HOMOZYGOSITY FOR 2 POINT MUTATIONS IN THE LIPOPROTEIN-LIPASE (LPL) GENE IN A PATIENT WITH FAMILIAL LPL DEFICIENCY - LPL(ASP(9)-]ASN, TYR(262)-]HIS)

Familial lipoprotein lipase (LPL) deficiency is an inherited disorder of lipoprotein metabolism characterized by hypertriglyceridemia and re current episodes of abdominal pain and pancreatitis. We have studied t he genetic basis of LPL deficiency in a 62-year-old black male with un detectable pre- and post-heparin plasma LPL mass and activity. DNA seq uence analysis of the patient's LPL cDNA and gene as well as digestion with Bcl I and Asu I revealed that the proband is a homozygote for tw o separate gene defects. One mutation changed a G to an A, resulting i n the conversion of amino acid 9 of the mature protein, aspartic acid (GAG), to asparagine (AAC). The second substitution, a C for a T, repl aced tyrosine (TAC) at residue 262 with histidine (CAC). Northern blot analysis of monocyte-derived macrophage RNA demonstrated the presence of LPL mRNA of approximately normal size and quantity when compared t o control. Expression of both mutations separately (pCMV-9 and pCMV-26 2) or in combination (pCMV-9+262) in human embryonal kidney-293 cells demonstrated that LPL-9 had approximately 80% the specific activity of wild type LPI,. but LPL-262 and LPL-9+262 had no enzymic activity, th us establishing the functional significance of the LPL-262 defect. Des pite an absolute deficiency of LPL mass and activity demonstrated by a nalysis of patient post-heparin plasma, in vitro expression of both LP L mutants was normal, suggesting that the absence of LPL in patient po st-heparin plasma was a result of altered in vivo processing. Analysis of the heparin binding properties of the mutant enzymes by heparin-Se pharose affinity chromatography indicated that most of the LPL-262 mas s was present in an inactive peak, which like the normal LPL monomer, eluted at 0.8 M NaCl. Thus, the Tyr(262) --> His mutation may alter th e stability of the LPL dimer, leading to the formation of inactive LPL -262 monomer which exhibits reduced heparin affinity. Based on these r esults, we propose that, in vivo, enhanced formation of LPL-9+262 mono mer leads to abnormal binding of the mutant lipase to endothelial glyc osaminoglycans ultimately resulting in enhanced catabolism of the muta nt enzyme and lower enzyme mass in post-heparin plasma.