H e m a t o lo g y E d u c a t io n - European Hematology Association

16 th Congress of the European Hematology Association
regarding selection of units. However, they remain frequently
puzzled because they do not really know the
clinical significance of the variant they have found.
After a long period of describing new variant alleles, the
next goal is to determine which variants represent a
risk for the SCD patients frequently exposed to foreign
antigens, to adapt DNA-based typing to the clinical situation,
14 and to implement prevention only when necessary.
The RH blood group and the variants
The RH blood group is composed of two highly
homozygous genes, the RHD and the RHCE genes.
The RHD gene produces the D antigen, and the RHCE
gene produces a polypeptide carrying two antigens: C or
c and E or e. 15 In Caucasians, the main haplotype is DCe,
in Afro-Americans and Afro-Caribbeans, the main haplotype
is Dce. 16 The result of this distribution is a high frequency
of SCD patients with the D+C-E-c+e+ phenotype.
Only 20% of patients express the C antigen. 5 Within
the five main antigens (D, C, E, c, e), schematically, two
types of variants are described, and they are encoded by
point mutations, multiple missense mutations or hybrid
alleles. 17 First, those named partial, because they lack some
immunogenic epitopes, as shown by allo-immunization
of the carriers against missing epitopes when exposed to
the complete antigen through transfusion or pregnancy.
Second, are the weak antigens. Individuals carrying weak
antigens do not get immunized when exposed to the normal
antigen. Therefore, a carrier of a partial antigen
should receive RBCs which do not express the antigen to
prevent allo-immunization. When a new variant is discovered
because the carrier has developed an antibody, the
variant is classified as partial on an immunological point of
view. When a new variant is described, because of a weak
reactivity, it is much more difficult to decide the category.
18 There is a consensus to classify the variant based on
the predicted localization of the substitution. The Rhesus
index determining the antigen density can help also to
decide. 19 Schematically, when localized at the outer surface
of the membrane, the variant is considered partial,
when localized in the intra membrane or in the intra cellular
domain, the variant is considered only weak. But it
has been shown that carriers of some variant categorized
as weak did get immunized. It is the case for the weak D
type 11, type 15, and also for the weak D type 4.2, which
is found in SCD patients. 20,21 Then, as stated by the Bristol
team, the weakD/partial D dichotomy is artificial. 18
Therefore, the risk of allo-immunization in a “variant” situation
is fundamental to know to manage safely and efficiently
the transfusion of the carrier. There is already a
register for anti-D immunization (http://www.uniulm.de/~wflegel/RH/),
but there are no data regarding the
clinical significance of the antibodies. There is no register
for RHCE variants prone to immunization.
The RH variants in SCD patients and
the associated risk of allo immunization
Sickle cell disease patients are highly polymorphic at
the RH locus compared with individuals of European
ancestry. In the RHD gene, the first difference is the
molecular basis of D-negative individuals. As compared
with D-negative European individuals who display
mainly a deletion of the RHD gene, 22 D-negative individuals
of African ancestry exhibit a silent gene produced
either by a 37 base pair insertion that lead to a premature
stop codon or a hybrid RHD-CE-D gene characterized
by the production of a partial C antigen but no
D. 23,24 These differences do not bring any specific risk for
SCD patients, as serologically there is no difference
between D negative individuals from European or
African ancestry. This background has to be considered
only when determination of D is based on DNA-based
typing methods. The variants that have to be taken into
account are those prone to allo-immunization. They are
the partial variants of the five main antigens (D, C, E, c,
e), the variants characterized by absence of expression a
high frequency antigen, mainly produced by abnormalities
within the RHCE gene, and variants characterized
by the expression of a new antigen (low incidence antigens).
Management of prevention of allo-immunization
can be extremely difficult in certain situations because
of the shortage of identical blood supply.
The partial D variants (Figure 1)
In Europeans, 1 to 2% carries RHD variant alleles,
which can produce weak D with no risk of allo-immunization
or partial D. In most of the cases, a D antigen
with a weak expression is frequently due to the weak D
type 1, 2, and 3 alleles, which are not prone to anti-D
immunization. 21 There are some major differences within
D variants among individuals of African ancestry:
firstly, the frequency of D variants is probably much
higher; secondly, a D antigen with a weak expression is
frequently a partial antigen prone to anti-D immunization;
thirdly, altered RHCE alleles are generally associated
with altered RHD alleles. We show the results of the
investigation of weak D samples from Afro-Caribbeans
that have been referred in our laboratory in 2010 (Table
1A). These results can be extrapolated to our SCD population
in term of the incidence of the variants that are
detected because of a decreased expression. In the
weak D type 4 cluster, many variants have been documented
to prone anti-D in the carriers. 21 In this category,
the DAR allele (weak D type 4.2.2) is inherited with ceAR
allele, and in some cases, with other similar alleles (ceEK
or ceBI). In the DIVa cluster, the DIII and DIII type 5 alleles
can appear normal or depressed, and are frequently
inherited with RHce variant alleles, mainly the ce S
allele. 25 The DAU allele cluster produces also weak D
antigens, which are partial for some of them. 26
It is well known that D antigen is the most immunogenic
antigen, but in case of partial D, the rate of alloimmunization
cannot be compared with the rate in Dnegative
patients receiving D-positive units, as only one
part of the epitopes of the complete antigen is missing.
Some variant remained controversial regarding the alloimmunization
risk. It is the case for the weak D type 4.0,
as well as for the DAU-0, which are not supposed to
prone anti-D allo-immunization. 26,27 The DAU-0 allele is
very frequent in some populations of Africa, reaching
almost 20% in Mali. 28 The antigen and Rhesus index of
the DAU-0 phenotype are about normal, rendering it
indistinguishable from the normal antigen-D-positive
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