The North American Shwachman-Diamond Syndrome Registry: Genetically Undefined Shwachman-Diamond Syndrome

Shwachman-Diamond syndrome (SDS) is an inherited marrow failure syndrome associated with exocrine pancreatic dysfunction and an increased risk of myelodysplasia and leukemia. The majority of individuals with SDS carry biallelic SBDS gene mutations, however a subset of patients remain genetically undefined.  The objective of this study was to compare the clinical characteristics of patients with and without SBDS mutations. To address these questions, we conducted a retrospective study of patients enrolled on the North American Shwachman-Diamond Syndrome Registry (SDSR). Clinical data from the SDSR were available for 55 individuals with biallelic SBDS mutations and 16 individuals who fulfilled clinical diagnostic criteria for SDS but lacked biallelic mutations in the SBDS gene.  Study subject ages for SBDS mutation positive and negative cohorts span 2-52.4 and 2.8-21.4 years with median ages of 12.4 and 10.9 years respectively.  Cytopenias were present for both SBDS mutation positive and negative cohorts, with neutropenia the most common event in 94% and 81% respectively.  Bone marrow hypocellularity was reported in 91% of those with SBDS mutations and 69% of those without.  Marrow dysplasia was reported in 65% of those with SBDS mutations and none of those without.  Clonal abnormalities were present in 44% and 25% of those with and without SBDS mutations with median age of initial appearance at 9 years (0.8-45.1) and 7 years (1.2-14) respectively. Abnormalities included del7q and del20q in both groups as well as iso7q, trisomy 8 and others in the SBDS mutation positive group. Clonal abnormalities were all transient in the SBDS mutation negative cohort. One SBDS mutation positive individual developed AML. None of the SBDS mutation negative individuals developed malignancy or progressed to require HSCT thus far. Pancreatic dysfunction determined by low serum trypsinogen or pancreatic isoamylase was similar in both cohorts 79% vs 80%.  However, only 27% (15/55) of SBDS mutation positive individuals reported requiring enzyme therapy with 33% (18/55) documenting failure to thrive, in contrast to 75% (12/16) of SBDS mutation negative individuals with 73% (11/15) having failure to thrive.  A broad spectrum of congenital anomalies were reported in 55% and 56% of SBDS mutation positive and negative individuals respectively, with skeletal anomalies being the most common in both groups.  Medical comorbidities commonly reported in both groups included eczema and endocrinopathies.  Elevated liver transaminases were seen in 27% of SBDS mutation positive individuals but this was not seen in the SBDS mutation  negative cohort. Conclusion:  Patients with genetically undefined (SBDS mutation negative) SDS share clinical characteristics with SBDS mutation positive patients; however, the risk of leukemia in the genetically undefined patients remains unclear due to low patient numbers with short follow-up.  Further studies of this young cohort are required to inform medical management and to advance our understanding of genetic etiology, mechanism, disease pathophysiology and treatment of these marrow failure disorders.