Germline and Somatic Genetic Characterization of Shwachman-Diamond Syndrome

The North American Shwachman-Diamond Syndrome Registry (SDSR) opened in 2008 to improve our understanding of the natural history of Shwachman-Diamond Syndrome (SDS), improve medical outcomes, and facilitate research. The diagnosis of SDS was defined by either biallelic SBDS gene mutations or by the clinical combination of exocrine pancreatic dysfunction with bone marrow failure. Median age of study subjects is 11.2 years (range, 0.6-52.8). SBDS genetic reports were available for 168 subjects. Eighty-one had biallelic SBDS mutations, while 48 individuals lacked SBDS mutations. Ongoing characterization of SBDS mutation-negative individuals has identified subgroups of SDS individuals meeting clinical diagnostic criteria as outlined above, as well as a more heterogenous subgroup with features of SDS but in whom a clinical diagnosis could not be confirmed by classic diagnostic criteria. Amongst those with biallelic SBDS mutations, cytopenias were noted in all but 1 subject. Intermittent neutropenia was noted in 97% (n=70/72) and was the most frequent hematologic abnormality. Anemia was noted in 53% (n=39/73) and thrombocytopenia in 64% (n=48/75). Congenital anomalies were seen in 50% (n=39/78).

The mutational spectrum of SBDS was explored. In 80 of 81 patients harboring biallelic SBDS mutations, the c.258+2 T>C intron 2 splice donor mutation was found in at least one of the two mutant SBDS alleles. In these cases, the mutation spectrum of the second mutant SBDS allele included missense mutations, splice site mutations, and truncating mutations. The one patient lacking the intron 2 splice donor mutation had a c.183_184delTAinsCT mutation together with a c.523 C>T mutation in SBDS confirmed to be in trans. This results in the combination of a truncating p.Lys62X mutation with a p.R175W missense mutation at a conserved residue predicted to be deleterious. This patient presented with neonatal severe aplastic anemia requiring platelet and red cell transfusions with neutrophil counts of 0-200 unresponsive to G-CSF. The marrow showed irregular islands of cartilage surrounded by osteoid consistent with a disorder of bone formation. A low level of SBDS protein is expressed by the c.258+2T>C variant, so the absence of this hypomorphic allele may have contributed to this exceptionally severe phenotype.

Bone marrow reports were available for 67 subjects with biallelic SBDSmutations. Marrow hypocellularity was noted in 79% (n=49/62). Mild morphologic marrow dysplasia was observed in 58% (n=35/60). Clonal abnormalities developed in 36%. The most common clonal abnormality was del20q in 16% (n=10/64). Isochromosome 7 was noted in 2% (n=1/64). Three individuals developed AML at ages 19.5, 38, and 39 years. Eleven (14%) have undergone hematopoietic stem cell transplantation (HSCT), 10 for MDS or AML and 1 for severe aplastic anemia.

Given the frequency of del20q clones in SDS, we studied clinical features of the 10 SDSR subjects with del20q clones. Median age of this group was 17 years (range, 10.8-29.3). Congenital anomalies were noted in 80% (n=8/10). Frequency of cytopenias was similar to that of non-del20q subjects, with neutropenia, anemia and thrombocytopenia seen in 90%, 50%, and 80% respectively. Bone marrow pathology reports were available for 9 subjects. All were noted to have hypocellular marrows as well as mild marrow dysplasia. In many patients, the clone was persistent or grew over time, with longest duration of 14.6 years. Progression to MDS was reported in 3 subjects who initially had an isolated del20q clone. Two developed an additional loss of chromosome 7. Progressive marrow dysplasia and falling blood counts were seen in two subjects. All three were treated with HSCT at 5.4, 5.7 and 7 years. Deletion of 20q in SDS has been hypothesized to result in a milder hematologic phenotype due to compensatory deletion of eIF6. eIF6 binds to the nascent 60S ribosomal subunit and sterically inhibits joining of the 60S to the 40S ribosomal subunits. SBDS functions to facilitate release of eIF6 thus promoting assembly of the mature 80S ribosome. These data from the SDSR suggest that SDS patients with del20q clones remain at risk for clonal evolution. Higher patient numbers are needed to quantitate risk of MDS in SDS patients with del20q.

The SDSR continues to expand and mature as a resource for biological and clinical studies in this rare disorder advancing our understanding of marrow failure and clonal evolution.