(Received for publication, November 8, 1994; and in revised form, December 2, 1994)
From the
Synthesis of soluble A, B, H, and Lewis b blood group antigens
in humans is determined by the Secretor (Se) (FUT2) blood group locus. Genetic, biochemical, and molecular
analyses indicate that this locus corresponds to an
(1,2)fucosyltransferase gene distinct from the genetically-linked H blood group
(1,2)fucosyltransferase locus. The
accompanying paper (Rouquier, S., Lowe, J. B., Kelly, R. J., Fertitta,
A. L., Lennon, G. G., and Giorgi, D.(1995) J. Biol. Chem. 270,
4632-4639) describes the molecular cloning and mapping of two
human DNA segments that are physically linked to, and cross-hybridize
with, the H locus. We present here an analysis of these two
new DNA segments. One of these, termed Sec1, is a pseudogene, because
translational frameshifts and termination codons interrupt potential
open reading frames that would otherwise share primary sequence
similarity with the H
(1,2)fucosyltransferase. The other
DNA segment, termed Sec2, predicts a 332-amino acid-long polypeptide,
and a longer isoform, that share 68% sequence identity with the
COOH-terminal 292 residues of the human H blood group
(1,2)fucosyltransferase. Sec2 encodes an
(1,2)fucosyltransferase with catalytic properties that mirror
those ascribed to the Secretor locus-encoded
(1,2)fucosyltransferase. Approximately 20% of randomly-selected
individuals were found to be apparently homozygous for an
enzyme-inactivating nonsense allele (Trp
ter) at
this locus, in correspondence to the frequency of the non-secretor
phenotype in most human populations. Furthermore, each of six unrelated
non-secretor individuals are also apparently homozygous for this null
allele. These results indicate that Sec2 corresponds to the human Secretor blood group locus (FUT2) and indicate that
homozygosity for a common nonsense allele is responsible for the
non-secretor phenotype in many non-secretor individuals.
The ABO blood group antigens consist of oligosaccharides
synthesized by the sequential action of glycosyltransferases. While
these molecules were classically defined as polymorphic red cell
antigens, subsequent studies demonstrated that soluble forms of these
molecules may be found in the saliva and in other secretions in some,
although not all, humans (reviewed in (1, 2, 3) ). The ability to elaborate soluble
A, B, and H antigens is determined by the Secretor blood group
locus. Homozygosity for null alleles at this locus occurs in
approximately 20% of most populations (4) and yields the
non-secretory phenotype, characterized by absence of normal amounts of
soluble A, B, and H substance in the saliva(1) . By contrast,
most humans maintain the secretor phenotype and elaborate soluble blood
group substance in amounts easily detectable by standard
hemagglutination-inhibition assays(5, 6) . The
penultimate step in the pathway leading to synthesis of soluble and
membrane-associated A and B antigens is catalyzed by
(1,2)fucosyltransferases(
)(1, 2, 3) .
These enzymes form a precursor oligosaccharide substrate (Fuc
(1,2)Gal
-) termed the H antigen, which is an essential
substrate for the final step in the pathway, catalyzed by allelic
glycosyltransferases encoded by the ABO locus(1) .
Genetic (7, 8, 9, 10) and
biochemical (10, 11, 12, 13) analyses indicate
that the H blood group locus represents an
(1,2)fucosyltransferase gene expressed in the erythroid lineage
and determines expression of the H antigen (along with A and/or B
antigens) on red cells. By contrast, these studies are consistent with
a hypothesis (7) that the Secretor locus corresponds
to a closely linked but distinct
(1,2)fucosyltransferase gene
whose expression is restricted to secretory epithelial cells in the
salivary glands, gastrointestinal tract, and elsewhere, where it
controls expression of soluble H antigen (and thus A or B antigen
synthesis) in saliva and other secretions (reviewed in (3) ).
In the accompanying paper(14) , we report the isolation of
two human DNA segments that represent candidates for the Secretor (1,2)fucosyltransferase locus because they cross-hybridize
with the H blood group
(1,2)fucosyltransferase gene and
are in close physical proximity to this locus on chromosome 19. We
report here the results of our structural and functional analysis of
these two newly identified sequences. One of these, termed Sec1,
represents a pseudogene. The other sequence, termed Sec2, encodes a
polypeptide of 332 amino acids in length (and an isoform that is longer
by 11 NH
-terminal amino acids) that in turn functions as an
(1,2)fucosyltransferase with catalytic properties corresponding to
those assigned previously to the Secretor locus-encoded
(1,2)fucosyltransferase. We furthermore find that each of six
unrelated non-secretor individuals maintains homozygosity for an allele
at this locus which contains an enzyme-inactivating translational
termination codon corresponding to amino acid residue 143. 10 of 52
(19%) randomly chosen individuals were determined to be homozygous for
the mutant allele, in close correspondence to the frequency of the
non-secretor phenotype in most populations. These results confirm that
this newly described
(1,2)fucosyltransferase gene is the FUT2 gene corresponding to the human Secretor blood group
locus and provide molecular confirmation of the hypothesis (7) that the H and Secretor loci correspond
to two distinct
(1,2)fucosyltransferase genes.
COS-7 cells were grown in Dulbecco's
modified Eagle's medium supplemented with 10% fetal calf serum
and transfected with expression vectors using a DEAE-dextran procedure,
all as described previously(8, 9) . A control plasmid
(pCDM7-CAT; (9) ), encoding bacterial chloramphenicol
acetyltransferase, was co-transfected to allow normalization for
transfection efficiency. Cell extracts were prepared 72 h after
transfection and were used in (1,2)fucosyltransferase assays as
described below. An aliquot of the cell culture medium was also
subjected to assay for
(1,2)fucosyltransferase activity. Cell
extracts were also subjected to chloramphenicol acetyltransferase
activity assays(17) .
For pH
optimum determination, reactions were buffered with either 25 mM sodium acetate (pH 4.5-6.0), 25 mM sodium phosphate
(pH 5.5-7.5), or 25 mM Tris-HCl (pH 7.0-9.0),
using concentrated solutions of these buffers previously titrated to
the various pH values. The final pH of each reaction was directly
determined with a micro pH electrode as described
previously(19) . Phenyl--D-galactoside was used
as the acceptor in these assays. These experiments were completed using
the enzyme derived from the pcDNAI-
(1,2)FTSe-short
vector.
In assays to determine the apparent K values, the concentration of the acceptors was varied as follows:
phenyl-
-D-galactoside, 0-170 mM;
lacto-N-biose I, 0-10 mM; N-acetyllactosamine, 0-12.5 mM. These assays
were performed in 25 mM sodium phosphate, pH 6.0, using 3
µM GDP-[
C]fucose. To determine the
apparent K
value for GDP-fucose, 3 µM GDP-[
C]fucose was supplemented with
different amounts of unlabeled GDP-fucose (19) to achieve final
GDP-fucose concentrations that ranged from 3 µM to 400
µM. The GDP-fucose K
determination
was completed in reactions containing 25 mM phenyl-
-D-galactoside and buffered with 25 mM sodium phosphate, pH 6.0. Apparent Michaelis constants were
derived from Lineweaver-Burk plots of substrate concentration-rate
determinations. These experiments were completed using the enzyme
derived from the pcDNAI-
(1,2)FTSe-short vector.
Molecular cloning and mapping studies described in the accompanying paper (14) identify two cross-hybridizing human sequences, termed Sec1 and Sec2, that represent candidates for the human Secretor blood group locus. The cross-hybridizing portions of each of these sequences were subcloned, sequenced, and tested for function, in order to determine if either represent the Secretor blood group fucosyltransferase locus.
Figure 1:
Comparison between the DNA and derived
protein sequences of the H blood group locus and the Sec1
sequence. The DNA and derived protein sequences for the Sec1 DNA
segment and the H gene (8, 9) are aligned to
maximize DNA sequence identity using the GCG program
Align(23) . Amino acid sequence residue numbers are indicated
at the left and right of the sequence listings.
Nucleotide sequences are numbered in a similar manner, assigning the A
residue of the translation initiation codon as position number 1.
Nucleotide sequence identity is denoted by a vertical line between
the two sequences. Dots denote gaps introduced to
maximize sequence identity between the DNA or protein sequences. The
derived protein sequences are displayed above (for Sec1) or below (for H) the corresponding DNA sequence. The
reading frame used to predict the maximally similar Sec1-derived amino
acid sequence is indicated at left by a, b,
or c. Amino acid residues within the Sec1-derived protein
sequence that are identical with corresponding residues in the H (1,2)fucosyltransferase sequence are indicated by a dash in the Sec1-derived amino acid sequence. Amino acid residues
within putative membrane-spanning hydrophobic segments in each amino
acid sequence are doubly underlined. A methionine codon that
yields an open reading frame with amino acid sequence similarity to the H
(1,2)fucosyltransferase is singly underlined,
as is the initiator methionine in the H
(1,2)fucosyltransferase. Potential asparagine-linked
glycosylation sites are indicated by dotted underlining. The
frameshift and nonsense mutations that must be conceptually suppressed
to maintain amino acid sequence similarity between the protein sequence
predicted by the Sec1 DNA sequence are indicated above that
sequence.
Figure 2:
Sequence and predicted structure of the
enzyme encoded by Sec2. A, comparison between the DNA and
derived protein sequences of the H blood group locus and the
Sec2 sequence. The DNA and derived protein sequences for the Sec2 and H genes are aligned to maximize DNA sequence identity, using
the GCG program Align(23) . Amino acid sequence residue numbers
are indicated at the left and right of the sequence
listings. Nucleotide sequences are numbered in a similar manner,
assigning the A residue of the translation initiation codon of the
shortest predicted polypeptide (indicated by the downward pointing
arrow) as position number 1. Nucleotide sequence corresponding to
coding sequence is displayed by capital letters, whereas
sequence predicted to correspond to nontranslated regions is denoted by lower case letters. Nucleotide sequence identity is denoted by
a vertical line between the two sequences. Gaps introduced to
maximize sequence identity between the DNA sequences are indicated by dotted lines. The derived protein sequences are displayed above (for Sec2) or below (for H) the corresponding
DNA sequence. Amino acid residues within the Sec2-derived sequence that
are identical with corresponding residues in the H (1,2)fucosyltransferase sequence are indicated by the
symbol above the Sec2-encoded residue. Amino acid residues within
putative membrane-spanning hydrophobic segments in each protein are doubly underlined. The methionine codon that may initiate the
longest of two potential Sec2-encoded polypeptides is singly
underlined, as is the initiator codon for the H gene.
This predicted protein is 11 amino acid residues longer (indicated in lower case letters) than the shorter Sec2-encoded
(1,2)fucosyltransferase (see text for details). The (second)
methionine codon tentatively assigned as an initiator codon for this
latter protein is singly underlined, and the corresponding
methionine residue is indicated by an arrow. Potential
asparagine-linked glycosylation sites are indicated by dotted
underlining. The polymorphic DNA sequence residues that either
inactivate the enzyme (Trp
ter), or that are
functionally neutral (Gly
Ser), are underlined (wild type nucleotide sequence is shown). The corresponding amino
acid residue is indicated in bold type. The sequence of the
alternative allele and its corresponding protein sequence change are
indicated above these positions. Protein sequence-neutral DNA sequence
polymorphisms are also indicated (wild type nucleotide sequence is
shown). The sequence of the alternative allele is indicated above these positions. The singly underlined section of the
Sec2 sequence 3` to the coding region corresponds exactly to the DNA
sequence of the partial cDNA derived from this locus and reported in
the companion paper(14) . B, schematic diagram of the
(1,2)fucosyltransferase encoded by the Sec2 sequence. Proposed
domain structure, above, and hydropathy plot(26) , below, for the 332-amino acid polypeptide predicted by the
Sec2 DNA sequence. The relative positions of the potential
asparagine-linked glycosylation sites (
),
the inactivating nonsense mutation (Trp
stop), and
transmembrane segment (&cjs2108;) are indicated on the rectangular
schematic representation of the fucosyltransferase. Hydrophobic regions
within the predicted protein are indicated by the shading above the
horizontal axis of the hydropathy plot. The predicted
intracellular locations of the NH
-terminal (Cytosol), hydrophobic transmembrane (T.M.), and
catalytic (Golgi Lumen) domains are indicated with their amino
acid lengths. The potential 11-amino acid-long NH
-terminal
extension is indicated by the dotted rectangle appended to the
shorter Sec2-encoded
(1,2)fucosyltransferase. The positions of the AgeI and PstI restriction sites and synthetic (see
``Materials and Methods'') AgeI and EcoRI
sites (in parentheses) used to create various expression
vectors are also displayed.
Translation from
these putative initiator codons predicts the synthesis of either a
332-amino acid-long polypeptide, or a 343-amino acid-long polypeptide,
each of which shares 68% sequence identity with the human
(1,2)fucosyltransferase protein sequence, across 292 corresponding
amino acid residues distal to the conserved alanine residue (Fig. 2). The NH
terminus of the protein(s) consists
of 3 residues (or 14 residues) that precede a 14-residue hydrophobic
segment. This hydrophobic segment is flanked by charged residues and is
predicted to function as a signal-anchor sequence (Fig. 2), in a
motif that corresponds to the type 2 transmembrane topology typical of
mammalian glycosyltransferases(24) . By analogy to these
enzymes, it is predicted that the 315 residues that comprise the
COOH-terminal catalytic domain reside within the lumen of the Golgi
apparatus. Three potential asparagine-linked glycosylation sites are
present in this COOH-terminal domain; two of these sites are at
positions corresponding to the two potential asparagine-linked
glycosylation sites previously identified in the H
(1,2)fucosyltransferase (Fig. 2). Sequence analysis
also identifies a region of sequence identity between a region 3` to
the predicted termination codon in the Sec2 sequence and the
hybrid-selected cDNA derived from human small intestinal tissue
described in the accompanying manuscript (14) (Fig. 2).
This confirms data reported in the accompanying manuscript indicating
that the Sec2 sequence is transcribed in the small bowel and suggests
that the Sec2 sequence encodes an
(1,2)fucosyltransferase gene
expressed in some gastrointestinal epithelial cells.
Figure 3:
Catalytic properties of the Sec2-encoded
(1,2)fucosyltransferase activity. A, effect of pH on
(1,2)fucosyltransferase activity. Enzyme activity encoded by the
Sec2 segment in transfected COS-7 cell extracts was measured using 25
mM phenyl-
-D-galactoside and 3 µM GDP-[
C]fucose, as described under
``Materials and Methods.'' Buffers used in assays at various
pH values are indicated by the symbols in the boxed
legend. The pH values displayed here were determined by measuring
the pH of the assay solution, as described under ``Materials and
Methods.'' B, apparent Michaelis constant for
phenyl-
-D-galactoside. The apparent K
(K
= 11.5 mM)
for the Sec2-encoded
(1,2)fucosyltransferase was determined using
3 µM GDP-[
C]fucose, as described
under ``Materials and Methods.'' C, apparent
Michaelis constant for lacto-N-biose I. The apparent K
(K
= 3.6 mM) for the Sec2-encoded
(1,2)fucosyltransferase was determined using 3 µM GDP-[
C]fucose as described under
``Materials and Methods.'' D, apparent Michaelis
constant for N-acetyllactosamine. The apparent K
(K
= 3.8 mM) for the Sec2-encoded
(1,2)fucosyltransferase was determined using 3 µM GDP-[
C]fucose as described under
``Materials and Methods.'' E, apparent Michaelis
constant for GDP-fucose. The apparent K
(K
= 197 µM)
for the Sec2-encoded
(1,2)fucosyltransferase was determined using
25 mM phenyl-
-D-galactoside as described under
``Materials and Methods.''
The Secretor locus-determined (1,2)fucosyltransferase activity is
detectable in soluble form in milk and in other body fluids (3, 29) . To determine if this biosynthetic process is
recapitulated in COS-7 cells, we transfected COS-7 cells with
pcDNAI-
(1,2)FTSe-short, or with
pcDNAI-
(1,2)FTSe-long, and quantitated the
cell-associated and soluble
(1,2)fucosyltransferase activity
produced by each vector (see ``Materials and Methods''). We
found that media isolated from a 100-mm dish of COS-7 cells transfected
with pcDNAI-
(1,2)FTSe-short contained approximately 868
total units of activity. By contrast, the cell extract prepared from
the same plate of transfected cells contained approximately 118 total
units of
(1,2)fucosyltransferase activity, for a media/cell
extract ratio of approximately 7.4. Similar results were obtained when
pcDNAI-
(1,2)FTSe-long was used in these experiments (357
units in media versus 55 units in cell extract; media/cell
extract ratio of approximately 6.5). These data indicate that both
forms of this enzyme are released from the transfected COS-7 cells in a
relatively efficient manner. This contrasts with the H blood
group cDNA-encoded
(1,2)fucosyltransferase activity, which is
largely cell-associated when expressed in COS-7 cells(8) .
In aggregate, these data confirm that the Sec2 segment encodes an
(1,2)fucosyltransferase. They are also consistent with the
hypothesis that this sequence corresponds to an
(1,2)fucosyltransferase locus, presumed to be the Secretor blood group locus, that encodes an
(1,2)fucosyltransferase
found in the serum and milk of secretor-positive individuals.
To determine if this
polymorphism yields an inactive allele, the expression vector
pcDNAI-(1,2)FTse (see ``Materials and
Methods''), containing the termination codon at position 143, was
tested by transfection for its ability to encode a functional
(1,2)fucosyltransferase. No
(1,2)fucosyltransferase activity
was detected in COS-7 cells transfected with vector
pcDNAI-
(1,2)FTse (data not shown). One other DNA sequence
polymorphism that yields a protein coding sequence alteration is also
present in the Trp
ter allele derived from cosmid
31553 (Gly
Ser; Fig. 2). An expression
vector (pcDNAI-
(1,2)FTSe-int, see ``Materials and
Methods'') containing this other amino acid sequence polymorphism,
on the wild type sequence background, was constructed and tested for
its ability to express
(1,2)fucosyltransferase activity. This
vector determined expression of wild type levels of
(1,2)fucosyltransferase activity when expressed in COS-7 cells.
The results indicate that the DNA sequence polymorphism that creates
the translation termination codon inactivates this allele, whereas the
other polymorphism is functionally neutral in a qualitative
(1,2)fucosyltransferase activity assay.
Figure 4:
Allele-specific oligonucleotide (ASO) hybridization analysis of the Trp
ter codon polymorphism. A, ASO analysis of 52 random
individuals and 8 control samples. A DNA segment encompassing the
polymorphic site was amplified from genomic DNA obtained from each of
52 randomly sampled individuals and probed with a radiolabeled
oligonucleotide corresponding to the Trp
allele (Wild-type probe) or with a radiolabeled oligonucleotide
corresponding to the ter
allele (Mutant probe),
using reagents and conditions described under ``Materials and
Methods.'' Results obtained with control samples derived from
cloned versions of each allele, or from individuals with known
phenotypes, are displayed in the boxed area at right. B, ASO analysis of six non-secretor individuals, two secretor
individuals, and cloned control samples, using reagents and procedures
identical with those used in A,
above.
To
further explore the possibility that homozygosity for the Trp
ter null allele is commonly responsible for the
non-secretor phenotype, we analyzed the Sec2 sequence at this position
in a group of individuals whose secretor phenotypes had been previously
determined (see ``Materials and Methods''). Each of six
non-secretor individuals were found to be homozygous for the
Trp
ter null allele, whereas all secretor-positive
persons were found to maintain at least one wild type allele at codon
143 (Fig. 4B). Taken together with the physical linkage
analyses reported in the accompanying paper(14) , and the
biochemical analyses reported here, these genetic results lead us to
conclude that the Sec2 locus corresponds to a structural gene encoding
the Secretor locus
(1,2)fucosyltransferase. They further
indicate that the non-secretor phenotype is often, although perhaps not
exclusively, due to homozygosity for a common Trp
ter null allele at this locus.
The results reported here, and in the accompanying paper,
indicate that the Sec2 DNA segment corresponds to the human Secretor blood group locus. They also confirm the hypothesis (3) that the H and Secretor loci represent
two distinct but closely linked (1,2)fucosyltransferase loci and
demonstrate, with other data(9) , that homozygosity for null
alleles at these two loci can explain virtually all known recessively
inherited H-deficient phenotypes in humans. Nonetheless, it will be
useful to obtain additional genetic support for this assignment, by
completing genetic studies of this locus in families informative for
the secretor and non-secretor phenotypes, analogous to previous studies
performed with the H(9) and Lewis(31) blood group loci.
It will also be interesting to determine if this locus maintains structurally distinct alleles with idiosyncratic catalytic properties, analogous to the allelic transferases encoded by the ABO (A and B subgroups; refs. 32-34) and Lewis(31) blood group loci. In particular, the molecular cloning of this locus will facilitate a determination of the molecular basis for the postulated weak Secretor allele (Se(w)) proposed to account for the unusual Lewis blood group phenotype observed in selected individuals of Polynesian ancestry(35, 36) .
The results reported
here raise a number of additional issues to be explored. For example,
while the Sec1 sequence we analyzed most probably represents a
pseudogene, it remains a formal possibility some individuals may
maintain an Sec1 allele in which the inactivating frameshift and
termination codon ``mutations'' we found are instead
represented by DNA sequence (``suppressed'') that encodes a
functional (1,2)fucosyltransferase. It is also interesting to
consider the possibility that a ``corrected'' Sec1 allele,
present either as an endogenous sequence, or derived by
``correction'' events associated with mutation or RNA editing
(37-39, reviewed in (40) ), may account for the
observation that a cultured human carcinoma cell line expresses an
(1,2)fucosyltransferase activity that is catalytically distinct
from the H and Secretor locus-encoded
enzymes(41) . These issues may be resolved by the construction
and testing of an expression vector in which the translational
frameshift and termination codons in the Sec1 sequence have been
corrected by site-directed in vitro mutagenesis procedures and
by a thorough search for transcripts corresponding to the Sec1 locus in
human tissues and cell lines.
The presence of two functional
initiation codons in the Sec2 locus, in the context of two potential
splice acceptor sites, suggests the possibility that alternative
splicing events might yield two distinct polypeptides. Since these two
polypeptides are predicted to maintain identical catalytic domains, it
seems unlikely that the catalytic activities of the corresponding
enzymes would differ. However, it is conceivable that splice acceptor
site-dependent polymorphism in the length of the proteins'
NH termini might in turn influence sub-Golgi localization
processes, post-translational proteolytic processing events that
liberate catalytically active fucosyltransferase fragments, and
functionally relevant consequences of these events (including
differential access to different glycoconjugate acceptors).
Alternatively, the hypothetical alternatively spliced transcripts might
be translated with differential efficiencies. Resolution of these
questions will require an exploration of the structures and functions
of the transcript(s) and corresponding polypeptides derived from this
locus.
Unfortunately, and perhaps not surprisingly, these studies do not address the biological relevance of the oligosaccharides whose expression is determined by the Secretor locus. While there is evidence for associations between secretor status and susceptibility to a variety of malignant and infectious diseases, virtually all of these associations are rather weak (reviewed in (42) ). However, one especially intriguing association concerns the observation that non-secretors maintain an increased relative risk (1.5-fold) for duodenal ulcers(43) . This observation is especially interesting in the context of recent work demonstrating that the bacterial organism Helicobactor pylori can attach to gastric epithelium via type I H and Lewis b oligosaccharide determinants(44) , whose expression in gastrointestinal tissues is determined by the Secretor locus, or by Secretor and Lewis blood group loci, respectively(3) . Since colonization of the gastrointestinal tract by this organism in humans has been associated with histologic gastritis(45) , gastric lymphoma(46) , gastric carcinoma(47, 48) , gastric ulcer(49) , and recurrence of duodenal ulcers(50) , it is possible to speculate that Secretor locus-dependent expression of soluble H-active and Lewis b-active glycoconjugates in the gastrointestinal tract prevents (or allows) H. pylori colonization and indirectly determines susceptibility to the gastrointestinal diseases noted above. The relationship between secretor phenotype and susceptibility to these gastrointestinal diseases remains weak, however, and there is evidence that H. pylori colonization and secretor phenotype may be independent risk factors for these disorders(51) .
In
summary, the evolutionary basis for polymorphism at the Secretor locus remains a mystery. Given that there seems to be no obvious
or strong selective disadvantage associated with the non-secretor
phenotype, one might have initially presumed that the Trp
ter null allele would be no more common than any other
inactivated allele and would be found, therefore, in only a small
fraction of the individuals we examined. Instead, we observed a high
frequency of the Trp
ter null allele in our
studies, high enough to suggest the surprising possibility that most,
if not all, non-secretors are homozygous for this allele. It is also
surprising that this was the only inactivating DNA sequence alteration
we found in the cosmid-derived Sec2 allele. These observations suggest
the possibility that this particular inactivated allele once conferred,
or still does confer, some specific selective advantage upon members of
the human species. Alternatively, it is intriguing to speculate that
this locus may indeed have an essential function and that this specific
termination codon is suppressed in a tissue-specific manner to yield a
functional enzyme in some tissues or at a particular developmental
stage, although not in tissues where the secretor phenotype is usually
assayed. In particular, it is formally possible that RNA editing
mechanisms (40) might suppress this nonsense codon, by changing
the A residue in the termination codon at position 143 in the se-derived transcript to a G residue, to yield a wild type
tryptophan codon at this position. We note that there is precedence for
an RNA editing activity in rodent and human neuronal cells that is
capable of catalyzing an A to G change in glutamate receptor
transcripts(37, 38, 39) . We also note that
there is evidence for editing of a mammalian glycosyltransferase
transcript(52) . A better understanding of these issues may be
obtained by a systematic determination of the frequency of the
Trp
ter null allele in a large number of
ethnically diverse non-secretor individuals and by determining if
non-secretor alleles generally maintain an otherwise wild type coding
sequence, in consideration of the possibility that other null alleles
can also yield the non-secretor phenotype. Experiments designed to to
determine if Se locus-derived transcripts are subject to RNA
editing mechanisms might also be informative.
This paper is dedicated to the memory of Dr. Mark S. Roth.
The nucleotide sequence(s) reported in this paper has been submitted to the GenBank(TM)/EMBL Data Bank with accession number(s) U17894 [GenBank]and 17895.