Positive regulation of apoptosis by HCA66, a new Apaf-1 interacting protein, and its putative role in the physiopathology of NF1 microdeletion syndrome patients

Cell Death Differ. 2007 Jun;14(6):1222-33. doi: 10.1038/sj.cdd.4402122. Epub 2007 Mar 23.


As a component of the apoptosome, a caspase-activating complex, Apaf-1 plays a central role in the mitochondrial caspase activation pathway of apoptosis. We report here the identification of a novel Apaf-1 interacting protein, hepatocellular carcinoma antigen 66 (HCA66) that is able to modulate selectively Apaf-1-dependent apoptosis through its direct association with the CED4 domain of Apaf-1. Expression of HCA66 was able to potentiate Apaf-1, but not receptor-mediated apoptosis, by increasing downstream caspase activity following cytochrome c release from the mitochondria. Conversely, cells depleted of HCA66 were severely impaired for apoptosome-dependent apoptosis. Interestingly, expression of the Apaf-1-interacting domain of HCA66 had the opposite effect of the full-length protein, interfering with the Apaf-1 apoptotic pathway. Using a cell-free system, we showed that reduction of HCA66 expression was associated with a diminished amount of caspase-9 in the apoptosome, resulting in a lower ability of the apoptosome to activate caspase-3. HCA66 maps to chromosome 17q11.2 and is among the genes heterozygously deleted in neurofibromatosis type 1 (NF1) microdeletion syndrome patients. These patients often have a distinct phenotype compared to other NF1 patients, including a more severe tumour burden. Our results suggest that reduced expression of HCA66, owing to haploinsufficiency of HCA66 gene, could render NF1 microdeleted patients-derived cells less susceptible to apoptosis.

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Antigens, Neoplasm / genetics
  • Antigens, Neoplasm / metabolism
  • Antigens, Neoplasm / physiology*
  • Apoptosis / physiology*
  • Apoptotic Protease-Activating Factor 1 / genetics
  • Apoptotic Protease-Activating Factor 1 / metabolism
  • Carrier Proteins / genetics
  • Carrier Proteins / metabolism*
  • Caspase 3 / genetics
  • Caspase 3 / metabolism
  • Caspase 9 / genetics
  • Caspase 9 / metabolism
  • Cell Line
  • Cells, Cultured
  • Chromatography, Gel
  • Gene Deletion
  • HeLa Cells
  • Humans
  • Immunoblotting
  • Immunoprecipitation
  • Mice
  • Molecular Sequence Data
  • Neurofibromatosis 1 / genetics
  • Neurofibromatosis 1 / pathology
  • Neurofibromatosis 1 / physiopathology*
  • Neurofibromin 1 / genetics*
  • RNA, Small Interfering / genetics
  • RNA-Binding Proteins
  • Reverse Transcriptase Polymerase Chain Reaction
  • Sequence Homology, Amino Acid
  • Syndrome
  • Transfection


  • APAF1 protein, human
  • Antigens, Neoplasm
  • Apoptotic Protease-Activating Factor 1
  • Carrier Proteins
  • Neurofibromin 1
  • RNA, Small Interfering
  • RNA-Binding Proteins
  • UTP6 protein, human
  • Caspase 3
  • Caspase 9