Thalassemias, Abnormal Hemoglobins and Glucose-6-Phosphate Dehydrogenase Deficiency in the Arta Area of Greece: Diagnostic and Genetic Aspects of Complete Village Studies*

Genome sequencing and implications for rare disorders

The practice of genomic medicine stands to revolutionize our approach to medical care, and to realize this goal will require discovery of the relationship between rare variation at each of the ~ 20,000 protein-coding genes and their consequent impact on individual health and expression of Mendelian disease. The step-wise evolution of broad-based, genome-wide cytogenetic and molecular genomic testing approaches (karyotyping, chromosomal microarray [CMA], exome sequencing [ES]) has driven much of the rare disease discovery to this point, with genome sequencing representing the newest member of this team. Each step has brought increased sensitivity to interrogate individual genomic variation in an unbiased method that does not require clinical prediction of the locus or loci involved. Notably, each step has also brought unique limitations in variant detection, for example, the low sensitivity of ES for detection of triploidy, and of CMA for detection of copy neutral structural variants. The utility of genome sequencing (GS) as a clinical molecular diagnostic test, and the increased sensitivity afforded by addition of long-read sequencing or other -omics technologies such as RNAseq or metabolomics, are not yet fully explored, though recent work supports improved sensitivity of variant detection, at least in a subset of cases. The utility of GS will also rely upon further elucidation of the complexities of genetic and allelic heterogeneity, multilocus rare variation, and the impact of rare and common variation at a locus, as well as advances in functional annotation of identified variants. Much discovery remains to be done before the potential utility of GS is fully appreciated.

L'hypérostose poreuse du crâne, les anémies héréditaires et le paludisme en Grèce

De prime abord on pourrait s'étonner de trouver réunis sous un seul titre des états pathologiques aussi disparates qu'une modification morphologique particulière des crânes anciens, un groupe d'anomalies héréditaires du sang et une fièvre d'origine parasitaire. Ce rapprochement n'est point fortuit : l'étude de chacun des volets de ce triptyque médico-historique peut éclairer singulièrement les deux autres.

On pourrait s'étonner également de voir figurer cet article dans un ensemble de textes consacrés à l'Histoire des Sciences, car on s'aperçoit d'emblée que son objet n'est point la pensée scientifique d'antan. Il s'agit pourtant d'un essai qui relève essentiellement de cette discipline dans la mesure où elle est le lieu privilégié de rencontre des recherches scientifiques (au sens restreint) et des études historiques. Le domaine d'une investigation se définit au moins autant par les méthodes utilisées que par l'objet examiné.

Phenotypic expansion illuminates multilocus pathogenic variation

Purpose:

Multilocus variation, pathogenic variants in two or more disease genes, can potentially explain the underlying genetic basis for apparent phenotypic expansion in cases for which the observed clinical features extend beyond those reported in association with a “known” disease gene.

Methods:

Analyses focused on 106 patients, 19 for which apparent phenotypic expansion was previously attributed to variation at known disease genes. We performed a retrospective computational re-analysis of whole exome sequencing data using stringent Variant Call File filtering criteria to determine whether molecular diagnoses involving additional disease loci might explain the observed expanded phenotypes.

Results:

Multilocus variation was identified in 31.6% (6/19) of families with phenotypic expansion and 2.3% (2/87) without phenotypic expansion. Intrafamilial clinical variability within 2 families was explained by multilocus variation identified in the more severely affected sibling.

Conclusions:

Our findings underscore the role of multiple rare variants at different loci in the etiology of genetically and clinically heterogeneous cohorts. Intrafamilial phenotypic and genotypic variability allowed a dissection of genotype-phenotype relationships in 2 families. Our data emphasize the critical role of the clinician in diagnostic genomic analyses and demonstrate that apparent phenotypic expansion may represent blended phenotypes resulting from pathogenic variation at more than one locus.

Hemoglobin Knossos: a clinical, laboratory, and epidemiological study

Hb Knossos is a beta-chain variant (beta 27 Ser----Ala) that is unrecognizable by conventional separation methods but detectable by globin electrophoresis on urea-Triton X-acrylamide gels or by IEF. Hb Knossos is characterized by reduced synthesis and by interaction with beta-thalassemia, in which the double heterozygotes display typical features of thalassemia intermedia. The present paper summarizes the salient genetic, clinical, and biochemical characteristics of five such cases hitherto identified in three families along with the same features on 12 heterozygous Hb Knossos carriers. Hb Knossos displays a slightly decreased oxygen affinity; this factor may compensate in part for the severe anemia of the double heterozygotes. Hb Knossos is relatively rare in our population, since a prospective survey on 610 individuals has failed to disclose any heterozygotes. However, the mutation appears to have spread over the Mediterranean countries and may be more common elsewhere.

Five families with homozygous delta-thalassaemia in Japan

Five families with delta-thalassaemia discovered in Ehime, Japan, are presented. The delta-thalassaemia was associated with a slight elevation of the level of Hb F in two families and with normal Hb F levels in three. Complete absence of HbA2 was found in the homozygous probands. No abnormal clinical or haematological findings were noted in the individuals with delta-thalassaemia.

Heterogeneity in beta 0 thalassemia from Algeria: genetic, clinical and molecular studies

Six Algerian patients with beta 0 thalassemia are presented, in addition to the two patients already reported (Godet et al., 1977). Family studies indicate that all the patients had homozygous beta thalassemia characterized by absence of beta globin chain synthesis in peripheral blood. The clinical severity varies from one family to the other and within the same family, from typical Cooley's anemia to thalassemia intermedia and appears to be related to the child death rate observed in each family. The gamma/alpha biosynthetic ratio was 0.36-0.40 in seven patients and 0.2 in the most seriously affected patient. The mRNA beta content in peripheral reticulocytes was less than 1.5% of mRNA alpha in seven patients and 13.3% in one patient. These results indicate that Algerians homozygous for beta 0 thalassemia are heterozygous at the clinical, biochemical and molecular levels.

Homozygous delta thalassemia in Japan

Six families with delta-thalassemia have been found in Japan, in which 10 individuals are delta-thalassemia homozygotes with complete deficiency of Hb A2 and 18 individuals are heterozygotes with low levels of Hb A2. In three families with 10 individuals among these, persistence of fetal hemoglobin of Swiss type was observed. Almost all the members of these families were free from clinical symptoms and hematological abnormalities of the red cells.

The heterogeneity of normal Hb A2-beta thalassaemia in Greece

Nine patients have been observed with homozygous beta thalassaemia in each of whom one parent has a normal level of Hb A2. On the basis of clinical, haematological and globin chain synthesis studies these families have been divided into two groups. Group 1 (six families). Heterozygotes for normal Hb A2-beta thalassaemia in this group showed minimal red cell abnormalities, normal osmotic fragility but imbalanced globin chain synthesis (alpha/beta=1.6), and appear to correspond to previous descriptions of 'silent' beta thalassaemia. Double heterozygotes with high Hb A2-beta thalassaemia have a clinical picture of mild beta thalassaemia intermedia characterized by relatively low levels of Hb F (less than 20%) and gamma chain synthesis. Group 2 (three families). beta Thalassaemia heterozygotes with normal HbA2 levels in this group showed more marked red cell abnormalities, decreased osmotic fragility and more imbalanced globin chain synthesis (alpha/beta=2.5) than those in group I. Double heterozygotes with high Hb A2-beta thalassaemia are more severely affected and are transfusion dependent. Haemoglobin F and gamma chain synthesis are high in these cases. The frequency of normal Hb A2-beta thalassaemia in Greece may be as high as 10% of all beta thalassaemia genes and this poses a significant problem for genetic counselling. Various molecular mechanisms are discussed which could account for the heterogeneity within these disorders.

Variation in hemoglobin A2

The structure, properties and function of, and some biosynthetic and genetic aspects of, Hb A2 are described. The structural variants of Hb A2 are reviewed and their geographical distribution presented. Hb A2, Hb A2-Flatbush and Hb A2-Babinga are characteristic of negro populations and may have originated in Western or Central Africa. Hb A2-Sphakia is characteristic of Canadian Amerindian and Hb A2-Indonesia of Indonesian/Malay populations. Hb A2-NYU has only been found sporadically and most frequently in persons of Eastern European origin. The other three variants of Hb A2 have only been reported in a single person or in single families. Some conditions which are associated with changes in Hb A2 levels are reviewed.

Haemoglobin Lepore in Cyprus

Structural analysis documented the presence of haemoglobin LeporeWashington (=LeporeBoston) in a Greek Cypriot family and provided further evidence that, of the various types of Lepore mutants, only one is common in the Mediterranean area. Two individuals in this family were heterozygous for both Hb Lepore and beta thalassaemia, but they exhibited striking differences in the clinical severity and course of the disease. The data illustrate that additional environmental or genetic factors play roles in determining or modifying the pathophysiological consequences of highly specific molecular defects and, thus, their ultimate clinical phenotypes.