Response of Patients with Transfusion-Dependent β-Thalassemia (TDT) to Betibeglogene Autotemcel (beti-cel; LentiGlobin for β-Thalassemia) Gene Therapy Based on HBB Genotype and Disease Genetic Modifiers

Introduction

We investigated the impact of β-thalassemia genotypes and disease genetic modifiers including HBA and KLF1 genotype and sentinel single-nucleotide polymorphism (SNP) genotypes at 3 major HbF quantitative trait loci (QTL) on clinical outcomes of TDT patients treated with beti-cel gene therapy in two phase 3 studies, HGB-207 (NCT02906202) and HGB-212 (NCT03207009).

Methods

HBA deletions and triplications were determined by gap-polymerase chain reactions. HBG2 (including rs7482144, Xmn1 site) and HBG1 promoters, HBA2, HBA1, and KLF1 underwent individual nucleotide sequencing. Multiplex amplification refractory mutation system (ARMS) tests were used to identify HbF QTL SNPs (rs10128556 in HBBP1; rs766432, rs1427407, rs10189857 in BCL11A; rs9399137, rs66650371 in HMIP). Thalassemia severity score (TSS) was calculated as defined by Danjou et al, Haematologica, 2015, considering gender, HBB and HBA genotypes, and 4 SNPs in HbF QTL (HBG2, BCL11A, HMIP). Correlative analyses were performed to assess relationships between genotype, presence/absence of non-HBB mutations (HBA2, HBA1, KLF1), presence/absence of HbF QTL SNPs (HBG2, BCL11A, HMIP), and TSS with the achievement of transfusion independence (TI; weighted average hemoglobin [Hb] ≥9 g/dL without red blood cell [RBC] transfusions for ≥12 months). Correlation coefficients used percentage bend correlation. Statistical significance threshold was p ≤ 0.05.

Results

As of 3 March 2020, 38 patients were treated in HGB-207 and HGB-212 (β⁰/β⁰ genotype n=9; non-β⁰/β⁰ genotype n=29 [β⁺/β⁺ n=8; β⁰/β⁺ n=15; β^E/β⁰ n=6]). All patients were heterozygous or homozygous for mutations or SNPs that may modulate disease severity; 20 patients were homozygous for ≥1 mutation or SNP. Patients had the following alleles associated with higher HbF synthesis: HBG2 rs7482144 C>T (Xmn1 site), C/T n=9, T/T n=1; BCL11A rs1427407 G>T, G/T n=8; BCL11A rs10189857 A>G, A/G n=16, G/G n=18; HMIP rs9399137 T>C, T/C n=9, C/C n=2. Three patients were heterozygous for single α-globin gene deletion (-α/αα) and 2 were heterozygous for α-globin gene triplication (αα/ααα). Median TSS was 3.65 (min - max 0.4 - 8.1).

TI was achieved by 23/27 (85%) evaluable patients; 4 patients with ≥ 12 months follow-up have been transfusion free for > 10 months but were not yet evaluable for TI (Figure). β-thalassemia genotype did not strongly correlate with TI (two-sided Fisher's Exact Test, p-value = 0.78). Month 12 median (min - max) peripheral blood vector copy number (PB VCN) was 1.5 (0.2 - 5.0) c/dg in TI or transfusion-free patients (n=27) and 0.2 (0.2 - 0.4) c/dg in patients who did not achieve TI (n=4). The transfusion-free patient (β⁰/β⁰) with the lowest month 12 PB VCN was homozygous for T/T at rs7482144 (HBG2Xmn1 site) and G/G at rs10189857 (BCL11A), and heterozygous for single α-globin gene deletion. Endogenous Hb (5.9 g/dL HbF + 0.2 g/dL HbA₂) and gene therapy-derived HbA^T87Q (4.4 g/dL) at month 12 enabled this patient to stop transfusions.

Tests of association of SNPs and mutations with TI were not significant; no p-value < 0.31 (chi-squared test). As only 4 patients did not achieve TI, the power to detect an association was limited. Larger sample sizes are needed to determine if individual SNPs and mutations may have an impact on TI.

In TI or transfusion-free patients (n=27), TSS correlated strongly with month 12 endogenous unsupported Hb (HbA + HbA₂ + HbF + HbE without RBC transfusions for 60 days) (correlation coeff. = -0.76, p < 0.0001), but not with HbA^T87Q (correlation coeff. = 0.26, p = 0.19) or unsupported total Hb (correlation coeff. = 0.32, p = 0.10).

Beti-cel-related adverse events (AE) in >1 patient were abdominal pain (n=2 non-β⁰/β⁰; n=1 β⁰/β⁰), thrombocytopenia (n=3 non-b⁰/b⁰). Serious AEs in ≥3 patients post-infusion were thrombocytopenia (n=2 non-β⁰/β⁰; n=1 β⁰/β⁰), pyrexia (n=1 non-b⁰/b⁰; n=2 β⁰/β⁰), veno-occlusive disease (n=3 non-β⁰/β⁰).

Summary

Genetic characterization of TDT patients treated with beti-cel revealed diverse HBB and non-HBB mutations and polymorphisms that may influence disease severity. Higher PB VCN and HbA^T87Q levels were associated with increased likelihood of TI. In instances of lower HbA^T87Q, higher endogenous HbF might be a determinant in whether TI is achieved. Despite genetic heterogeneity, beti-cel enabled patients to achieve TI regardless of β-thalassemia genotype, TSS, disease genetic modifiers including HBA, and HbF QTL SNP genotypes.

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