LETTER TO THE EDITOR TCR αβ and CD19-depleted haploidentical stem cell transplant with reduced intensity conditioning for Hoyeraal–Hreidarsson syndrome with RTEL1 mutation Bone Marrow Transplantation (2016) 51, 753–754; doi:10.1038/ bmt.2015.352; published online 25 January 2016 The eponym Hoyeraal–Hreidarsson (HH) syndrome was first coined by Aalfs et al. describing similar cases previously reported by Hoyeraal in 1970 and then Hreidarsson in 1988. 1 HH syndrome is a multisystem telomere biology disorder that represents the most severe clinical variant of dyskeratosis congenita (DC). 2 A clinical diagnosis can be made if patients have at least four of the following six features associated with this disorder that includes bone marrow failure, intrauterine growth retardation, microcephaly, developmental delay, cerebellar hypoplasia and immunodeficiency. 3 There is significant genetic overlap between HH syndrome and DC, and the genetic defect can be identified in ~ 60% of patients with HH syndrome. Mutations in genes required for both telomere maintenance and genomic stability such as RTEL1 (regulator of telomere elongation helicase 1) and DKC1 (dyskerin) are classically associated with HH syndrome. Early-onset severe bone marrow failure and immunodeficiency (classically T + B - NK - ) may necessitate urgent stem cell transplantation that can be curative for both marrow failure and immunodeficiency but is unlikely to have any beneficial impact on the other aspects of this disease. 4 Initial attempts at stem cell transplantation using conventional myeloablative conditioning in DC were disappointing due to the severe systemic toxicity of conditioning chemotherapy. Reduced intensity conditioning (RIC) has been a fairly successful strategy with reasonable outcomes. 5–8 Combination of RIC with T-cell depletion from grafts raises the concerns of non-engraftment but successful CD34 + cell selected haploidentical transplant has been recently reported. 9 Novel ex vivo T-cell depletion techniques using TCR αβ and CD19 depletion have many potential advantages. Removal of αβ-T cells by 4–5 logs can reduce the risk of GvHD but at the same time facilitates excellent CD34 + cell recovery while retaining γδ T cells, dendritic cells, natural killer (NK) cells and monocyte/myeloid cells, thereby ensuring early robust engraftment and immune reconstitution. This technique has demonstrated excellent early results in paediatric nonmalignant stem cell transplantation. 10,11 The strategy of combining RIC with T-cell depletion technique using TCR-αβ and CD19 depletion appears attractive in DC patients for whom stem cell transplantation has been challenging. We describe an infant who was born at 34-week gestation with a birth weight of 800 g and a platelet count of 80 × 10 9 /L. The severe growth retardation was initially thought to be due to placental insufficiency. The thrombocytopenia resolved tempora- rily but progressed to severe bone marrow failure by the age of 4 months. Bone marrow aspiration showed a hypocellular marrow with no clonal cytogenetic changes. Quantitative serum Ig estimation was normal but lymphocyte subset analysis showed low NK cell numbers with normal T and B cells. Combination of these features with microcephaly, developmental delay, severe cerebellar hypoplasia on magnetic resonance imaging scan and short telomere length confirmed a clinical diagnosis of HH syndrome. Mutation analyses for known genes (NHP2, NOP10, TERT, TINF2 and WRAP53) associated with DC was negative. Trio whole-exome sequencing (WES) for proband and biological parents was performed through an institutional review board approved research project. DNA was extracted from peripheral leukocytes, and WES was performed on Illumina’s HiSeq 2000 (San Diego, CA, USA) as per the manufacturer’s protocol. The reads were mapped to hg19, and variants were identified using Broad Institute’s GATK and annotated with ANNOVAR. The results were then filtered to include only protein altering variants that were absent or rare (allele frequency o0.01) from dbSNP, 1000 genomes, exome variant server and in-house exome database of Asian individuals, and were consistent with the phenotype and mode of inheritance. WES revealed biallelic mutations in RTEL1 (chr20:62319369G4A; c.1561G4A; p.Asp521Asn and chr20:62319116A4G; c.1474A4G; p.Met492Val). Both these variants were absent from control populations, altered highly conserved amino-acid residue and were predicted to be pathogenic by in silico software (Polyphen-2 (http://genetics. bwh.harvard.edu/pph2/) and SIFT (http://sift.bii.a-star.edu.sg/)). Neither variant has been reported previously. The variant Met492Val was inherited from his mother, whereas the variant Asp521Asn was de novo. The variants were validated by Sanger sequencing. We proceeded with haploidentical transplantation due to severe pancytopenia needing frequent blood product infusions, and the lack of a suitable donor. The transplant was performed at 10 months of age using his father as the stem cell donor. The father was killer Ig-like receptor (KIR) mismatched and of B haplotype on KIR genotyping. RIC chemotherapy with ATG (Thymoglobulin, 2.5 mg/kg per day × 4 days), fludarabine (40 mg/m 2 per day × 4 days) and treosulfan (12 g/m 2 per day × 3 days) was used. GCSF-mobilized peripheral blood stem cells were collected and processed by negative selection for TCR-αβ and CD19. Graft engineering was achieved using the Miltenyi Biotec CliniMACS system (CliniMACS; Miltenyi Biotec, Bergisch Gladbach, Germany). Infused product had CD34+ cells of 36 × 10 6 /kg and TCR-αβ cells 4.5 × 10 4 /kg. No GvHD prophylaxis was used. Rapid platelet engraftment on D+11 and neutrophil engraftment on D+15 were noted. He developed mild mucositis and feeding difficulty needing prolonged nasogastric tube feeding. Apart from one episode of culture negative fever, no major complications were observed in immediate post-transplant period. There were no episodes of peri-transplant viral or fungal infections. He is currently 10 months post transplant with no major infections or GvHD. Day 30, 120 and 245 lymphocyte subset analysis continues to demonstrate excellent immune reconstitution (Figures 1 and 2). Chimerism analyses were performed initially on day 30 from bone marrow and subsequently from peripheral blood. These showed 100% donor chimerism and we did not observe any mixed chimerism. Isolated mild thrombocytopenia in a neonate followed by temporary normalization of platelet count has been described in children with congenital amegakaryocytic thrombocytopenia but is not a well-described feature of DC. Different mutations have been identified in both HH syndrome and DC; however, RTEL1 Bone Marrow Transplantation (2016) 51, 753 – 754 © 2016 Macmillan Publishers Limited All rights reserved 0268-3369/16 www.nature.com/bmt