Targeted conditioning to maximize prenatal HSC engraftment for SCD

PROJECT ABSTRACT:
Treatment of genetic disorders by in utero transplantation (IUTx) has safely been performed for decades in
humans. The first IUTx cure, in the US, used hematopoietic stem cells (HSC) and corrected a child with X-SCID.
Since this groundbreaking moment >25 yrs. ago, >50 patients have now been treated with this procedure for 14
different genetic disorders. However, for reasons that are still not well understood, full therapeutic success has
only been achieved in X-SCID patients. Thus, a better understanding of the mechanisms by which HSC
engraftment is hindered after IUTx is required, so that strategies can be developed to achieve therapeutic levels
of HSC engraftment in other genetic disorders, such as hemoglobinopathies, that could benefit from IUTx. We
and others have identified several characteristics of the developing fetus that may negatively impact its ability to
serve as an amenable HSC recipient. Among these factors are competition from highly proliferative host HSC,
more significant fetal immune barriers than initially known, and the degree of maturity and receptivity of nascent
BM niches required for engraftment of donor (adult) HSC. Here, using fetal sheep as a model, we propose to:
(Aim1) define the nature of, and overcome, the barriers to engraftment by using non-genotoxic conditioning to
dissect the role that niche availability, host HSC competition, and fetal immunity play in the engraftment of adult
donor HSC following IUTx, and (Aim 2) determine the impact of the phenotype and functionality of the donor
HSC on the levels of engraftment following IUTx. We hope that, upon completion of these first 2 Aims, we will
not only have identified the mechanisms involved in resistance to HSC engraftment, but we will also have
achieved a minimum target of 20-25% HSC engraftment, which would allow IUTX to become a viable therapeutic
approach for hemoglobinopathies. Among these, sickle cell disease (SCD) is the most common inherited blood
disorder in the US, and one of the diseases that could benefit from IUTx, since even though the fetus is protected
from sickling by the presence of fetal hemoglobin (Hb), clinical manifestations of SCD start during early infancy,
placing the child at risk of complications such as stroke, splenic crisis, pain episodes, life-threatening infections,
and episodes of acute chest syndrome, which can cause permanent lung damage. Of direct relevance to SCD,
sheep exhibit the same developmental pattern of fetal to adult Hb switching as humans. Recently, using CRISPR/
Cas editing and subsequent somatic cell nuclear transfer, we produced SCD sheep with a disease phenotype
mirroring that of human patients, displaying sickled cells in blood smears, positive Hb solubility test, and HbS
detected by Hb electrophoresis. In Aim 3, we propose to validate the sheep SCD model by monitoring the
animals over time, determining the stressors that induce sickle cell crises, and defining acute and chronic disease
complications. In addition, we will test the therapeutic efficacy of IUTx for treating/curing SCD. Upon completion,
we hope these studies will contribute to the development of novel strategies to achieve curative levels of HSC
engraftment after IUTX and will validate a highly clinically relevant model for the SCD community in general.

Grant Number: 5R01HL165247-03
NIH Institute/Center: NIH
Principal Investigator: Graca Almeida-Porada