are believed to result from gene recombination events in
which SBDSP1 acts as the donor. These recombinational events result
Future Directions in three common gene conversion mutations in exon 2 that account
for 75% of SDS alleles: (1) a splice-site mutation, c.258+2T>C,
The premise for gene therapy in FA is based on the assumption that which may either cause premature truncation of the SBDS protein
corrected hematopoietic cells would have a growth advantage. by frameshift (p,C84fs3) or use an alternative splice site; (2) a non-
Strengthening this supposition are patients with FA with hematopoi- sense mutation, c.183_184TA>CT that introduces an in-frame stop
etic somatic mosaicism who show spontaneous disappearance of cells codon (p.K62X); and (3) an extended conversion mutation,
with the FA phenotype. These mosaic patients may show spontaneous c.183_184TA>CT and c.258+2T>C, encompasses both mutations.
hematologic improvement, suggesting that hematopoiesis was derived In the Toronto database of 210 SDS families, 89% of unrelated SDS
from stem cells with a normal phenotype. In the context of gene individuals carry a gene conversion mutation on one allele, and 60%
therapy, evidence suggests that even one genetically corrected HSC carry conversion mutations on both alleles. Thus the vast majority of
may be able to repopulate the BM of a patient with FA. patients are compound heterozygotes with respect to p.K62X and
Despite encouraging preclinical studies more than a decade ago p.C84fsx3. Additional rare mutations in the SBDS gene have been
using retroviral vectors showing that wild-type FANCC and FANCA identified in patients with SDS. These include dozens of insertion,
+
could be integrated into normal and FA CD34 cells, the ensuing deletion, and missense mutations that have not arisen from gene
clinical trials in FANCC and FANCA patients using retrovectors were conversion events. Most SBDS mutations alter the N-terminal
disappointing. A central problem was suboptimal wild-type gene domain of the protein and lead to markedly reduced protein levels.
integration into FA cells in culture. Because of the apoptotic pheno- SBDS protein is essential for life because no patients with homo-
type and the sensitivity to oxidative stress, FA cells die rapidly in vitro zygous null mutations have been reported, and small levels of residual
before efficient gene transfer is accomplished. Changing the tissue protein can usually be detected in patients with SDS. Furthermore,
culture conditions (e.g., usage of low oxygen condition) and introduc- a complete loss of the protein in mice causes developmental arrest
ing lentiviral vectors that can infect noncycling human cells were before embryonic day 6.5 and early lethality. SBDS seems to be
deemed the solutions, and the first clinical trial for FA group A has multifunctional and play a role in several cellular pathways, including
been opened. Ongoing research is directed at improving vector design, ribosomal biogenesis, cell survival, chemotaxis, mitotic spindle for-
transduction methodology, and improved strategies for preparing mation, and protection from cellular stress.
HSCs. One caveat: a successful FA gene therapy protocol may correct The SBDS protein phylogeny is shared with proteins that are
BM failure and possibly the propensity for MDS and AML, but the enriched for RNA metabolism and/or ribosome-associated functions.
predisposition for cancer in other tissues will continue unchecked. The SBDS protein can be detected in human cell nuclei and cyto-
plasm. It concentrates in the nucleolus during G1 and G2. Synthetic
genetic arrays of YHR087W, a yeast homolog of the N-terminal
Shwachman-Diamond Syndrome domain of SBDS, suggested interactions with several genes involved
in RNA and rRNA processing. Loss of the protein in humans and
SDS is an autosomal recessive multisystem disorder characterized by yeast results in failure to remove eukaryotic initiation factor 6, eIf6
varying degrees of BM failure, a high risk of leukemia, and exocrine or its homologue in yeast, Tif6, from the ribosomal large subunit in
pancreatic insufficiency. Additional features may include short stature the cytoplasm and impairs the assembly of the large and small ribo-
and skeletal abnormalities. The mutant gene responsible for this some subunits to form the mature ribosomes. SBDS directly interacts
complex pleiotropic phenotype, termed Shwachman-Bodian-Diamond with the GTPase elongation factor-like 1 (EFL1). The interaction