There is probably no more canonical example of the relevance of evolutionary genetics to clinical medicine than sickle cell disease. The relevance of the sickle allele, in heterozygous form, at the beta-globin locus for resistance to falciparum malaria was published by Allison in 1954 (Lancet), and the precise amino acid substitution responsible for the phenotype of sickle cell disease, when the mutation is present in homozygous form, was identified by Ingram in 1956 (Nature). Two recent papers (Xu et al. Science 2011; Cyrklaff et al. Science 2011) offer interesting insights pertinent to both plausible future treatment strategies for the often-devastating condition arising from homozygosity of the sickle allele and the pathophysiology of the malaria resistance associated with possession of a single copy of the sickle allele of beta-hemoglobin. In this installment, I will focus on the findings of Xu et al. and their implications for devising improved therapies for sickle cell disease.
In the 1960s it was discovered that continued expression of fetal hemoglobin tended to diminish symptoms of those with sickle cell disease and prevent sickling of red cells in the presence of sickle hemoglobin (Conley et al., 1963). Subsequent studies demonstrated that increased erythrocyte expression of fetal hemoglobin inhibited sickling upon deoxygenation in vitro (Sewchand et al., 1983) and was correlated with milder clinical symptoms (Perrine et al. 1978; Powars et al., 1989). Thus as noted by Lander and Schork (1994), “To some extent, the category of complex traits is all-inclusive. Even the simplest genetic disease is complex, when looked at closely.”
Xu et al. (the Orkin lab and collaborators) took note of the evidence that hemoglobin F (HbF) expression varies as a quantitative trait in adults and of the findings recent findings indicating that three polymorphic loci exerted major influence on HbF expression (Menzel et al., 2007; Uda et al., 2008; Lettre et al., 2008). Orkin and colleagues reasoned that a molecule involved in regulating fetal hemoglobin expression might serve as a useful target for pharmacologic manipulation. They decided to focus on one these three loci cited above, the locus encoding BCL11A, a zinc-finger protein that controls the switch from embryonic to adult beta-globin expression in the mouse and that mediates the inhibition of HbF expression from human beta-globin transgenes in mouse fetal liver and in primary human eryhtroid cells in culture.
The authors first demonstrated that erythroid-specific knockout of the BCL11A locus in mice carrying a yeast artificial chromosome containing the human beta-globin gene cluster (which includes the locus encoding the beta-like chain of fetal hemoglobin) increased HbF expression without substantially perturbing erythropoiesis in either fetal liver or adult bone marrow. Expression patterns for multiple transcription factors relevant to erythroid development were comparable for BCL11A-null and wild-type erythroid cells. Blood counts in BCL11A-deficient and wild-type mice were comparable except for a lower number of B cells in the BCL11A-deficient mice, suggesting a role for BCL11A in lymphopoiesis. Elimination of expression of BCL11A also enhanced the effects of inhibitors of DNA methylation and histone deacetylation on increasing HbF expression.
Xu et al. then assessed the effects of introducing the BCL11A-null alleles into a strain of mice that serves as a model of sickle cell disease, referred to as “Berkeley” SCD mice. In these mice, elimination of BCL11A expression increased HbF expression dramatically and corrected numerous hematologic parameters, including red cell counts and Hb content, greatly improved red cell survival, normalized renal function, and reversed histopathological manifestations in lung, liver, spleen, and kidney. Similar results were obtained upon introducing the BCL11A-null alleles into a second strain of mice that serves as a model of sickle cell disease.
The authors are realistic about the challenges involved in transforming these exciting findings into clinical therapies, but they point to recent findings in various fields that improve the prospects for ultimate success. They conclude by offering alternative therapeutic approaches that might be explored, inlcuding inhibition of BCL11A mRNA function by siRNA or shRNA and inhibition of BCL11A function by small molecule drugs or peptides. It will be interesting to determine if the effect on B cell development of eliminating BCL11A production, noted above, represents a significant obstacle to exploiting this approach to therapy of sickle cell disease. If in the future an effective therapeutic is developed by building on the findings of Xu et al., this treatment will represent the transformation of the fruits of human evolution (i.e. genetic variation) into useful medicine.
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