A complex phenotype in a child with familial HDL deficiency due to a novel frameshift mutation in APOA1 gene (apoA-IGuastalla).

•A boy with hypertriglyceridemia, metabolic syndrome, and HDL-deficiency is described.•No mutations were found in LPL, APOC2, APOA5, GPIHBP1, and LMF1 genes.•The patient carries a novel APOA1 mutation producing a truncated protein.•The truncated apoA-I was detectable in plasma in minute amounts.•Patient's apoB-depleted serum has greatly reduced cholesterol efflux capacity.