Page 256 - 2021_06-Haematologica-web

Letters to the Editor
Homozygous Southeast Asian ovalocytosis in five
live-born neonates
Southeast Asian ovalocytosis (SAO) is an autosomal dominant inherited red blood cell (RBC) membrane dis- order caused by the heterozygous deletion of codons 400–408 in SLC4A1/band 3/anion exchanger 1 (AE1).1 This deletion leads to misfolding of the protein, creating an inactive anion-transporter and altering the mechanical stability of the RBC. Heterozygous SAO is characterized by the presence of stomatocytes, theta cells (RBC with two stomas), macro-ovalocytes and ≥25% ovalocytes in the peripheral blood smears.2 Although heterozygous SAO carriers are generally asymptomatic, homozygous SAO was considered to be lethal.3 However, one success- ful birth of a homozygous SAO individual, born to asymptomatic heterozygous SAO Comorian parents, was reported in 2014 showing an association with severe dyserythropoietic anemia.4 Here, we report the birth of five unrelated homozygous SAO babies in Malaysia showing that homozygous SAO is not as rare as previ- ously thought. These babies belong to an SAO cohort collected from January 2007 to March 2020 at University Sains Malaysia (USM), which includes a total of 68 patients. The study was instigated to further understand the physiological implications of heterozygous SAO within the Malay population. The detection of numerous homozygous SAO births has highlighted the need for the development of good practice to support the survival of these babies.
SAO has a distinctive geographical distribution, occur- ring mostly in areas of Southeast Asia including neighbor- ing regions of the Malay Peninsula, coastal regions of Papua New Guinea, Thailand, Indonesia, Taiwan and even Madagascar in the western Indian Ocean.5,6 Investigations into the high incidence of SAO in malari- ous areas revealed that the SAO mutation confers protec- tion from severe malaria caused by Plasmodium vivax and Plasmodium falciparum.7,8
showed expression of SAO AE1 during erythropoiesis (in
vitro) affected trafficking and cytokinesis resulting in
numerous multinucleated erythroblasts and reduced pro-
liferation and enucleation. Unexpectedly, even the in vitro
cultured heterozygous SAO erythroblasts displayed
multinuclearity and reduced enucleation, with an inter-
mediate phenotype part way between the homozygous
and control cells,13 suggesting that heterozygous SAO
individuals may have a mild dyserythropoiesis. Mature
homozygous SAO RBC show signs of altered membrane
composition, oxidative damage, are very large, oval and
4,13
In Malaysia, where the prevalence of SAO is around 4% within the Malay ethnic group,15 SAO diagnostic tests are performed mostly upon still birth or recurring miscarriages of the mother. Several heterozygous SAO mothers in this cohort have a history of past miscarriages and complications during pregnancy such as hydrops fetalis as well as intrauterine death (Figure 1). Furthermore, SAO was identified in 18 of 22 dRTA patients (81.8%) when the association between SAO and dRTA was studied in the Malaysian dRTA cohort.15 Here, we provide an overview of a Malaysian heterozygous SAO cohort and describe the clinical characteristics of five live-born homozygous SAO cases in Malaysia.
This SAO cohort includes a total of 107 patients whose blood samples were tested for the 27 base pair deletion of AE1 using polymerase chain reaction (PCR) and blood smears (Online Supplementary Appendix; Online Supplementary Table S1). Among the 107 tested patients, 68 cases were positive where 93% (63 of 68 cases) of SAO positive cases belonged to Malay ethnicity. Within this Malaysian SAO cohort, 81 cases studied belong to a total of 28 families. Among these 28, 23 families had at least one heterozygous SAO family member. The remaining 26 samples were from individual cases, of which 15 were heterozygous SAO. The majority of the SAO positive cases (43%, n=29) are from Penang, fol- lowed by Kuala Lumpur (22%, n=15), Johor (10%, n=7), Sabah (9%, n=6), Kedah (9%, n=6), and Melaka (7%, n=5) (Online Supplementary Figure S3). A variety of RBC morphologies were observed from peripheral blood film examinations of the SAO positive cases, which includes macro-ovalocytes, stomatocytes, theta cells, spherocytes and target cells (Online Supplementary Table S1; Online Supplementary Figure S1).
Surprisingly, a total of six homozygous SAO were found in our cohort, five of whom were live-born (Figure 1, Table 1, Online Supplementary Appendix, and Homozygous SAO case histories). All parents of live- born homozygous SAO cases, with the exception of one (GO 009/16), were confirmed heterozygous for the SAO AE1 deletion; case GO 012/19 was a homozygous fetus that suffered intra-uterine death at 29 weeks. All the five live-born cases were delivered prematurely, at 30 to 35 weeks of gestation. Similar to previous homozygous SAO live-borns, our five homozygous SAO live-borns suffered a severe phenotype requiring intra-uterine trans- fusions, post-delivery transfusions and ventilation sup-
AE1 is the most abundant RBC membrane protein and mediates the efficient transport of carbon dioxide in the blood.1 Heterozygous SAO individuals have only 50% normal RBC anion transport activity and thus less effi- cient gas transport. The heterozygous expression of the SAO AE1 in the red cell membrane not only affects the folding and structure of AE1 but also affects the structure of other red cell membrane proteins depressing a relative- ly high number of red cell antigens.9 Neonatal anemia has also been reported in SAO families.2
A truncated form of AE1 is expressed in the α-interca- lated cells of the kidneys and is essential for acid secre- tion in the urine.1 Loss of AE1 anion-transport activity in the kidney causes distal renal tubular acidosis (dRTA) and occurs when AE1 expression is severely reduced as in hereditary spherocytosis (HS)10,11 or when kidney AE1 activity is severely reduced1 or trafficking of kidney AE1 is impeded in the α-intercalated cell.1 Many dRTA muta- tions are recessive. When SAO AE1 is inherited in trans to a recessive dRTA AE1 mutation then the compound heterozygote will completely lack AE1 anion transport activity. This phenomenon explains the prevalence of dRTA in South East Asia where there are a number of recessive dRTA AE1 mutations maintained in the popula- tion.12
Homozygous SAO individuals, with no functioning AE1, and homozygous HS individuals, with no AE1, suf- fer from hemolytic anemia, dRTA and failure to thrive.3,10,11 An in-depth study of homozygous SAO cells,13
unstable. AE1 null HS individuals sometimes have a mild dyserythropoiesis10 and their RBC are unstable.11 Extensive clinical management of AE1 null individuals, including splenectomy, has improved the survival of these children, some of whom have now reached adoles- cence. Similarly, the Comorian homozygous SAO child is now 10 years old and quite well, receiving regular trans- fusions, iron chelation and acidosis treatment.4,13 From Malaysia, a live-born homozygous SAO neonate with dRTA was reported in 2015 but the child failed to thrive after 2 years.14
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haematologica | 2021; 106(6)