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SAMD9L-related familial MDS
ground (Table 2). An additional somatic SAMD9L muta- tion p.R1188X (co-occurring in cis with p.V1512M) was found in a minor clonal fraction of approximately 6%.
In contrast, P2 and P7 showed an unexpected clinical course with spontaneous hematologic recovery, disap- pearing monosomy 7 and the presence of a large double wild-type UPD7q clone in the bone marrow (Figures 4 and 5). Both patients remained healthy, had normal follow-up bone marrow examinations with no signs of dysplasia (Table 1), and normal compete blood counts until their last follow-ups, 20 (P1) and 16 (P7) years after initial diagnosis.
SAMD9L mutations inhibit cell proliferation
An inhibitory effect on cell proliferation reported for SAMD9L mutants overexpressed in 293FT cells in vitro was termed as gain-of-function. In contrast, ectopic expression of p.T233N was shown to mitigate cell prolif- eration to a lesser extent in comparison to wild-type SAMD9L, and was categorized as a disease-protective or loss-of-function variant.20 SAMD9L p.R986H was previ- ously functionally studied and shown to be gain-of-func- tion,20 while p.H880Q was shown to induce loss of het- erozygosity by -7/del(7q) or UPD7q in an Epstein-Barr virus-transformed cell line in vitro.19 To determine the effect of the SAMD9L mutation identified in families I and III, we transiently transfected 293FT cells with vec- tors containing disease-associated mutations p.V1512M and p.R986C respectively, along with the loss-of-function variant p.T233N. Cell proliferation was assessed in dye dilution assays. Both SAMD9L p.V1512M and p.R986C decreased dye dilution in comparison to wild-type SAMD9L and p.T233N (Figure 6A,B), pointing to an
amplified growth restrictive effect of the disease-associat- ed variants (Figure 6B).
Discussion
In this study, we describe a familial MDS syndrome caused by heterozygous missense mutations in the SAMD9L gene located on chromosome 7q21. We present seven individuals from four unrelated pedigrees who developed MDS/-7 from the age of 1 to 42 years without any neurological involvement. Unlike other reported SAMD9L mutation carriers,19,20 the sole clinical manifesta- tion in our cases was hematologic. Other novel findings outlined here are the description of the somatic mutation- al landscape likely contributing to the progression of MDS, the observation of transient monosomy 7, and final- ly the occurrence of non-random revertant mosaicism leading to complete hematologic recovery.
The SAMD9L mutations p.R986H, p.R986C, and p.V1512M identified in this cohort affect evolutionarily highly conserved amino acid residues and are assessed as pathogenic by in silico prediction. The mutation p.H880Q (P7) shows a weak conservation score; however, this mutation had already been reported as causative for the ataxia-pancytopenia phenotype.19 We were not able to test SAMD9L genetics in P7, however the unclear ataxia that this patient had been evaluated for at his last visit points to a carrier status and indicates that there must be an over- lap between sole hematologic and ataxia phenotypes in SAMD9L disease. Summarizing all SAMD9L mutations recently reported or identified in our cohort, a total of six
Figure 3. Mechanisms of clonal escape from SAMD9L germline mutations. Multiple mechanisms of clonal escape from damaging germline missense SAMD9L mutations are observed and lead to complete (monosomy 7) or partial (dele- tion 7q) loss of chromosome 7 with decreasing mutant SAMD9L allele (red cir- cles), both situations can lead to MDS development; UPD7q and truncating somatic SAMD9L mutations (green cir- cles), which have a benign outcome and contribute to normal hematopoiesis. Multiple clonal outcomes can occur in a single patient.
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