Surfactant metabolism in lung tissue deficient in surfactant protein B (SP-B)

Inherited deficiency of SP-B causes fatal respiratory failure in term infants. The most common mutation is a 2 bp insertion (121ins2) resulting in a reading frame shift and introduction of a premature translation stop codon. Surfactant of these infants is not surface active and has reduced content of phosphatidylglycerol (PG). We have studied the synthesis of surfactant components in lung tissue of a 121ins2 infant who was the recipient (R) of a donor (D) lung transplant at 10 wk of age. Tissue was cultured as explants in serum-free Waymouth's medium under air/5% CO₂. The rates of ³⁵S methionine incorporation into protein by R and D tissues were similar on both days 1 and 5 of culture and tissue viability was confirmed by histology. SP-B protein was abundant in D tissue but was not detected by Western analysis in R tissue, confirming SP-B deficiency diagnosed in lavage fluid. SP-B mRNA in R preculture lung was 8% of that for D tissue whereas there were comparable levels of mRNA for SP-A, SP-C and β-actin. The transcription rates for all three SPs, normalized to β-actin, were ∼2-fold greater for R tissue vs. D, and the relative rates for SP-A/SP-B/SP-C were comparable in the two lungs. Immunoprecipitation of ³⁵Smet-labelled protein with anti-NPROSP-C antibody showed appearance of precursor SP-C proteins (21 & 24 kDa) in both tissues and persistence of a 6-9 kDa form in R lung. Incorporation of labelled precursors into total phospholipid and % phosphatidylcholine were comparable in the two lungs but there was a ∼2-fold greater % incorporation of both glycerol and acetate into PG and a ∼2-fold decrease into phosphatidylinositol (PI) of R tissue. We conclude that the marked decrease in SP-B mRNA in infants with the 121ins2 mutation is the result of mRNA instability rather than decreased transcription rate. The absence of SP-B protein and lamellar bodies and the abnormal processing of SP-C do not affect phosphatidylcholine synthesis but increase the PG/PI synthetic ratio.