Am. J. Respir. Cell Mol. Biol., Vol 16, No. 3, 03 1997, 225-231.
alpha1-antitrypsin gene mutation hot spot associated with the formation of a retained and degraded null variant
M Brantly, JH Lee, J Hildeshiem, CS Uhm, UB Prakash, BA Staats and RG Crystal
Pulmonary-Critical Care Medicine Branch, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892-1590, USA.
Null alpha1-antitrypsin (alpha1AT) alleles represent the end of a continuum
of variants associated with profound alpha1AT deficiency and an increased
risk of emphysema. This study characterizes the molecular basis of
QOclayton, a new example of an alpha1AT null allele arising from a
mutational hot spot in the alpha1AT gene. The QOclayton allele is identical
to the normal M1(V213) alpha1AT allele except for an insertion of a
cytosine. This insertion occurs in the alpha1AT sequence which normally has
seven cytosines corresponding to amino acid residues 360 to 362. The
QOclayton mutation is located in the same reiterated DNA sequence as the
alpha1AT QObolton deletion mutation and the insertion mutation allele
QOsaarbruecken. The QOclayton cytosine insertion causes a 3' frameshift and
results in the formation of a termination codon at residue 376, the same
consequence as the alpha1AT QOmattawa mutation (L353 T-insertion with a 3'
frameshift). To determine the molecular mechanisms responsible for the
absence of alpha1AT associated with the QOclayton gene, an in vitro model
of QOclayton was established using Chinese hamster ovary cells (CHO)
transfected with the QOclayton gene. These cells were evaluated for
alpha1AT mRNA expression, protein synthesis and secretion. Although the
QOclayton gene expresses a similar amount of alpha1AT mRNA as compared with
the normal alpha1AT gene, no QOclayton protein is secreted. Protein
trafficking and double-label immunofluorescence demonstrate that the
QOclayton protein is retained in the rough endoplasmic reticulum or
pre-Golgi compartment and is degraded (t1/2 = 6.5 h). Since QOmattawa,
QObolton, and QOsaarbruecken have similar termination sites in the alpha1AT
mRNA, they may share a similar intracellular fate.
