Hereditary persistence of fetal hemoglobin (HPFH) is a benign condition defined as an increased proportion of fetal hemoglobin which persists beyond infancy. HPFH has been identified in a wide range of ethnic groups but is more common in populations with a high prevalence of hemoglobinopathies and thalassemias. This is thought to be due to selective pressure resulting from the ameliorating effect which HPFH can have in some of these disorders. Consequently, although it can occur as a single entity it is often observed as a co-existing condition in a variety of hemoglobin disorders. Although several well established genetic factors have been identified that can cause the HPFH phenotype, they only account for <50% of the Hb F variance observed in adults. More recently, mutations in the erythroid transcription gene KLF1 have been found to be associated with a HPFH phenotype in a single family from Malta (Borg et.al. Nat Genet. 2010; 42(9): 801–805) and in a single family from Sardinia (Satta et.al. Haematologica 2010; 96(5):767–70).
We therefore decided to investigate whether KLF1 mutations could be associated with increased Hb F levels observed in patients referred to our laboratory for hemoglobinopathy investigation. 130 adult patient samples with an elevated Hb F level above 1.5% (range 1.5 – 25%) were studied for KLF1 mutations. 55 of these samples had co-existing alpha thalassemia trait, 6 were carriers for sickle cell trait, 28 were beta thalassemia carriers and 41 had no evidence of any other hemoglobinopathy. The results showed that 8% of the cohort had a mutation in the KLF1 gene. 10 different mutations which were predicted to be deleterious (PolyPhen-2 and SIFT) were identified in 10 patients, 8 of which are previously unreported (c.159_169 del GAAGTCTGAGG, c.901C→T, c.902G→A, c.939G→T, c.1001C→A, c.913+1G→A, c.526_527Ins CGGCGCC, c.152T→G). No deleterious KLF1 mutations were identified in a matched cohort of 101 samples referred for hemoglobinopathy investigation that had normal Hb F levels (<1.0%), strongly suggesting that the KLF1 mutations are associated with a HPFH phenotype.
In addition, 35 sickle cell disease patients with Hb F levels greater than 10% were investigated. One further unreported deleterious KLF1 mutation (c.914–4_914–1 del CTAG) was found in a sickle cell disease patient with a Hb F level of 20.3%. Interestingly this patient appeared to have a particularly mild phenotype and maintained a hemoglobin level of 12.7g/dl.
The KLF1 mutations were predominantly found in individuals of African, Indian and Southeast Asian descent, indicating that KLF1 mutations could be a widespread cause of HPFH in malarial regions where hemogobinopathies are common, possibly making a significant contribution to Hb F variance in these populations. All the mutations identified were heterozygous, with only one case of compound heterozygosity, suggesting that haploinsufficiency for KLF1 may be sufficient to increase Hb F levels. This in turn provides further in-vivo evidence that controlled reduction of KLF1 gene expression could activate fetal hemoglobin expression and provide a potential therapeutic target.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.