Ataxia-Telangiectasia (A-T) is a recessive hereditary syndrome characterized by cerebellar degeneration, telangiectasia, precocious aging, immunodeficiency, cancer predisposition and insulin-resistant diabetes. A-T is caused by defects in Ataxia-Telagiectasia Mutated (Atm) gene. Atm encodes a ser/thr kinase (ATM) of the PI3 kinase family that plays a crucial role in the DNA damage response (DDR). Although some A-T features are easily explained by defects in DDR, others, like precocious aging, insulin-resistant diabetes and the recently described whole chromosome instability, are not. We, thus, searched for possible DDR-independent roles of ATM. To this end, we analyzed human cells in which ATM was chemically or genetically downregulated in the absence of any DNA damaging insult. We observed that ATM downregulation induced abnormal mitotic figures. The phenotypes ranged from abnormal mitotic spindles that were displaced from the cell centre, indicating loss of cortical interaction of astral microtubules, to spindles with extra poles that slowly coalesced to form pseudo-bipolar spindles. Often the extra poles lacked the centrosome marker gamma-tubulin. These alterations resembled that of previously described mitotic phenotypes induced by genetic knock-down of the poly(ADP-ribosyl)-polymerase Tankyrase 1 (TNKS1). The mitotic phenotypes have been ascribed to reduced TNKS1-dependent poly(ADP-ribosyl)ation of the Nuclear and Mitotic Apparatus protein 1 (NuMA1). Indeed, we found that ATM down-regulation impaired mitotic poly(ADP-ribosyl)ation of NuMA1. We further observed that ATM physically interacted with TNKS1. ATM activity was required for TNKS1-dependent poly(ADP-ribosyl)ation of NuMa1, perhaps through ATM-dependent phosphorylation of NuMa1, but not for ATM-TNKS1 interaction. Previous observations have shown that TNKS1 controls telomere length by (poly(ADP-ribosyl)ating TRF1 and glucose uptake by regulating insulin-dependent transport into the plasma-membrane of the glucose transporter Glut-4. We propose that the ATM-dependent control of TNKS1 function, independently of DDR, could explain several A-T features and shed light on new therapeutic approaches for the A-T syndrome.