Mandibuloacral dysplasia type A (MADA; OMIM # 248370) is a premature ageing disease caused by the homozygous R527H mutation in the LMNA gene. At the cellular level, MADA is characterized by unprocessed prelamin A accumulation, nuclear architecture alterations, chromatin defects and increased incidence of apoptosis. In some progeroid laminopathies (e.g., HGPS) it has been demonstrated that such biochemical and morphological alterations are strongly linked with genomic instability. To test this also in MADA fibroblasts, their response to the ionising radiation-induced damage was analysed. We observed that their ability to repair the damage was significantly impaired, as demonstrated by the increased chromosome damage and the higher percentage of residual gamma-H2AX foci, corresponding to unrepaired DNA-damage sites. Moreover, MADA fibroblasts showed a markedly reduced phosphorylation of p53 at Ser15(S15) and a lower induction of p53 and CDKN1A proteins after irradiation, compared to the control cell line. Upon irradiation, we also detected differences in the expression of some p53 downstream target genes. In addition, MADA cells showed partial defects in the checkpoint response, particularly in G(1)/S transition. Our results indicate that accumulation of the lamin A precursor protein determines a defect in DNA damage response after X-ray exposure, supporting a crucial role of lamin A in regulating DNA repair process and cell cycle control.