Symptoms vary for the different diseases: in sickle cell disease, for example, the red blood cells tend to assume a different shape under anaerobic conditions, leading to organ damage and circulatory problems while in alpha/beta thalassemias there is a reduced synthesis of the alpha/beta hemoglobin chain that results in microcytic hypochromic anemia, an abnormal peripheral blood smear with nucleated red blood cells, and reduced amounts of hemoglobin A.
The genes involved are HBA1, HBA2, HBB, HBZ, HBQ1, HBG1, HBG2, HBD, HBE1, with the first three playing the major role. The globin genes are located in two clusters at chromosome 11p15.5 and chromosome 16pter-p13.3, respectively. Despite the marked molecular heterogeneity, the prevalent molecular defects are limited in each at-risk population, albeit there is a greater than ever population heterogeneity in Europe and in all other developed countries. HbVar (http://globin.bx.psu.edu/hbvar) is a database of human hemoglobin variants and thalassemias that now (Aug 23, 2007) includes 1,323 entries of which 1.078 (81%) are substitution mutations, 13% small insertion/deletions, while 4% are larger (<200bp) deletions or fusion events.
The genotyping methods are mainly based on PCR and sequencing, with MLPA and DNA chips for rapid genotyping of commonest mutations. The existing platforms include the arrayed primer extension developed on the APEX technology (http://www.asperbio.com) that provides the parallel detection of forty common mutations of the HBB and glucose 6-phosphate genes and six mutations from the HBD genes.
One of the Techgene partners has already experimented with microarray technology and melting curve analysis with the aim to develop possible applications of these methods to population screening and diagnostics. In a recent review by the EU project ITHANET the actual state of high throughput microarray technologies has been described.