Two Hematological Pre-Malignancies In a Pair Of Homozygous Twins Revealing Differences In Their Pre- and Postnatal Development
Authors: Hokland Peter , Roug Anne Stidsholt , Nederby Line , Kjeldsen Eigil , Villesen Palle , Nyvold Charlotte Guldborg 
Published: October 2019

Abstract
Prior studies have established that ALL can be present at birth. However, such inborn hematological malignancies are rare and contrast to the far more frequent hematological (pre-) malignancies, which are typically found in patients in their sixties or later. Thus, the point mutation V617F in the JAK2 gene in myeloid cells has been found to be a significant player for many chronic myeloproliferations, e.g. polycythemia vera (PCV). In fact, its prevalence in the elderly population is as high as 0.1%. In addition, studies in B-cell derived chronic lymphocytic leukemia (B-CLL) has found that this malignancy is preceded by a clinically silent phase of monoclonal B-cell lymphocytosis (MBL) in the majority of patients. In contrast to leukemic childhood twins, where a high (10-15%) concordance is thought to reflect the inborn initiating mutations, much less is known about the balance between genetics and environment for the pre-leukemic conditions mentioned above.
This report relates to a pair of homozygous twins, who came to our attention when they were 73 years old in 2009. Twin A presented at a hematological department for the first time in 2005 at the age of 70 years. He had experienced a 1 h episode of amaurosis fugax and during a neurological workup mild thrombocytopenia (126x109/L), increased hematocrit (0.52) and lymphocytosis (7.95x109/L) without abnormalities in other leukocyte subsets. Immunophenotyping at this time revealed CD5/CD19/CD23+ monoclonal B-cells at the 7-8 109/L levels, but no lymphadenopathy or organomegaly. While no therapy for the latter cells has been instituted, venesectio 3-4 times /year has been deemed necessary to keep the hematocrit below 0.45. Twin B went for a checkup at his GP subsequent to the brother experiencing the above episode. He, too, was diagnosed with lymphocytosis, but his hematocrit was only slightly increased levels. No intervention has been instituted in this twin. While the extent of lymphocytosis in twin A is consistently between 5 and 10 bill/L, that of twin B remains below 5 bill/L. Remarkably, the B-cell light chain expression differed between these identical twins, with twin A being kappa- and twin B lambda positive. High-resolution arrayCGH on blood sample revealed that twin A had microdeletions on chromosome 2p11.2 involving the Ig kappa gene cluster, on chromosome 7p11.2 involving the processed pseudogene ENST00000448242, chromosome 13 involving the RB gene, and on chromosome 17 involving the azacytidine induced gene. In contrast, twin B exhibited no clear aberrations by arrayCGH. For both twins, we have performed whole exome sequencing using FACS purified T-lymphocytes as surrogate germline cells and compared to whole the BM MNC exome in order to identify mutated genes. This analysis revealed (Figure) that twin A predominantly displayed mutations, which have been related to myeloid cells in the literature, while twin B mainly exhibited alterations in genes linked to lymphoid cells. In fact, only the three genes, NTSR2, ATP2C2 and ACOX3, were altered in the BM cells and present on both twins, neither of these readily explaining the shared monoclonal B-cell disorders. Interestingly, mutations in the genes TET2 and RUNX1, which have been closely related to myeloid cells, were solely found in twin A.
In conclusion, we have described a pair of homozygous twins who in their seventies were diagnosed with benign monoclonal B-cell lymphocytosis (MBL) with different B-cell light chain expression. Additionally, one twin, but not the other, was found to be JAK2+ with clear polycythemia vera (PCV) features requiring frequent phlebotomies. Extensive molecular analyses including whole exome sequencing have revealed that while both twins harbored point mutation in NTSR2, ATP2C2 and ACOX3, the mutations displayed in twin A with PCV were predominantly genetic abnormalities that have been associated with the myeloid cells underlining that extensive alterations in myelopoiesis have taken place postnatally in this twin. Thus, for these two pre-leukemic conditions, while predisposition to MBL seems to be prenatal, that related to JAK2 is postnatal. Ongoing studies are aimed at discovering a link between the 3 shared genes and B-cell development in an effort to link them to BML development.

Disclosures:
No relevant conflicts of interest to declare.