Successful HSCT Slows Brain Volume Loss in Adults With Sickle Cell Disease

Protecting the Brain in Sickle Cell Disease: The Role of Stem Cell Transplant

Sickle cell disease (SCD) is a monogenic blood disorder characterized by chronic pain, organ damage, and a reduced life expectancy, with significant neurological morbidities affecting patients across their lifespan [1]. These complications can manifest as brain atrophy and white matter hyperintensities, contributing to cognitive decline even without overt stroke [2]. Hematopoietic stem cell transplantation (HSCT) is recognized as a curative treatment for SCD, offering the potential to alleviate disease burden and improve quality of life [3, 4]. However, HSCT involves conditioning regimens that carry risks, especially for adults with pre-existing organ damage [5, 6]. The emergence of gene therapies, including CRISPR-Cas9 editing, also offers promising avenues for reactivating fetal hemoglobin synthesis and potentially correcting the underlying genetic defect, further expanding curative options [7, 8, 9]. A new study now offers fresh insights into the neurological benefits of successful HSCT in this vulnerable population [2].

Understanding Neurological Risks in SCD

Adults with sickle cell disease (SCD) face a significant risk of decline in brain health and cognition, even in the absence of overt clinical stroke. This neurological vulnerability is often reflected in specific changes observable on magnetic resonance imaging (MRI). These SCD-related MRI findings include brain atrophy, which is a reduction in brain tissue volume, and T2 fluid attenuated inversion recovery (FLAIR) white matter hyperintensity (WMH) growth, indicating areas of increased water content in the white matter that can signify damage.

Hematopoietic stem cell transplant (HSCT) offers a potentially curative treatment for SCD. However, the conditioning regimens required for HSCT carry inherent risks of new complications, particularly for adult patients who may have accumulated organ damage from their disease over time. Given these considerations, the researchers in this cohort study specifically investigated whether successful HSCT in adults with SCD was associated with a reduction in brain atrophy or a slowing of WMH growth, aiming to understand the neurological impact of this therapeutic intervention.

Study Design and Methodology

This cohort study investigated the neurological impact of hematopoietic stem cell transplant (HSCT) in adults with severe sickle cell disease (SCD). Participants, comprising 77 individuals with a mean age of 32 ± 9 years and 43% female, underwent HSCT as part of five distinct research protocols conducted at the National Institutes of Health Clinical Center. These protocols encompassed various HSCT approaches, including autologous gene therapy, transplantation with HLA-matched donor hematopoietic stem cells (HSCs), and haploidentical HSCs. Participants were followed for a median of 5.3 years, with an interquartile range (IQR) of 4.6 years. The specific clinical trials contributing data to this study included ClinicalTrials.gov NCT02140554 (registered 2014-05-14, first enrollment February 2015), NCT00061568 (registered 2004-07-16, first enrollment July 2004), NCT02105766 (registered 2014-04-01, first enrollment April 2014), NCT03077542 (registered 2017-03-10, first enrollment April 2017), and NCT00977691 (registered 2009-12-14, first enrollment December 2009).

To assess brain health, serial brain magnetic resonance imaging (MRI) scans were acquired both before and after transplant, with follow-up extending for up to 12 years. The MRI protocol included 3D T1-weighted and axial fluid attenuated inversion recovery (FLAIR) sequences. Longitudinal brain volumes were meticulously segmented using the FreeSurfer software package and subsequently corrected for total intracranial volume (TICV), with brain parenchymal fraction (BPF) serving as the primary measure of overall brain volume. White matter hyperintensity (WMH) volumes were segmented from the FLAIR sequences using a specialized deep learning algorithm. All measured volumes were then log2-transformed and expressed as a percentage of TICV. The researchers employed mixed-effects linear regression to determine whether HSCT success, defined as persistent engraftment versus graft failure, or the change in hemoglobin (Hb) levels (post-transplant minus pre-transplant) was associated with the rate of brain atrophy or WMH growth. The statistical models incorporated random effects for scanner, participant intercept, and slope, while fixed effects included age and sex.

HSCT Success and Brain Volume Preservation

The study's primary findings centered on the impact of hematopoietic stem cell transplant (HSCT) success on brain parenchymal fraction (BPF), a measure of overall brain volume. Among the 65 participants who achieved successful HSCT with persistent engraftment, the rate of BPF decline was found to be -0.055% per year (95% CI -0.12, 0.006), which was not statistically significant (p = 0.08). This indicates a relative preservation of brain volume in individuals where the transplant was successful.

In stark contrast, the 12 participants who experienced graft failure demonstrated a substantially accelerated rate of brain volume loss. This group exhibited a 7-fold greater BPF decline, averaging -0.42% per year (95% CI -0.57, -0.26), a statistically significant rate (p < 0.001). The difference in BPF decline between the successful HSCT group and the graft failure group was also highly significant, with a group difference p < 0.001, underscoring the protective effect of successful engraftment on brain volume. Furthermore, the researchers observed a direct relationship between the change in hemoglobin (Hb) levels after HSCT and the rate of BPF decline: a smaller increase or even a decrease in Hb after transplant was associated with a steeper decline in BPF, specifically 0.035% per year per unit Hb (95% CI 0.017, 0.053), with a significant interaction (p < 0.001).

Impact on White Matter Hyperintensities

While successful hematopoietic stem cell transplant (HSCT) demonstrated a clear association with preserved brain volume, its effect on white matter hyperintensities (WMHs) differed. The researchers found that WMH growth was similar between participants with successful HSCT and those who experienced graft failure. Specifically, the annual WMH growth rate was 0.038% per year in the successful HSCT group and 0.030% per year in the graft failure group, with an interaction p = 0.50, indicating no statistically significant difference between the groups. This suggests that HSCT success, defined by persistent engraftment, did not significantly alter the progression of WMHs.

Further analysis revealed that WMH growth was associated with other factors, namely older age and the presence of baseline WMHs, but it was not associated with treatment success of the HSCT. Therefore, while successful HSCT was associated with preserved brain volume, it was not associated with WMH growth. These findings highlight a differential impact of successful HSCT on distinct biomarkers of brain health in adults with sickle cell disease, suggesting that the mechanisms influencing brain parenchymal fraction and white matter hyperintensity progression may be distinct.

Clinical Implications and Future Directions

In summary, this cohort study provides Class III evidence that in adults with sickle cell disease, successful hematopoietic stem cell transplant (HSCT) is associated with slower declines in brain parenchymal fraction (BPF) over a median period of 5 years, when compared with patients who experience transplant failure. This finding suggests a potential mechanism by which successful HSCT may mitigate the progressive brain volume loss often observed in adults with sickle cell disease, offering a tangible benefit for brain health in this population.

While the study clearly demonstrates an association between successful HSCT and preserved brain volume, the researchers emphasize that whether these HSCT-related brain structural changes translate to cognitive preservation remains to be determined. This critical unanswered question highlights an important area for future research, as understanding the functional impact of these structural benefits is essential for fully assessing the long-term clinical utility of HSCT in preventing neurological decline in adults with sickle cell disease.

References

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