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The cblD defect causes either isolated or combined deficiency of methylcobalamin and adenosylcobalamin synthesis.
[homocystinuria without methylmalonic aciduria]
Intracellular
cobalamin
is
converted
to
adenosylcobalamin
,
coenzyme
for
methylmalonyl-
CoA
mutase
and
to
methylcobalamin
,
coenzyme
for
methionine
synthase
,
in
an
incompletely
understood
sequence
of
reactions
.
Genetic
defects
of
these
steps
are
defined
as
cbl
complementation
groups
of
which
cblC
,
cblD
(
described
in
only
two
siblings
)
,
and
cblF
are
associated
with
combined
homocystinuria
and
methylmalonic
aciduria
.
Here
we
describe
three
unrelated
patients
belonging
to
the
cblD
complementation
group
but
with
distinct
biochemical
phenotypes
different
from
that
described
in
the
original
cblD
siblings
.
Two
patients
presented
with
isolated
homocystinuria
and
reduced
formation
of
methionine
and
methylcobalamin
in
cultured
fibroblasts
,
defined
as
cblD-variant
1
,
and
one
patient
with
isolated
methylmalonic
aciduria
and
deficient
adenosylcobalamin
synthesis
in
fibroblasts
,
defined
as
cblD-variant
2
.
Cell
lines
from
the
cblD-variant
1
patients
clearly
complemented
reference
lines
with
the
same
biochemical
phenotype
,
i
.
e
.
cblE
and
cblG
,
and
the
cblD-variant
2
cell
line
complemented
cells
from
the
mutant
classes
with
isolated
deficiency
of
adenosylcobalamin
synthesis
,
i
.
e
.
cblA
and
cblB
.
Also
,
no
pathogenic
sequence
changes
in
the
coding
regions
of
genes
associated
with
the
respective
biochemical
phenotypes
were
found
.
These
findings
indicate
heterogeneity
within
the
previously
defined
cblD
mutant
class
and
point
to
further
complexity
of
intracellular
cobalamin
metabolism
.
Diseases
Validation
Diseases presenting
"deficiency of adenosylcobalamin synthesis"
symptom
homocystinuria without methylmalonic aciduria
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