Molecular Characterization Of a Patient With Thrombocytopenia-Absent Radii (TAR) Syndrome and Diffuse Langerhans Cell Histiocytosis (LCH): Novel Genetic Findings

TAR is a rare bone marrow failure syndrome comprised of thrombocytopenia and a spectrum of bony abnormalities, the most common being bilateral radial aplasia in the presence of thumbs. Recent research has implicated alterations in the RBM8A gene in the pathogenesis of this disorder. Diffuse LCH is a neoplasm of mature Langerhans cells, a subset of dendritic cells, that has now been demonstrated to be clonal in nature. The co-occurrence of these two rare disorders has only been reported twice (Ingram, BMT 2006; Guastadisegni EJMG 2012). We report the third case of a patient with TAR and LCH and present the molecular and genetic characterization of this patient. A 52 year woman with TAR presented with abdominal mass, splenomegaly and left axillary adenopathy. CT scan showed a right pelvic mass measuring 11.5 x 8.6 cm, retroperitoneal lymphadenopathy, enlarged spleen and patchy areas of radiolucency throughout the spine, pelvis and left femoral head. Excisional biopsy of the left axillary lymph node revealed complete effacement of the normal lymph node architecture by an atypical proliferation of Langerhans cell histiocytes. Bone marrow involvement was documented as well. On immunohistochemistry, these cells were positive for CD1a (membrane staining) and S-100 (nuclear and cytoplasmic staining). As part of the diagnostic evaluation, CGH microarray (Signature Genomics) was performed. A diagnosis of diffuse LCH was made. She was treated with vinblastine and prednisone for a year and remains in complete remission from her disease 14 months later. Genomic DNA was extracted from peripheral blood (normal mononuclear cells) and paraffin-embedded tumor tissue, with written informed consent following the approval of the Institutional Review Board of Thomas Jefferson University. The CGH result on the patient's uninvolved tissue as well as the tumor sample showed a hemizygous deletion of 1q21.1 encompassing the RBM8A gene, consistent with recent TAR literature (Albers, Nat Genet 2012). Sequence analysis of the other RBM8A allele revealed minor allele A at SNP rs139428292 at position chr1:145,507,646 (hg19) in the 5'UTR of RBM8A and nt G at chr1:145,507,777 in the first intron (int1 +44). The coding sequence was normal. Minor allele A at the 5'UTR SNP has been associated with TAR syndrome, along with minor allele C at intronic SNP (rs 201779890, int1 +32) which was major allele G in our patient. However, nt G at intron 1 +44 is a novel variant that has not been reported before in dbSNP137, HapMap or 1000 Genomes (UCSC Genome Browser, August 1, 2013). The CGH result on the patient's tumor sample showed a 1.82 Kb deletion within the MAF gene. We identified in normal and tumor DNA one intact MAF gene and one deleted of all of the coding sequences of MAF isoform 1 (alternatively, complete deletion of exon 1 of MAF isoform 2). In addition, a copy loss in the 14q32.2 region was found in the tumor sample but not in the uninvolved tissue. This region contains the BCL11b gene. Finally, we identified a tumor-specific missense mutation (GTG to GAG) resulting in a change in amino acid from valine to glutamine at codon 600. This is the BRAF V600E mutation previously described in LCH and other malignancies.

In summary, we report the third ever reported case of the co-occurrence of two rare disorders, TAR and LCH, in a single patient and identify novel molecular changes. We have found the patient to have the recently reported RBM8A genetic alterations of TAR (microdeletion of 1q21 for one allele, minor 5’UTR SNP on the other allele), plus a novel RBM8A intronic SNP. We found a somatic MAF deletion in our patient, possibly a predisposing factor towards LCH. In addition, BRAF V600E mutation is a tumor-specific mutation in our patient. Whether BRAF inhibitors will be of therapeutic benefit in primary or relapsed LCH is currently unknown. Future research is needed to elucidate the function, if any, of the novel RBM8A SNP, the mechanism(s) by which the identified SNPs lead to reduced Y14 levels, and the potential contribution of Y14 deficiency in tumorigenesis.

The authors wish to acknowledge the invaluable role of Dr. Elena Gitelson in the care of this patient.