Homozygosity for
alpha 1-antitrypsin deficiency, usually of the genotype PIZZ, is one of the more common
single gene defects in infants of European origin, occurring in about 1 in 2000 to 1 in 7000 of the newborn population. About 17% of such infants present with
neonatal hepatitis and a small number with intracranial haemorrhage thought to be caused by
vitamin K deficiency associated with
cholestasis. At least 3% of PIZZ infants will die of
cirrhosis in later childhood unless successfully treated by
liver transplant. The pathogenesis of the
liver disease is not understood and this is unsatisfactory both for treatment and for genetic counselling. The locus coding for
alpha 1-antitrypsin (alpha 1AT) is designated PI for
proteinase inhibitor. Careful study of the genotypes at this locus in
neonatal disease shows that the only certain association is with the homozygous PIZZ genotype. The mutation results in a normal rate of synthesis of a
polypeptide that becomes entrapped in the endoplasmic reticulum of the hepatocyte. Some other factor (or factors), as yet unidentified, determines whether severe liver damage results. The low level of alpha 1AT in the plasma seems unlikely to be the primary cause of damage but may play a secondary role. There is some evidence that the other factor(s) may be familial since in one study, though not in all, a high correlation for severity of
liver disease was found between PIZZ siblings. The heterogeneity of the
clinical course does not result from heterogeneity of PIZ alleles and there is no evidence that it is determined by variation in other related genes on chromosome 14. Only two possible clues have emerged so far. There is some evidence of a protective effect of breast-feeding, and a recent study has found the HLA class II DR3
antigen to be more common than expected in children with
alpha 1-antitrypsin deficiency and
liver disease. Accumulation of alpha 1AT
protein in the hepatocytes may predispose them to some unidentified alteration of the immune response. It is possible that lack of
antiprotease activity in the plasma might exacerbate the original damage, so the possibility of useful
therapy with alpha 1AT cannot be ruled out entirely. At present, there is no valid way of predicting the severity of disease in a PIZZ child; hence, it is common for parents of a severely affected child to wish to terminate any future PIZZ pregnancy. The most direct method to diagnose the PIZZ genotype of a chorion villus sample is by allele-specific hybridization or sequencing of amplified
DNA.(ABSTRACT TRUNCATED AT 400 WORDS)