Nodal follicular helper T-cell (T <sub>FH</sub>) lymphoma of the angioimmunoblastic (AITL) subtype has a dismal prognosis. Using whole-exome sequencing (n = 124), transcriptomic (n = 78), and methylation (n = 40) analysis, we identified recurrent mutations in known epigenetic drivers (TET2, DNMT3A, IDH2 <sup>R172</sup> ) and novel ones (TET3, KMT2D). TET2, IDH2 <sup>R172</sup> , DNMT3A co-mutated AITLs had poor prognosis (p < 0.0001). Genes regulating T-cell receptor (TCR) signaling (CD28, PLCG1, VAV1, FYN) or activation (RHOA <sup>G17V</sup> ) or regulators of the PI3K-pathway (PIK(3)C members, PTEN, PHLPP1, PHLPP2) were mutated. CD28 mutation/fusion was associated with poor prognosis (p = 0.02). WES of purified, neoplastic T-cell (CD3<sup>+</sup>PD1<sup>+</sup>) demonstrated high concordance with whole tumor biopsies and validated the presence of TET2 and DNMT3A in tumor and non-lymphoid cells, but other mutations (CD28, RHOA <sup>G17V</sup> , IDH2 <sup>R172</sup> , PLCG1) in neoplastic cells. Integrated DNA-methylation and mRNA expression analysis revealed epigenetic alterations in genes regulating TCR, cytokines, PI3K-signaling, and apoptosis. RNA-seq analysis identified fusion transcripts regulating TCR-activation (8%), revealed a restricted TCR-repertoire (α = 87%, β = 72%), and showed the presence of Epstein-Barr virus transcriptome (73%). GEP demonstrated the association of B-cells or dendritic cells in the tumor milieu with prognosis (p < 0.01). RNA-seq and WES analysis of 12 AITL-patient-derived-xenografts (PDX) showed that bi-allelic TET2 and DNMT3A mutations or sub-clonal mutations (PLCG1, PHLPP2) propagated in sequential passages, and gene signatures related to T <sub>FH</sub> and T <sub>CM</sub> (central-memory) were well-maintained through passages. Gene expression signatures associated with late PDX passages (3rd-5th) were enriched with proliferation and metabolic reprogramming-related genes and predicted prognosis in an independent AITL series. Low PHLPP2 mRNA expression predicted poor prognosis (p = 0.05) and engineered PHLPP2 or TET2 loss in CD4<sup>+</sup> T-cells showed enhanced PI(3)K activation, thus uncovering a therapeutic target for clinical trials.