Alpha Thalassemia
Alpha thalassemia refers to a group of disorders characterized by inactivation of
alpha globin genes. This results in a relative increase in nonfunctional beta globin
or gamma globin tetramers and subsequent cell damage. Normally, there are four alpha genes.
Absence or non-function of three alpha genes results in hemoglobin H disease, and the loss
of all four alpha genes usually results in intrauterine death.
The alpha thalassemias are caused by a decrease in production of alpha
globin chains due to a deletion or mutation of one or more of the four
alpha globin genes located on chromosome 16. Alpha gene mapping can be
obtained to determine the specific mutation. The alpha thalassemias can
be generally categorized as: Silent Carrier, Alpha Thalassemia Trait,
Hemoglobin H disease, Hemoglobin H-Constant Spring, and Alpha Thalassemia
major. Frequently, the diagnosis of alpha thalassemia trait in a parent
is discovered after the birth of an affected child.
The Silent Carrier status is characterized by three functional
genes that code for the production of alpha globins
(-a/aa).
Outside the newborn period, it is not possible to make this diagnosis
by conventional methods. There is overlap between the red blood cell
indices of these individuals and normals, although the MCV
may be slightly lower. The silent carrier will experience no health problems
in his/her lifetime. This carrier state is diagnosed by deduction when
a 'normal' individual has a child with Hgb H disease or with microcytic
anemia consistent with alpha thalassemia trait. An unusual case of the
silent carrier state is the individual who carries the Hemoglobin Constant
Spring
mutation [(acsa/aa)
or (aacs/aa)].
This is an elongated a-globin due to a
termination codon mutation. Individuals who have this mutation have normal
red blood cell indices, but can have children who have Hgb H-Constant
Spring disease if the other parent has alpha thalassemia trait (--/aa).
Generally, children with Hgb H-Constant Spring are more affected clinically
than children who have classic Hgb H disease. Two Constant Spring carriers
can also pass on their genes to have a child with Homozygous Constant
Spring, a condition that has similar clinical implications as Hemoglobin
H disease.
Alpha thalassemia trait is characterized by two functional genes
that code for the production of alpha globins [(-a/-a)
or (--/aa)]. The two genes can either
occur on the same chromosome (cis-type) or on each of the pair (trans-type).
Cis-type a-thalassemia trait tends to be
found in individuals of Asian descent, while trans-type tends to run in
individuals of African descent. Cis-type can be co-inherited with another
cis-type or hemoglobin H disease to result in alpha thalassemia major,
or hydrops fetalis. Individuals who have alpha thalassemia trait are identified
by microcytosis,
erythrocytosis,
hypochromia,
and mild anemia. The diagnosis is made by a combination of family studies
and the ruling out of both iron deficiency anemia and beta thalassemia
trait. In the neonatal
period, when hemoglobin Bart's (g4)
is present, the diagnosis can be strongly suspected. In children, there
are no markers such as Hgb A2 and Hgb F to make the diagnosis.
(One exception is the case where both of the deletions occur on the same
chromosome and zeta [z] globin is expressed
in carriers. This is most common in Southeast Asians.) The diagnosis is
one of exclusion. The clinician should be satisfied with the presumed
diagnosis if the above criteria are met. During pregnancy, the microcytic
anemia can be mistaken for anemia of pregnancy.
The individual with a thalassemia trait
will experience no significant health problems except a possible slight
anemia which cannot be treated with iron.
Hemoglobin H disease is characterized by one functional gene that
codes for the production of alpha globins (--/-a).
Hgb H disease should be considered in the case of a neonate in whom all
of the red blood cells are very hypochromic. These neonates have a high
percentage of hemoglobin Bart's on the their newborn screening results.
In older children, this hemoglobinopathy is characterized by moderate
anemia with a hemoglobin in the 8 to 10 gm/dL range, hypochromia, microcytosis,
red cell fragmentation, and a fast migrating hemoglobin (Hgb H) on electrophoresis.
Hemoglobin H does not function as a normal hemoglobin and has a high oxygen
affinity
(holds onto oxygen longer making it unavaible for use by the body), so
the measured hemoglobin in these children is misleading. Individuals who
have Hgb H generally have a persistent stable state of anemia, which may
be accentuated by increased hemolysis during viral infections and by exposure
to oxidant medications, chemicals and foods such as sulfa drugs, benzene,
and fava beans (similar to individuals who have G6PD deficiency). As the
red cells mature they loose their ability to withstand oxidant stress
and Hgb H precipitates, leading to hemolysis. Therapy for individuals
who have Hgb H disease includes folate, avoidance of oxidant drugs and
foods, genetic counseling education and frequent medical care. Uncommon
occurrences in a child with Hgb H would be severe anemia,
skin ulceration, and splenomegaly
requiring splenectomy. Unlike individuals who have beta thalassemia,
hemosiderosis
is rare in Hgb H disease.
Children with Hemoglobin H-Constant Spring (--/acsa)
have a more severe course than children who have Hgb H. They have a more
severe anemia, with a steady state hemoglobin ranging between 7 and 8
gm/dl. They more frequently have splenomegaly and severe anemia with
febrile
illnesses and viral infections, often requiring transfusion. If anemia
is chronically severe and the child has splenomegaly, a splenectomy may
be performed. If splenectomy is
anticipated, the complication of severe post-splenectomy
thrombocytosis
with hypercoagulability
can occur, leading to thrombosis
of the splenic vein or hepatic
veins. This complication has also been reported as recurrent
pulmonary
emboli and clotting
diathesis.
At Children's Hospital Oakland, children who are scheduled to have surgery
are treated pre-surgically
with low molecular weight heparin, followed by low dose aspirin, continued
indefinitely.
The most severe form of alpha thalassemia is Alpha Thalassemia Major
or hydrops fetalis, characterized by a deletion of all four genes that
code for alpha globins (--/--). This diagnosis is frequently made in the
last months of pregnancy when fetal ultrasound indicates a hydropic fetus.
The mother frequently exhibits
toxemia
and can develop severe postpartum hemorrhage.
These infants are usually stillborn. There can be other congenital anomalies,
though none are
pathognomonic
for alpha thalassemia major. Since alpha globins are required for production
of fetal and adult hemoglobin, the fetus suffers from signifiant in utero
hypoxia.
The only hemoglobins found in these infants are:
Hgb Portland (d2g
2),
Hgb H (b),
and Hgb Bart's (g4),
and no Hgb A or A2. These babies can have other complications
associated with hydrops, such as heart failure and
pulmonary edema.
If the diagnosis is made early, intrauterine transfusions can be performed.
There are reports of survival and chronic transfusion in these infants;
CHO cares for one alpha thalassemia major baby with chronic transfusion
therapy. Undoubtedly, more of these infants could be saved if the diagnosis
is anticipated by prenatal diagnosis and treatment provided.
