Gas7 Induces The Proliferation Of Ph+ ALL Cells and Prevents The Differentiation Of Early B Cell Progenitors Into CD25high Small Pre-B Cells
Authors: Lee Jaewoong , Buchner Maike , Geng Huimin , Swaminathan Srividya , Park Eugene , Klemm Lars , Lin-Chao Sue , So Chi Wai , Muschen Markus 
Published: October 2019

Abstract

Background
Growth arrest-specific gene 7 (Gas7) first discovered in growth-arrested NIH3T3 cells possesses WW, Fes/CIP4 homology (FCH), and coiled-coil domains, which can act as an adaptor for SH2 or 3-containing proteins. Gas7 is abundantly expressed in the brain and is involved in neuronal differentiation. Recently, Gas7-deficient mouse transiently expressing truncated form of Gas7 mutant protein shows motor activity defects due to reduced motor neuron number (Huang BT et al., PLoS One 2012). Interestingly, MLL-GAS7 resulting from t(11;17)(q23;p13) has been reported in a treat-related acute myeloid leukemia (AML) and in a pediatric acute lymphoblastic leukemia (ALL). However the specific role of Gas7 in an area of hematology has not been determined yet.

Results
We found that Gas7-deficient bone marrow (BM)-derived progenitor B cells, grown in vitro in the presence of interleukin 7 (IL-7), display two distinct populations of CD43highCD25- and CD43lowCD25high representing the small pre-B cells compared to their normal counterparts which show only CD43highCD25- population. As expected, CD43lowCD25high population showed reduced IL7R expression on the cell surface and increased expression of intracellular µHC, Igα and surface Igβ compared to CD43highCD25- population. Consistent with the high expression of CD25, Gas7 deficient B cell progenitors showed significantly increased expression of κ light chains on cell surface with decreased levels of P-AktS473 as well as increased levels of P-Erk T202/Y20, p53 and p21. Moreover, Gas7 mRNA expression was specifically upregulated by >13-fold in pre pro-B (Hardy fraction A) and small pre-B (Hardy fraction D) subsets compared to other subsets of B cell progenitors, suggesting that Gas7 may be involved in the regulation of early B-cell development.
To elucidate the function of Gas7 in Ph+ B cell lineage leukemia, we transformed bone marrow B cell progenitors from Gas7-deficient mice with BCR-ABL1. Gas7 deficient Ph+ ALL cells showed decreased proliferation with reduced S phase and increased apoptosis through the inhibition of Stat5Y694 phosphorylation as well as increased levels of p21. We found that Imatinib-mediated suppression of Stat5Y694 phosphorylation dramatically upregulates the expression of Gas7. In agreement with effects of Stat5 on the sensitivity of Ph+ ALL cells against tyrosine kinase inhibitors (TKIs), Gas7 deficient Ph+ ALL cells showed high susceptibility to Imatinib-induced apoptosis. In addition, absence of Gas7 leads to loss of self-renewal capacity and failure to form colonies in methylcellulose assay.

Conclusions
Here we show that Gas7 may play critical roles in early B-cell development and BCR-ABL1-driven leukemia cell survival. Pathways affected by Gas7 include Stat5, AKT and Erk signaling which is known as a downstream of BCR-ABL1 and as major regulators of B-cell development.

Disclosures:
No relevant conflicts of interest to declare.