Mucocutaneous Engraftment and Type VII Collagen (C7) Replacement after Allogeneic Hematopoietic Cell Transplantation (HCT) in Patients with Recessive Dystrophic Epidermolysis Bullosa (RDEB)
Authors: Tolar Jakub , McGrath John A , Osborn Mark J. , Keene Douglas R , Hook Kristen , Hordinsky Maria , Woodley David , Chen Mei , Hovnanian Alain , Tamai Katsuto , Blazar Bruce R. , Wagner John E 
Published: October 2019

Abstract
In RDEB-an incurable, often fatal, blistering genodermatosis-loss-of-function mutations in the COL7A1 gene result in diminished or absent C7, lack of attachment between the epidermal basement membrane and the dermal matrix, and a poor quality of life and early death as a result of chronic mucocutaneous blistering.
In 2010, we reported the results of a 'first-in-human' study on the use of HCT in 6 patients with RDEB, suggesting that increased C7 expression and mucocutaneous wound healing paralleled integumentary donor cell engraftment (NEJM 363:629, 2010). However, myeloablative conditioning (MAC) resulted in high incidence of renal, pulmonary and hepatic complications, as well as opportunistic infections due to chemotherapy-induced mucocutaneous toxicity. Therefore, subsequent patients with RDEB received reduced intensity conditioning (RIC) and the addition of bone marrow (BM)-derived mesenchymal stem/stromal cells (MSC) in an attempt to reduce the conditioning-related toxicities and enhance tissue recovery.
After 2010, 20 patients aged 0.4-20 years with generalized severe RDEB underwent allogeneic HCT at the University of Minnesota. Of these, 7 were treated with MAC (busulfan IV 0.8 mg/kg for >12 kg and 1.1 mg/kg for those <12 kg of body weight q6h on days -9 to -6, with pharmacokinetic targeting 1,000 mmol/min for the area under curve; fludarabine [FLU] IV 25 mg/m2/day on days -5 to -3, and cyclophosphamide [CY] IV 50 mg/kg/day on days -5 to -2). Subsequently, 13 were treated with RIC (CY IV 50 mg/kg/day on day -6, FLU IV 40 mg/m2/day on days -6 to -2, anti-thymocyte globulin IV 30 mg/kg/day on days -4 to -2, and total body irradiation [TBI] 200 cGy or 300 cGy on day -1). Uniform GvHD prophylaxis used was cyclosporine A and mycophenolate mofetil. The hematopoietic graft was HLA-matched related BM (N=10), unrelated BM (N=7) or umbilical cord blood (UCB, N=3). In addition, a single infusion of third-party HLA-mismatched MSC (dose: 2 x 106/kg) was administered on day 0 (7 with MAC; 13 with RIC).
The probability of 2-year survival was 90% (95% CI, 47-99%) in recipients of RIC as compared to 69% (95% CI, 37-87%) in recipients of MAC (p=0.3). 3 individuals (2 MAC; 1 RIC) died within one year after treatment as a result of veno-occlusive disease (N=1) and fungal sepsis (N=2), and one patient treated with MAC died after one year from extensive chronic GvHD. Interestingly, acute GVHD was rare (overall grade 2, N=2). There were no statistically significant survival differences in comparison of related vs. unrelated grafts, BM vs. UCB grafts, and those who received additional MSC vs. those who did not. Complete hematopoietic chimerism was achieved in all patients after MAC; however, RIC was associated with partial engraftment-mean 31% (range, 0-93%) in the CD15 compartment and 55% (range, 25-100%) in the CD3 compartment by day 100 after HCT. Wild-type levels of C7 in the skin believed to be sufficient to protect from extensive blistering are 10-30%. The skin chimerism was a mean 37% (range, 0-97%) after MAC in contrast to 13% (range, 0-47%) after RIC with a trend to slightly higher proportion in recipients of TBI 300cGy relative to 200cGy TBI. The primary endpoint of the analysis was survival with objective evidence of reduced mucocutaneous blistering. When compared to MAC, RIC was better tolerated with reduced risks of end organ toxicities and infections: cumulative incidence was 63% (95% CI, 34-92%) in recipients of RIC as compared to 92% (95% CI, 65-100%) in recipients of MAC, (p=0.06). For the entire cohort of engrafted, surviving subjects with RDEB thus far (N=18 of 26), 14 individuals demonstrated partial to marked biochemical and clinical improvement in mucocutaneous disease as evidenced by >50% reduction in affected body surface area, and/or C7 expression; example from RIC-treated individual is shown in Figure 1 (nuclei are blue, epidermis [top] and dermis [bottom of each panel] with C7 expression [red] at the dermal-epidermal junction).
These results suggest that [1] RIC is associated with less toxicity and potentially improved survival; [2] HCT can result in improved expression of C7 protein, with reduced mucocutaneous blistering characteristic of generalized severe RDEB; and [3] these observations set the stage for exploratory studies with hematopoietic cell therapy for tissue repair and regeneration in extramedullary sites, extending their use into treatment of a broad spectrum of non-hematologic disorders.
Figure 1. Figure 1.

Disclosures
No relevant conflicts of interest to declare.