Genotypes and Phenotypes — Another Lesson from the Hemoglobinopathies
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《新英格兰医药杂志》
The human genome project has ignited considerable excitement about the future of medicine by revealing insights into diseases that seemed impossible to unravel only a few years ago. But knowledge of genes and proteins cannot by itself explain clinical phenomena — for this, molecular genetics requires the context of pathophysiology. The puzzling ways in which diseases manifest themselves in individual patients serve as a reminder that using genomics to improve clinical medicine will not be easy.
The report by Geva and colleagues in this issue of the Journal (pages 1532–1538) illustrates nicely how an astute combination of molecular biology and pathophysiology can explain a disease. The authors describe a child who, at 19 months of age, had a classic symptom of sickle cell anemia, splenic sequestration crisis, during an airplane flight. This event was puzzling, however, because the child did not appear to have inherited the classic abnormal globin genotype (S/S) or any of the other gene combinations associated with sickling syndromes, such as coinheritance of hemoglobin S (HbS) and hemoglobin C (HbC) — or HbSC (2SC) disease. Patients with typical sickle cell anemia are homozygous for HbS, an allele of the globin gene that encodes valine instead of glutamic acid at amino acid position 6 (Glu6Val). In the low oxygen tension of capillary–venous blood, HbS polymerizes into fibers that disrupt red-cell homeostasis, damage the membrane, and distort the shape of red cells. The rigid, adherent erythrocytes clog small venules, causing episodic tissue ischemia. These malformations of the erythrocyte can cause hemolytic anemia, painful crises, retinopathy, aseptic bone necrosis, and nephropathy. Acute sickling in the splenic sinuses produces the sequestration crisis, in which inflowing blood cannot traverse the obstructed microvasculature, causing a life-threatening trapping of blood.
Using powerful methods to analyze their patient's globin genes, Geva et al. found that she had inherited a normal globin allele from her mother and an apparent S allele from her father. This combination produces the sickle cell trait, which is asymptomatic except under conditions of severe hypoxic stress. Why, then, did the girl become so ill?
The explanation was found in the sequence of the S allele. A second mutation was present, coding for leucine instead of phenylalanine at amino acid 68. The authors dubbed the doubly mutated globin "hemoglobin Jamaica Plain" (Hb JP, or Glu6Val,Leu68Phe) and demonstrated convincingly that the second alteration was due to a mutation of the S allele that most likely occurred in a paternal spermatid.
By itself, Leu68Phe is known as hemoglobin Rockford, a member of a class of "low-affinity hemoglobins" with reduced affinity for oxygen. These hemoglobins cause few symptoms — and often none at all. Why? Normal hemoglobin (hemoglobin A, or 22) becomes fully saturated with oxygen at a partial pressure of oxygen of 60 to 70 mm Hg, well below the partial pressure of oxygen in alveoli. Low-affinity hemoglobins become adequately saturated in the lungs and release oxygen much more readily at capillary oxygen tension, supplying plenty of oxygen to the tissues. Thus, they are usually harmless.
What happens when these two amino acid substitutions coexist on the same globin molecule? The Leu68Phe mutation causes Hb JP to desaturate easily and therefore to sickle more readily than HbS (see Figure). This mechanism accounts for the patient's clinical picture. The child had a respiratory infection, and the sequestration crisis occurred in the lowered air pressure of an airplane cabin at high altitude. These stresses probably uncovered the latent pathologic features of the doubly mutated globin.
Figure. Desaturation of Normal and Variant Hemoglobins during Passage from Artery to Vein.
Hemoglobins lose oxygen during the journey from artery to vein. Cells with HbS sickle only when desaturated. The likelihood that they will obstruct the vessel depends on the concentration of HbS and the extent of desaturation.
Two opposing messages may be taken from this case report. On the one hand, it validates the power of modern molecular medicine. Geva et al. were able to describe this child's genotype precisely and to formulate an elegant and logical explanation for her symptoms. On the other hand, their report is a sobering reminder that currently we can link mutations to clinical manifestations in only a very few diseases.
Consider the critical elements used to explain the child's illness. Genotypic characterization was essential but hardly sufficient. The genotype explained the "misbehavior" of the mutant hemoglobin only because we have a deep understanding of the relationships between structure and function in the globin proteins, the interactions among globin proteins that result in the formation of hemoglobin, their functions in vivo, and the myriad abnormal phenotypes associated with hundreds of variants arising from amino acid changes at almost every position of each globin. The behavior of Hb JP in the patient made sense only in the context of the detailed mechanisms of oxygen transport, the hemodynamics of normal and sickled red cells in the microvasculature, and the physiological effects of the environmental stress precipitating the crisis.
In the parlance of genomics, contextual information about the physiologic and pathologic behavior of proteins encoded by a gene is called "annotation." This case illustrates how thorough and extensive annotation must be in order to provide a basis for a mechanistic connection between a gene and the clinical picture. No other genes are annotated as completely as the human globin genes. Massive efforts to develop similar annotations must be undertaken if we are to exploit fully our ability to implicate individual genes in the cause or modification of and susceptibility to specific illnesses.
The example of the hemoglobinopathies emphasizes the need for a balanced portfolio of research themes and styles. Investments in cutting-edge genomics must be matched by support for clinical research focused on achieving a deeper mechanistic understanding of human diseases as they behave in individual patients. We need intensified investment in each area in order to deliver on the promise of human genomics.
Source Information
From the Dana–Farber Cancer Institute, Boston.(Edward J. Benz, Jr., M.D.)
The report by Geva and colleagues in this issue of the Journal (pages 1532–1538) illustrates nicely how an astute combination of molecular biology and pathophysiology can explain a disease. The authors describe a child who, at 19 months of age, had a classic symptom of sickle cell anemia, splenic sequestration crisis, during an airplane flight. This event was puzzling, however, because the child did not appear to have inherited the classic abnormal globin genotype (S/S) or any of the other gene combinations associated with sickling syndromes, such as coinheritance of hemoglobin S (HbS) and hemoglobin C (HbC) — or HbSC (2SC) disease. Patients with typical sickle cell anemia are homozygous for HbS, an allele of the globin gene that encodes valine instead of glutamic acid at amino acid position 6 (Glu6Val). In the low oxygen tension of capillary–venous blood, HbS polymerizes into fibers that disrupt red-cell homeostasis, damage the membrane, and distort the shape of red cells. The rigid, adherent erythrocytes clog small venules, causing episodic tissue ischemia. These malformations of the erythrocyte can cause hemolytic anemia, painful crises, retinopathy, aseptic bone necrosis, and nephropathy. Acute sickling in the splenic sinuses produces the sequestration crisis, in which inflowing blood cannot traverse the obstructed microvasculature, causing a life-threatening trapping of blood.
Using powerful methods to analyze their patient's globin genes, Geva et al. found that she had inherited a normal globin allele from her mother and an apparent S allele from her father. This combination produces the sickle cell trait, which is asymptomatic except under conditions of severe hypoxic stress. Why, then, did the girl become so ill?
The explanation was found in the sequence of the S allele. A second mutation was present, coding for leucine instead of phenylalanine at amino acid 68. The authors dubbed the doubly mutated globin "hemoglobin Jamaica Plain" (Hb JP, or Glu6Val,Leu68Phe) and demonstrated convincingly that the second alteration was due to a mutation of the S allele that most likely occurred in a paternal spermatid.
By itself, Leu68Phe is known as hemoglobin Rockford, a member of a class of "low-affinity hemoglobins" with reduced affinity for oxygen. These hemoglobins cause few symptoms — and often none at all. Why? Normal hemoglobin (hemoglobin A, or 22) becomes fully saturated with oxygen at a partial pressure of oxygen of 60 to 70 mm Hg, well below the partial pressure of oxygen in alveoli. Low-affinity hemoglobins become adequately saturated in the lungs and release oxygen much more readily at capillary oxygen tension, supplying plenty of oxygen to the tissues. Thus, they are usually harmless.
What happens when these two amino acid substitutions coexist on the same globin molecule? The Leu68Phe mutation causes Hb JP to desaturate easily and therefore to sickle more readily than HbS (see Figure). This mechanism accounts for the patient's clinical picture. The child had a respiratory infection, and the sequestration crisis occurred in the lowered air pressure of an airplane cabin at high altitude. These stresses probably uncovered the latent pathologic features of the doubly mutated globin.
Figure. Desaturation of Normal and Variant Hemoglobins during Passage from Artery to Vein.
Hemoglobins lose oxygen during the journey from artery to vein. Cells with HbS sickle only when desaturated. The likelihood that they will obstruct the vessel depends on the concentration of HbS and the extent of desaturation.
Two opposing messages may be taken from this case report. On the one hand, it validates the power of modern molecular medicine. Geva et al. were able to describe this child's genotype precisely and to formulate an elegant and logical explanation for her symptoms. On the other hand, their report is a sobering reminder that currently we can link mutations to clinical manifestations in only a very few diseases.
Consider the critical elements used to explain the child's illness. Genotypic characterization was essential but hardly sufficient. The genotype explained the "misbehavior" of the mutant hemoglobin only because we have a deep understanding of the relationships between structure and function in the globin proteins, the interactions among globin proteins that result in the formation of hemoglobin, their functions in vivo, and the myriad abnormal phenotypes associated with hundreds of variants arising from amino acid changes at almost every position of each globin. The behavior of Hb JP in the patient made sense only in the context of the detailed mechanisms of oxygen transport, the hemodynamics of normal and sickled red cells in the microvasculature, and the physiological effects of the environmental stress precipitating the crisis.
In the parlance of genomics, contextual information about the physiologic and pathologic behavior of proteins encoded by a gene is called "annotation." This case illustrates how thorough and extensive annotation must be in order to provide a basis for a mechanistic connection between a gene and the clinical picture. No other genes are annotated as completely as the human globin genes. Massive efforts to develop similar annotations must be undertaken if we are to exploit fully our ability to implicate individual genes in the cause or modification of and susceptibility to specific illnesses.
The example of the hemoglobinopathies emphasizes the need for a balanced portfolio of research themes and styles. Investments in cutting-edge genomics must be matched by support for clinical research focused on achieving a deeper mechanistic understanding of human diseases as they behave in individual patients. We need intensified investment in each area in order to deliver on the promise of human genomics.
Source Information
From the Dana–Farber Cancer Institute, Boston.(Edward J. Benz, Jr., M.D.)