HOMOZYGOUS MISSENSE MUTATION (BAND-3 FUKUOKA-G130R) - A MILD FORM OF HEREDITARY SPHEROCYTOSIS WITH NEAR-NORMAL BAND-3 CONTENT AND MINIMAL CHANGES OF MEMBRANE ULTRASTRUCTURE DESPITE MODERATE PROTEIN-4.2 DEFICIENCY

The characteristics of phenotypic expression were studied in a Japanes e family with hereditary spherocytosis and an extremely rare homozygou s missense mutation of the band 3 gene (band 3 Fukuoka: G130R). The ho mozygous unsplenectomized proband was a 29-year-old male with compensa ted haemolytic anaemia (red cell count 4.21 x 10(12)/l, reticulocytes 278 x 10(9)/l, and indirect bilirubin 44 mu mol/l). His red cell band 3 (B3) protein demonstrated a 9.3% reduction and his protein 4.2 (P4.2 ) level was substantially reduced (45.0%), compared to normal subjects , P4.2 protein was composed mostly of a wild type (72 kD) with a trace of 68 kD peptide. The binding properties of the mutated B3 to normal P4.2 were significantly impaired, which probably resulted in the subst antial reduction of P4.2 in this proband, since no abnormalities were detected on the P4.2 gene. Electron microscopy (EM) using the freeze-f racture method demonstrated a mild decrease in intramembrane particles (IMPs) of near-normal size (8 nm in diameter) with no substantial inc reases in their oligomerization, Their distribution on the membrane P face was almost normal, although most of the IMPs could represent the homozygously mutated B3 protein. EM (quick-freeze deep-etching method) disclosed a skeletal network of near-normal size and size distributio n of the skeletal units, suggesting that the mutated B3 protein itself did not have much effect on the skeletal network. in situ, Therefore the reduced P4.2 content (45% of that of normal subjects), which remai ned on the red cell membrane of this proband, appeared to be nearly su fficient for maintaining the normal structure of the skeletal network and IMPs in situ, contrary to the marked abnormalities in both IMPs an d the skeletal network in complete P4.2 deficiencies.