Sickle cell anaemia (SCA) is a clinically heterogeneous autosomal recessive anaemia characterised by chronic haemolysis and recurrent episodes of severe vaso-occlusion and infection. Several environmental and genetic determinants have been suggested to modulate the onset, course and outcome of SCA. The level of chronic haemolysis has been considered a critical measure of SCA severity and a possible proximate cause of some disease complications such as stroke, pulmonary hypertension, priapism, leg ulceration and cholelithiasis. Thus, we proposed to search for genetic modifiers of this sub-phenotype and gain insights into the underlying mechanisms.

We studied the association between commonly measured haemolysis biomarkers (LDH, total bilirubin and reticulocyte count) and the inheritance of 41 genetic variants (34 SNP, 6 indel, 1 STR) of 10 candidate genes in a longitudinally observed series of 99 paediatric homozygous SCA patients (median current age of 9.9 yr) followed up in two general hospitals in Greater Lisboa area (median follow-up per patient of 5.0 yr). Candidate gene genotyping was performed by PCR-RFLP, Sanger sequencing, Gene Scan or Gap-PCR.

The following significant associations were observed after correction for multiple comparisons: i) an increased serum LDH level was associated with haplotype 7 within VCAM1 gene; ii) a lower total bilirubin was associated with the 3.7-kb deletion at HBA gene, with the rs2070744_T allele at NOS3 gene and with haplotype 9 within VCAM1 promoter; and iii) a diminished reticulocyte count was associated with the 3.7-kb deletion at HBA, whereas an increased count was associated with rs1984112_G allele at CD36 gene.

On the whole, our findings suggest a complex genetic architecture for the SCA haemolysis process involving multiple pathways, namely control of vascular cell adhesion, NO synthesis and erythrocyte volume and haemoglobinisation.

Partially funded by FCT: PIC/IC/83084/2007 and PEst-OE/SAU/UI0009/2011