section 28.3
Inherited Disorders of Hemoglobin Structure and Synthesis
667
cannot fit into an
a
helix (Chapter 4). The disruption can
cause severe disease (Hbs Bibba, Abraham Lincoln, and
Genova) or be benign (Hb Saki). If proline is introduced
at one end of a helix, it does not disrupt it. However, in Hb
Syracuse, new proline at the end of the H helix eliminates
positive charge needed for 2,3-DPG binding.
Loss of a charged or polar group (Hb Syracuse) or in-
troduction of a charged or polar group into the hydropho-
bic interior of the molecule (Hbs Ann Arbor, St. Louis,
North Shore, and Indianapolis) usually distorts the struc-
ture and causes altered function. If the mutation occurs
at a site where the side chain is not very important or
where only the type of side chain is important, the muta-
tion may well be benign. A number of mutations of this
type have been detected during routine screening in many
populations.
Unstable Hemoglobins
Many of the substitutions mentioned above adversely af-
fect the stability of the tetrameric molecule and cause easy
denaturation and precipitation within erythrocytes to form
Heinz bodies. Deletion of one or more amino acids is also
likely to cause instability, precipitation (e.g., Hbs Leiden,
Tochigi, Tours, Gun Hill), and membrane damage. Con-
sequently, these variants are associated with intravascular
hemolysis, anemia, reticulocytosis, splenomegaly, and, in
some patients, intermittent urobilinuria.
If the hemoglobin precipitates soon after synthesis,
before the cells leave the bone marrow, a thalassemia-
like syndrome may occur (Hbs Nottingham and Indi-
anapolis). These mutations as well as Hbs Hammersmith,
Abraham Lincoln, Bibba, and others produce a severe
clinical disorder with massive hemolysis that is not im-
proved by splenectomy. Other mutations (e.g., Hbs Torino,
Ann Arbor, St. Louis, Koln, and Shepherd’s Bush) in
which precipitation occurs in the circulation cause se-
vere hemolysis but are improved by splenectomy, which
increases the erythrocyte lifetime. Although sometimes
difficult, it is important to distinguish these two groups
clinically when deciding whether splenectomy is indi-
cated. Most unstable hemoglobins are associated with
only mild hemolysis and occasional hemolytic crises, usu-
ally brought on by some stress such as a mild infection
or treatment with sulfonamide or other oxidizing drugs
(e.g., Hbs Philly, Sydney, North Shore, Leiden, Gun Hill,
Seattle, and Louisville). Another group (e.g., Hbs Saki and
Tacoma) is clinically benign, and the unstable hemoglobin
is
discovered incidentally by
routine electrophoretic
screening.
The difference in disease severity among the unstable
hemoglobins may reside in the number and time of
formation of Heinz bodies. Heinz body formation is
believed to start when hemoglobin is oxidized to methe-
moglobin. The unstable methemoglobins are degraded
to hemichromes, a process promoted by their tendency
to dissociate (separate subunits form hemichromes more
readily than tetramers). The tendency of the chains to
form hemichromes decreases in the order
a > /3
>
y.
Hemichromes precipitate and form Heinz bodies, which
attach hydrophobically to the erythrocyte membrane,
increasing membrane permeability and the rate of lysis.
Loss of heme and oxidation of sulfhydryl groups do not
appear to be necessary for precipitation.
The degree of morbidity associated with an unstable
hemoglobin is also determined by the effect of the mu-
tation on oxygen affinity. If the oxygen affinity is de-
creased, delivery of oxygen to the tissues will be higher
than normal and compensate, in part, for the lower
hemoglobin concentration. This is the case with Hbs
Leiden, Seattle, Louisville, and Peterborough. In con-
trast, Hbs Koln, St. Louis, and Shepherd’s Bush, which
cause severe hemolytic anemia and Heinz body forma-
tion, have increased oxygen affinity. The effect of the
anemia is exacerbated by the inability of erythrocytes to
unload the oxygen they carry. Both groups have simi-
larly low hemoglobin levels and rapid hemolytic rates.
Thus, the phenotype caused by a mutation is the net
result of all its effects on hemoglobin structure and
function.
Secondary Polycythemia Syndromes
Mutations thatincrease oxygen affinity (reduce P
5
o) can
decrease tissue oxygenation more than expected on the
basis of hemoglobin concentration. At low oxygen ten-
sion, production of erythropoietin is stimulated, causing a
secondary polycythemia that increases oxygen delivery to
the tissues and partially compensates for the hemoglobin
abnormality.
Any mutation that stabilizes the R (oxy) state (Hb
Chesapeake) or destabilizes the T (deoxy) state (Hb
Kempsey) can cause polycythemia. More than 28 such
variants
are
known.
Fourteen
of these
high-affinity
hemoglobins have mutations at the
a i/32
interface. Be-
cause the C terminus of the
ft
chain supplies a hydro-
gen bond that is important for stabilizing the T-state,
its replacement (Hb Bethesda), deletion (Hb McKees
Rocks), or alteration in its environment (Hbs Syracuse
and Cowtown) markedly decreases P
50
. Hb San Diego is
unique in having high oxygen affinity due to a substitution
in the
a
1
/5
j interface. Mutations that decrease the stability
of the hemoglobin and cause precipitation or dissociation
can also increase oxygen affinity.
