Angiotensinogen gene associated polymorphisms and risk of stroke in sickle cell anemia: Additional data supporting an association

Genetic, laboratory and clinical risk factors in the development of overt ischemic stroke in children with sickle cell disease

Cerebrovascular disease, particularly stroke, is one of the most severe clinical complications associated with sickle cell disease and is a significant cause of morbidity in both children and adults. Over the past two decades, considerable advances have been made in the understanding of its natural history and enabled early identification and treatment of children at the highest risk. Transcranial Doppler screening and regular blood transfusions have markedly reduced the risk of stroke in children. However, transcranial Doppler has a limited positive predictive value and the pathophysiology of cerebrovascular disease is not completely understood. In this review, we will focus on the current state of knowledge about risk factors associated with ischemic stroke in patients with sickle cell disease. A search of PubMed was performed to identify studies. Full texts of the included articles were reviewed and data were summarized in a table. The coinheritance of alpha-thalassemia plays a protective role against ischemic stroke. The influence of other genetic risk factors is controversial, still preliminary, and requires confirmatory studies. Recent advances have established the reticulocyte count as the most important laboratory risk factor. Clinical features associated with acute hypoxemia as well as silent infarcts seem to influence the development of strokes in children. However, transcranial Doppler remains the only available clinical prognostic tool to have been validated. If our understanding of the many risk factors associated with stroke advances further, it may be possible to develop useful tools to detect patients at the highest risk early, improving the selection of children requiring intensification therapy.

The epidemiology, evaluation and treatment of stroke in adults with sickle cell disease

Stroke is a frequent and severe complication in adults with sickle cell disease. Ischemic stroke often causes physical and cognitive disability, while hemorrhagic stroke has a high mortality rate. As more children survive, the number of strokes in adults is increasing, yet stroke remains poorly understood. We review the epidemiology of ischemic and hemorrhagic stroke in adults with sickle cell disease and outline a practical approach to the evaluation of stroke including both sickle cell disease specific and general risk factors. We discuss the acute treatment and secondary prevention of stroke in this population based on the evidence in children with sickle cell disease and the general population, in addition to the limited studies in adults with sickle cell disease.

Sickle Cell Disease in the Post Genomic Era: A Monogenic Disease with a Polygenic Phenotype

More than half a century after the discovery of the molecular basis of Sickle Cell Disease (SCD), the causes of the phenotypic heterogeneity of the disease remain unclear. This heterogeneity manifests with different clinical outcomes such as stroke, vaso-occlusive episodes, acute chest syndrome, avascular necrosis, leg ulcers, priapism and retinopathy. These outcomes cannot be explained by the single mutation in the beta-globin gene alone but may be attributed to genetic modifiers and environmental effects. Recent advances in the post human genome sequence era have opened the door for the identification of novel genetic modifiers in SCD. Studies are showing that phenotypes of SCD seem to be modulated by polymorphisms in genes that are involved in inflammation, cell–cell interaction and modulators of oxidant injury and nitric oxide biology. The discovery of genes implicated in different phenotypes will help understanding of the physiopathology of the disease and aid in establishing targeted cures. However, caution is needed in asserting that genetic modifiers are the cause of all SCD phenotypes, because there are other factors such as genetic background of the population, environmental components, socio-economics and psychology that can play significant roles in the clinical heterogeneity.

Modifier genes and sickle cell anemia

PURPOSE OF REVIEW

With the completion of the human genome project and HapMap, previously unknown genetic polymorphisms associated with disease have been observed. This review highlights genetic polymorphisms that have provided insight into the pathophysiology underlying the many phenotypes of sickle cell disease.

RECENT FINDINGS

The phenotypes of sickle cell disease are likely to be modulated by polymorphisms in genes that are involved in inflammation, cell-cell interaction, and nitric oxide biology. Case-control studies are beginning to define the relationships between single-nucleotide polymorphisms in candidate genes and the many subphenotypes of sickle cell anemia. A common theme emerging from these studies is that single-nucleotide polymorphisms in genes of the transforming growth factor-beta/bone morphogenetic protein and a few other genes such as Klotho are associated with several subphenotypes of sickle cell disease.

SUMMARY

Genomic medicine is merging with clinical practice as our understanding of the structure and variability of the human genome increases. Patients with diseases caused by identical mutations in a single gene - sickle cell anemia is a prime example - can have clinical courses very different from one another, and when environmental influences are removed the phenotypic heterogeneity of mendelian single-gene disorders is best explained by single-nucleotide polymorphisms in genes that modulate the disease phenotype. As this field expands, insights will be gained into complex epistatic factors that influence the clinical presentation of sickle cell disease, enabling physicians to better predict and manage the many complications of this disease.

Early onset dactylitis associated with the occurrence of severe events in children with sickle cell anaemia. The Paediatric Cohort of Guadeloupe (1984-99)

Dactylitis or hand-foot syndrome is related to intravascular sickling and classically occurs around 6 months of life when haemoglobin S levels reach pathological significance. It has been hypothesised as a possible predictive factor of adverse outcome. The objective of this study was to estimate the survival without severe events (acute chest syndrome, stroke or death) according to early occurrence of dactylitis in the sickle cell paediatric cohort of Guadeloupe. The study period was 1 January 1984 to 31 December 1999. Medical data were recorded from birth for children with sickle cell anaemia diagnosed before 6 months of age. The Kaplan-Meier method was used to estimate the severe event-free survival and to draw related curves. We used proportional-hazard modelling to assess the risk of severe events (stroke or acute chest syndrome or death) in children with early dactylitis (before 6 months of age). The study sample comprised 114 children. The incidence [95% confidence interval] of clinical events per 100 child-years were 10.0 [7.63, 13.07] for dactylitis, 0.73 [0.33, 1.62] for stroke, 10.88 [8.35, 14.17] for acute chest syndrome and 1.06 [0.56, 2.05] for death. Thirteen of the 14 children who had dactylitis before 6 months of age had at least one severe event later in life. The median severe event-free survival time was 2.1 [1.6, 2.7] years in children with dactylitis before 6 months compared with 6.3 [4.5, 8.1] years in the other children; P < 0.0005. After adjustment for haemoglobin level <or=7 g/dL (25th percentile), leucocyte count >19 000/mm3 (75th percentile), transfusion before age of one year, acute splenic sequestration before the age of one year, the adjusted hazard ratio of severe event for children with early dactylitis was 2.13 [1.06, 4.40]; P < 0.03. Dactylitis occurring before 6 months of life identifies children at risk of severe complications who should benefit from close management.

Predicting clinical severity in sickle cell anaemia

The ability to predict the phenotype of an individual with sickle cell anaemia would allow a reliable prognosis and could guide therapeutic decision making. Some risk factors for individual disease complications are known but are insufficiently precise to use for prognostic purposes; predicting the global disease severity is not yet possible. Genetic association studies, which attempt to link gene polymorphisms with selected disease subphenotypes, may eventually provide useful methods of foretelling the likelihood of certain complications and allow better individualized treatment.