出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2014/04/30 22:51:14」(JST)
| Thalassemia | |
|---|---|
| Classification and external resources | |
| ICD-10 | D56 |
| ICD-9 | 282.4 |
| OMIM | 141800 141850 142310 604131 141800 141850 142310 604131 |
| DiseasesDB | 448 33334 33678 3087 |
| MedlinePlus | 000587 |
| eMedicine | article/958850 article/206490 article/955496 article/396792 |
| MeSH | D013789 |
| GeneReviews |
|
Thalassemia (British English: thalassaemia) are forms of inherited autosomal recessive blood disorders that originated in the Mediterranean region. In thalassemia, the disorder is caused by the weakening and destruction of red blood cells. Thalassemia is caused by variant or missing genes that affect how the body makes hemoglobin. Hemoglobin is the protein in red blood cells that carries oxygen. People with thalassemia make less hemoglobin and have fewer circulating red blood cells than normal, which results in mild or severe anemia. Thalassemia will be present as microcytic anemia.
Thalassemia can cause significant complications, including iron overload, bone deformities and cardiovascular illness. However this same inherited disease of red blood cells may confer a degree of protection against malaria, which is or was prevalent in the regions where the trait is common. This selective survival advantage of carriers (known as heterozygous advantage) may be responsible for perpetuating the mutation in populations. In that respect, the various thalassemias resemble another genetic disorder affecting hemoglobin, sickle-cell disease.[1] [2]
Both α and β thalassemias are often inherited in an autosomal recessive fashion, although this is not always the case. Cases of dominantly inherited α and β thalassemias have been reported, the first of which was in an Irish family with two deletions of 4 and 11 bp in exon 3 interrupted by an insertion of 5 bp in the β-globin gene. For the autosomal recessive forms of the disease, both parents must be carriers in order for a child to be affected. If both parents carry a hemoglobinopathy trait, there is a 25% risk with each pregnancy for an affected child. Genetic counseling and genetic testing is recommended for families that carry a thalassemia trait.
There are an estimated 60-80 million people in the world carrying the beta thalassemia trait.[citation needed] This is a rough estimate; the actual number of those thalassemia major is unknown due to the prevalence of thalassemia in less developed countries.[citation needed] Countries such as Nepal, Bangladesh and Pakistan are seeing a large increase of thalassemia patients due to lack of genetic counseling and screening.[citation needed] There is growing concern that thalassemia may become a very serious problem in the next 50 years, one that will burden the world's blood bank supplies and the health system in general.[citation needed] There are an estimated 1,001 people living with thalassemia major in the United States and an unknown number of carriers.[citation needed] Because of the prevalence of the disease in countries with little knowledge of thalassemia, access to proper treatment and diagnosis can be difficult.[citation needed]
Having a single gene for thalassemia may protect against malaria and thus be an advantage.[5]
People diagnosed with heterozygous (carrier) β thalassemia have some protection against coronary heart disease.[6]
Normally, the majority of adult hemoglobin (HbA) is composed of four protein chains, two α and two β globin chains arranged into a heterotetramer. In thalassemia, patients have defects in either the α or β globin chain causing production of abnormal red blood cells (In sickle-cell disease, the mutation is specific to β globin).
The thalassemias are classified according to which chain of the hemoglobin molecule is affected. In α thalassemias, production of the α globin chain is affected, while in β thalassemia production of the β globin chain is affected.
The β globin chains are encoded by a single gene on chromosome 11; α globin chains are encoded by two closely linked genes on chromosome 16.[7] Thus, in a normal person with two copies of each chromosome, there are two loci encoding the β chain, and four loci encoding the α chain. Deletion of one of the α loci has a high prevalence in people of African or Asian descent, making them more likely to develop α thalassemias. β Thalassemias are not only common in Africans, but also in Greeks and Italians.
The α thalassemias involve the genes HBA1[8] and HBA2,[9] inherited in a Mendelian recessive fashion. There are two gene loci and so four alleles. It is also connected to the deletion of the 16p chromosome. α Thalassemias result in decreased alpha-globin production, therefore fewer alpha-globin chains are produced, resulting in an excess of β chains in adults and excess γ chains in newborns. The excess β chains form unstable tetramers (called Hemoglobin H or HbH of 4 beta chains), which have abnormal oxygen dissociation curves.
Beta thalassemias are due to mutations in the HBB gene on chromosome 11,[10] also inherited in an autosomal-recessive fashion. The severity of the disease depends on the nature of the mutation. Mutations are characterized as either βo or β thalassemia major if they prevent any formation of β chains, the most severe form of β thalassemia. Also, they are characterized as β+ or β thalassemia intermedia if they allow some β chain formation to occur. In either case, there is a relative excess of α chains, but these do not form tetramers: Rather, they bind to the red blood cell membranes, producing membrane damage, and at high concentrations they form toxic aggregates.
| It has been suggested that Delta-thalassemia be merged into this article. (Discuss) Proposed since March 2014. |
As well as alpha and beta chains present in hemoglobin, about 3% of adult hemoglobin is made of alpha and delta chains. Just as with beta thalassemia, mutations that affect the ability of this gene to produce delta chains can occur.[citation needed]
Thalassemia can co-exist with other hemoglobinopathies. The most common of these are:
Mild thalassemia: people with thalassemia traits do not require medical or follow-up care after the initial diagnosis is made.[11] People with β-thalassemia trait should be warned that their condition can be misdiagnosed as the more common iron deficiency anemia. They should avoid routine use of iron supplements; yet iron deficiency can develop during pregnancy or from chronic bleeding.[12] Counseling is indicated in all persons with genetic disorders, especially when the family is at risk of a severe form of disease that may be prevented.[13]
Severe thalassemia: People with severe thalassemia require medical treatment. A blood transfusion regimen was the first measure effective in prolonging life.[11]
Multiple blood transfusions can result in iron overload. The iron overload related to thalassemia may be treated via chelation therapy with the medications deferoxamine, deferiprone or deferasirox.[14] These treatments have resulted in improved life expectancy in those with thalassemia major.[14]
Deferoxamine is only effective via daily injections which makes its long term use more difficult. It has the benefit of being inexpensive and decent long term safety. Adverse effects are primary skin reactions around the injection site and hearing loss.[14]
Deferasirox has the benefit of being an oral medication. Common side effects include: nausea, vomiting and diarrhea. It however is not effective in everyone and is probably not suitable in those with significant cardiac issues related to iron overload. The cost is also significant.[14]
Deferiprone is given as an oral medication. Nausea, vomiting and diarrhea is relatively common with its use. While available in Europe as of 2010 it is not available in North America. It appears to be the most effective agent when the heart is significantly involved.[14]
Bone marrow transplantation may offer the possibility of a cure in young people who have an HLA-matched donor.[17] Success rates have been in the 80–90% range.[17] Mortality from the procedure is about 3%.[18]
If the person does not have an HLA-matched compatible donor such as the first curative method requires, there is another curative method called Bone Marrow Transplantation(BMT) from haploidentical mother to child (mismatched donor), in which the donor is the mother. It was invented in 2002 by Dr. Pietro Sodani. The results are these: thalassemia-free survival rate 70%, rejection 23%, and mortality 7%. The best results are with very young patients.[19]
The beta form of thalassemia is particularly prevalent among Mediterranean peoples and this geographical association is responsible for its naming[citation needed]. Globally in 2010 it resulted in about 18,000 deaths.[20]
In Europe, the highest concentrations of the disease are found in Greece, coastal regions in Turkey (particularly the Aegean Region such as Izmir, Balikesir, Aydin, Mugla, and Mediterranean Region such as Antalya, Adana, Mersin), in parts of Italy, particularly Southern Italy and the lower Po valley. The major Mediterranean islands (except the Balearics) such as Sicily, Sardinia, Malta, Corsica, Cyprus, and Crete are heavily affected in particular. Other Mediterranean people, as well as those in the vicinity of the Mediterranean, also have high rates of thalassemia, including people from West Asia and North Africa. Far from the Mediterranean, South Asians are also affected, with the world's highest concentration of carriers (16% of the population) being in the Maldives.
Nowadays, it is found in populations living in Africa, the Americas and also, in Tharu people in the Terai region of Nepal and India.[21] It is believed to account for much lower malaria sicknesses and deaths,[22] accounting for the historic ability of Tharus to survive in areas with heavy malaria infestation, where others could not. Thalassemias are particularly associated with people of Mediterranean origin, Arabs (especially Palestinians and people of Palestinian descent), and Asians.[23] The Maldives has the highest incidence of Thalassemia in the world with a carrier rate of 18% of the population. The estimated prevalence is 16% in people from Cyprus, 1%[24] in Thailand, and 3–8% in populations from Bangladesh, China, India, Malaysia and Pakistan. Thalassemias also occur in descendants of people from Latin America and Mediterranean countries (e.g. Greece, Italy, Portugal, Spain, and others).
The name of this condition derives from the Greek Thalassa (θάλασσα), sea, and haema (αἷμα), blood. The term was first used in 1932.
In 2008, in Spain, a baby was selectively implanted in order to be a cure for his brother's thalassemia. The child was born from an embryo screened to be free of the disease before implantation with in vitro fertilization. The baby's supply of immunologically compatible cord blood was saved for transplantation to his brother. The transplantation was considered successful.[25] In 2009, a group of doctors and specialists in Chennai and Coimbatore registered the successful treatment of thalassemia in a child using an unaffected sibling's umbilical cord blood.[26]
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
全文を閲覧するには購読必要です。 To read the full text you will need to subscribe.
| リンク元 | 「重症型サラセミア」「thalassemia major」 |
| 関連記事 | 「major」 |