Perspectives in genetics and sickle cell disease prevention in Africa: beyond the preliminary data from Cameroon

Abstract

Cameroon is a country of 20 million inhabitants with a carrier frequency of Sickle Cell Disease (SCD) ranging from 8 to 34 % [1]. Despite the exceptional severity, illustrated by a high prevalence of stroke (6·7%) among SCD patients [2], there are no specialized centres for lifelong medical care in Cameroon. Therefore, management needs to be accompanied by various preventive strategies. Prevention also extends to early detection with the aim to identify SCD in the foetus, provide reproductive options and options for treatment and prognosis.

Ethical challenges of prenatal genetic diagnosis for SCD

Acceptance of prenatal genetic diagnosis for SCD

In Cameroon, we reported that the majority of pre-clinical, clinical medical students, and doctors described the principle of prenatal genetic diagnosis (PND) for SCD, as acceptable (77·6, 64·8, and 78·7 %, respectively), but in the three groups, the acceptance of termination of an affected pregnancy (TAP) for SCD was lower (22·4, 10·8, and 36·1%, respectively) [3]. Views of doctors could be influenced by prospects in treatments of SCD such as hydroxyurea [4], or possible cure with stem cell (cord blood or bone marrow) transplantation [5], even though, these options are largely not available to Cameroonian SCD patients [6]. However, the majority of parents that had children affected by SCD, will accept, in principle, PND (89·8%) and TAP (62·5%) for SCD [7] with the acceptance of the principle of TAP increasing with unemployment status and the total number of children in the family. The chronicity of SCD could impair the quality of life of caregivers. Using Likert-type statements in a survey instrument, including 38-item stress factors, we evaluated the general perceptions of stress and five main specific stressors among parents namely: disease factors (clinical severity), hospital factors, financial factors, family factors (life/dynamic) and SCD-child factors (perceived quality of life) [6]. There were four response options with increasing severity (0,1,2 and 3); “2” scored for “moderate difficulty” and “3” for “severe difficulty”. The majority of participants (88.3%) experienced moderate to severe difficulty coping with SCD. Median score of SCD clinical severity was the major factor which undermined the coping ability of parents (2.2); vaso-occlusive painful events (>3 per year) was the disease-related stressor that most impacted their coping ability [6]. Similar data were also reported in Nigeria, where a sample of doctors and a group of health professional would accept termination of an affected pregnancy for SCD in 21.4% [8] and 33% [9], respectively; while 92% of the SCD heterozygous carrier Nigerian mothers favored PND for SCD and 63% indicated they would opt for TAP [10]. Like in Cameroon, Nigerian parents with SCD-affected children disclosed a high burden of SCD on their families [11]. Suprinsingly, adult patients living with SCD in Cameroon were supportive of PND (89·2%) and a remarkably high number found TAP for SCD acceptable (40·9%) [12]. This was based on their assessment of their own relatively poor quality of life and also their assessment the future well-being of the child [12, 13]. Similarly, 85% of female SCD patients in Nigeria, acceptated PND for SCD, in principle, and 35% indicated they would opt for TAP [10]. It is possible that some SCD patients, in both Cameroon and Nigeria, had such poor quality of life that they did not find their own lives worth living, and did not want another child to have the same experience, an alarming finding that requires urgent attention of policy makers [12]. PND for SCD was introduced in Cameroon in 2007, following a series debate on value judgement of what is considered a life worth living, the possibility of stepping down the slippery slope towards eugenics, and the legality of TAP [14].

Attitude to termination of an affected pregnancy for SCD

In actual practice, most clients opted for termination of SCD-affected pregnancies (90%); amongst them, 93·9% had either a SCD-affected child or sibling [15, 16]. Similarly, in Nigeria, following introduction of PND for SCD up to 96 % the women with SCD-affected foetuses terminated the pregnancies; among them, 51% previously had children with SCD [17]. In Cameroon, The possibility of having a child with SCD was not realized by 85·7% before marriage. Up to 21·4% of clients admitted to having voluntarily terminated at least one pregnancy, in the past, for fear of having a SCD-affected child [16]. This was a surprising revelation; since under Cameroonian law, voluntary abortion is a criminal offence, whilst medical abortion is permitted “…if it is done by an authorised professional and justi ed by the need to save the mother from grave health jeopardy” (Act 339; exception 1; The Cameroon Penal Code). The studies signal potential value-based conflicts among Cameroonian students, doctors, parents and patients, regarding principle and practice of TAP for SCD. The conflicts could be partly embedded in the differential perceived severity and curability of SCD, the restrictive abortion laws, cultural values and beliefs, the major burden of SCD on patients and families, as well as the environment compounded by socio-economic difficulties and a dysfunctional health system [6, 13]. We also reported on the increasing burden of SCD and the introduction of PND for SCD in clinical practice in Cape Town, South Africa [18]. In practice, although the majority of parents would like to take the option of PND for SCD, they mostly disagreed with the idea of TAP for SCD [19]. Contrary to the Central Hospital in Yaoundé in Cameroon, at the Red Cross War Memorial Hospital in Cape Town, there is a provision of a specialized free of charge comprehensive clinic, with all possible available care and therapeutic options, for SCD. It is therefore possible that, a more favourable social environment and hospital services could improve the ability of parents and patients to cope with SCD, and lead to a lower desirability of TAP for SCD in Cameroon. This hypothesis will require further investigations in order to frame appropriate policies that allows parents to opt for reproductive options that do not conflicts their societal Ethics [19]. The ethical and legal challenges around the practice of PND prompted the need to develop and extend our studies to the potential use of genetics for secondary prevention in SCD.

Genomics and SCD secondary prevention: preliminary data and perspectives

Genomics of foetal haemoglobin

Despite SCD being a single gene disorder, foetal haemoglobin (HbF) is the most important disease modifier, which is genetically determined by three major loci (BCL11A, HBS1L-MYB and HBB cluster) that are amenable to therapeutic manipulation [20]. Among a group of 610 Cameroonian SCD patients (97% HbSS), the two principal known HbF-promoting loci were significantly associated with HbF in Cameroonian patients [21], as previously reported among African SCD patients from the USA [22], Brazil [22] or Tanzania [23]. In Cameroon, SNPs in BCL11A and HBS1L-MYB loci were, together, associated with about 10% variation in HbF level among SCD patients, and specially, rs28384513 (HMIP 1) and rs9494142 (HMIP 2) were associated with hospitalization rates, reflecting their influence to the severity of the disease in patients [21]. In addition, BCL11A rs4671393 was significantly associated with a wide range of haematological indices; rs28384513 (HMIP 1) with WBC counts, rs9399137 (HMIP 2) with Hb levels, rs9402686 (HMIP 2) and rs9494142 (HMIP 2) were both associated with MCV [21]. We also replicated the finding previously reported among African American SCD patients that, in combination, HBS1L-MYB rs9399137, and BCL11A rs4671393 SNPs affected platelet counts [22]. Equally, the effect of these HbF-promoting loci on the haematological phenotype in SCD was reported in Tanzania [24]. Understanding the effect of haematological variables-associated variants will require additional data from various SCD populations from Africa.

The co-inheritance of α-thalassemia and SCD

In Cameroon, the co-inheritance of 3·7 α-globin gene deletion and SCD was associated with late SCD onset and possibly improved patients’ survival, that could explained the much higher allele frequency of 3·7kb α-globin gene deletion among SCD patients (~40%) than HbAA controls (~10%) [25]. Co-inheritance of SCD and α-thalassemia was associated with lower consultation rate among Cameroonian (p = 0·038), implying a lesser severity of disease, that could be attributed to its association with improved haematological indices [26]. In generalised linear regression models, adjusted for age, sex and HbF-promoting SNPs, the positive effects of the co-inheritance of α-thalassemia on RBC count, MCV and lymphocytes count were still observed among Cameroonian [26]. In addition, among African Americans and Tanzanians, the co-inheritance of α-thalassemia and SCD was associated with a lower stroke risk [27, 28].

Others selected genomic variants associated to SCD phenotypes

Specific phenotypes have been associated with genomic variations in SCD among African American. One mutation in GOLGB1 (Y1212C) and another mutation in ENPP1 (K173Q) were confirmed as having significant associations with a decreased risk for stroke [29]. Seven SNPs in the myosin, heavy chain 9, non-muscle (MYH9) and one in apolipoprotein L1 (APOL1) have been associated with risk for focal segmental glomerulosclerosis and end-stage renal disease [30]. Genetic association with elevated tricuspid regurgitation jet velocity and pulmonary hypertension was revealed with five SNPs within GALNT13, and quantitative trait locus upstream of the adenosine-A2B receptor gene (ADORA2B) [31]. Children with two shorter alleles (4%; ≤ 25 repeats) of the is the highly polymorphic (GT)(n) dinucleotide repeat located in the promoter region of the HMOX1 gene had lower rates of hospitalization for acute chest syndrome (ACS) [32]. All these variants that affect cardiovascular phenotypes and ACS deserve to be investigated in SCD patients in Africa, to fully appreciate their potential clinical values.

Together, the preliminary data from Cameroon are the first evidence, from the African continent, of the clinical effect that is associated with selected HbF promoting SNPs and the co-inheritance of α-thalassemia and SCD. These results could suggest that clinical genotyping of these variants and others, may potentially be useful to risk stratify SCD patients, and to serve as a guide to adjust therapeutic and follow-up plans.

Public health policy actions

The postgenome era promises can offer new insights into the nature of SCD, its prevention and treatment; sub-Saharan African countries need urgent societal and policies adjustment to this [33]. The data indicate the urgent need to develop and implement, in Cameroon, policy actions at least five levels [19]: 1) the implementation of the national control program of SCD with screening policies that put emphasis on premarital detection to, hopefully, reduce the number of cases that will be referred to PND; 2) to implement policies and practices that improve the early detection and care of SCD, e.g. neonatal screening, the use of hydoxyurea and the development of specialized centre care for SCD patients; 3) to frame social policies for birth defects and disabilities incorporating socio-economic supports to alleviate the burden of SCD on affected families; 4) to address the appropriateness of the Cameroonian abortion laws e.g. with clarity on the maternal distress, due to foetal anomalies like SCD, as acceptable justification for medical abortion; 5) lastly to develop a national plan for genetic medicine, including a specific research program for SCD, and the related genetic education for professional and the general public.

Conclusion

The differential acceptability in principle and practice of TAP for SCD in Cameroon has revealed the potential value-based conflicts among various population groups. The molecular data support some perspectives in the clinical genotyping of α-globin gene deletion, HbF-promoting genomic variants, and others, yet to be found, that may potentially be useful to risk stratify SCD patients, and to serve as a guide for therapeutic and follow up strategies. The present report summarised a modest body of work from a single African country, which in addition to other report, could be of unique value to some developing settings willing to explore the use of genetics in SCD prevention; indeed, PND of haemoglobinopahies represents the point of entry of genetic medicine in many developing countries [34]. Finally, these data was generated in Africa by African scientists; this could contribute to fast-tract the most needed regional capacity-building in both social sciences coupled with genetic research [35], to offer different perspectives to improve the care of SCD patients Africa.