Identification of the SBDS Gene Mutation in Japanese Patients with Bone Marrow Failure Syndrome

Abstract

Shwachman-Diamond syndrome (SDS) which is caused by mutations in both alleles of the SBDS gene, is an inherited bone marrow failure syndrome (BMFS) typified by pancreatic exocrine insufficiency, skeletal defects, and an elevated risk for developing hematologic malignancy. The functional consequences of SBDS mutation have not yet been characterized, although short telomeres of leukocytes have been observed in affected patients. About one third of patients with aplastic anemia (AA) show significant reductions in telomere length. Several groups, including our own, have recently reported that some of the patients of acquired AA or myelodysplasia (MDS) with telomere shortening may be explained by mutations of the telomerase complex genes. However the responsible genes have not yet been identified in most of the patients of BMFS with telomere shortening. A recent study showed a heterozygous mutation of SBDS, 258+2T>C, in 4 out of 91 patients with apparently acquired AA (Blood . 2007; 110: 1141). Therefore, we carried out an investigation to determine whether mutations in SBDS are associated with the disease in our cohort of Japanese BMFS patients. We analyzed mutation of SBDS gene among 110 BMFS patients with acquired AA (n=38) or with MDS (RA) (n=72) diagnosed between 1993 to 2005 at the Nippon Medical School. We identified the 258+2 T>C mutation at intron 2 in an AA patient only, which was absent in 200 controls. This mutation found in SDS patients, is thought to disrupt the donor splice site in intron 2, engaging a cryptic upstream splice site at positions 251 to 252 and leading to protein truncation at codon 84 by frameshift. Previous study reported that the 4 AA patients with heterozygous for SBDS 258+2T>C mutation were younger than other AA patients without mutation (Blood . 2007; 110: 1141). However our patient with the mutation was a 64 yr-old female. She was clinically diagnosed with mild AA, and the medical history, family history, and the physical examination were not suggestive of an inherited disease. We observed this patient without treatment, because her hematological data was not severe, and blood transfusion was not necessary for her. Our study sowed that she was not found to have TERC and TERT mutation. These sequence polymorphisms were identified in both intronic and exonic regions of the gene. IVS2-70T/C, IVS2-71G/A, and 141C>T were identified at similar allele frequencies in patients and controls. Calado et al showed 201A>G in 13 out of 91 patients and 38 out of 276 controls (Blood . 2007; 110: 1141). However, we didn't identify 201A>G among our patients, as observed by previous Japanese population studies (Haematologica . 2007; 92: 1573). The incidence of 201A>G is much lower in Japanese population compared with that in western countries. Subsequently, we compared the length of telomeres of mononuclear cells using southern blot in the AA patient with heterozygous for SBDS 258+2T>C mutation with age-much controls. The AA patient with heterozygous for SBDS 258+2T>C mutation didn't show shorter telomere than age-much controls. In patients with acquired AA heterozygous for SBDS mutation, it was reported that granulocytes' telomeres were short via a telomerase-independent pathway, but lymphocytes' telomeres were in the normal range. Since this is an archival case, we could not examine the length of telomere of each leukocyte subsets from this patient. We speculated the reason of this patient not showing shorting telomere that lymphocytes were dominant in MNC of this patient's peripheral blood. Finally, our data presented here are consistent with the idea that SBDS mutation contributes to the pathogenesis of a subset of BMFS in a subset of these patients. The largest controlled epidemiologic studies reported that the incidence of AA in the Western countries is 2 per million per year comparing the 2- to 3-fold higher rate in Asia. However, our and another study (Haematologica . 2007; 92: 1573) showed that the mutational frequencies of SBDS gene among Japanese BMFS patients were lower than that for other ethnic groups and that this genetic difference could not explain the higher incidence of the disease in Asian populations.