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ژنتیک - سندرم کیوتی/QT syndrome

سندرم کیوتی/QT syndrome

نویسنده : ریحانه دره گزنی شنبه 16 شهریور 1392 07:01 ب.ظ  •   

سندرم کیوتی/QT syndrome

سندرم کیو.تی (به انگلیسی: QT syndrome) یک اختلال مادرزادی است، که به صورت افزایش اینتروالQT در نوارقلب(ECG) خود را نشان می‌دهد و ممکن است به آریتمی‌های بطنی و متعاقباً به سنکوپ و ایست قلبی و مرگ ناگهانی ختم شود. شایعترین تظاهر کلینیکی در بیماران دچار LQT، حملهٔ ناگهانی یا ایست قلبی می‌باشد. شیوع بیماری در حد ۱۰۰۰۰:۱ نفر می‌باشد وقوع این بیماری در تمام نقاط دنیا تقریباً یکسان است. بیماران دچار QT معمولاً دارای حملات قلبی مانند سنکوپ، ایست قلبی و یا مرگ ناگهانی در زمان کودکی و اوایل بلوغ می‌باشند اما در دههٔ پنجم نیز ممکن است ظاهر شود. خطر مرگ و میر LQTs در پسران بیشتر از دختران در سنین کمتر از ۱۰ سال است سالیانه حدود ۴۰۰۰ مورد مرگ به علت QT در ایالات متحدهٔ آمریکا رخ می‌دهد که ۶ درصد موارد تا سن ۴ سالگی می‌باشد.
---------------------------------ٌ
ژنتیک:
LQTS می توانیددر اثر یکی از چند جهش ژنی بوجود آید.
این جهش تمایل به طولانی تر کردن طول مدت پتانسیل عمل بطن و در نتیجه طولانی شدن فاصله QT دارد. این جهش می تواند به صورت اتوزومال غالب یا اتوزومال مغلوب به ارث برسد. فرم اتوزومال مغلوب LQTS ها تمایل دارند که یک فنوتیپ شدید تر همراه با ناشنوایی عصبی مادرزادی ایجاد کنند.
آزمایش ژنتیک بالینی برای LQTS ها در دسترس است و ممکن است به هدایت درمان های مناسب کمک کند.
علل شایع ترین LQTSها، جهش در ژن های KCNQ1 (LQT1)، KCNH2 (LQT2)، و SCN5A (LQT3) هستند!

در LQ نوع ۱ عامل محرک در ایست قلبی، ورزش و شنا کردن است و تماس ناگهانی آب سرد با صورت بیمار باعث تحریک رفلکس واگ می‌شود.

تشخیص:
LQT معمولاً بعد از اینکه شخصی یک حملهٔ قلبی مانند سنکوپ یا ایست قلبی داشته باشد تشخیص داده می‌شود و یا بعد از تشخیص LQTs در یک فرد فامیل یا مرگ ناگهانی مورد ارزیابی قرار می‌گیرد که بهترین و قطعی‌ترین روش بررسی ژنتیک می‌باشد.

علائم:
یکی از علائم این اختلال وجود یک نمودار طولانی تر شده از حد نرمال QT در نوار الکتروکاردیوگرام است. اما برخی از افرادی که در معرض خطرند این علامت را نشان نمی‌دهند و درنتیجه تشخیص داده نمی‌شوند. و ۱۰ درصد از آنها تا سن چهل سالگی دچار حمله قلبی می‌شوند.

Long QT syndrome
The long QT syndrome (LQTS) is a rare inherited heart condition in which delayed repolarization of the heart following a heartbeat increases the risk of episodes of torsades de pointes (TDP, a form of irregular heartbeat that originates from the ventricles). These episodes may lead to palpitations, fainting and sudden death due to ventricular fibrillation. Episodes may be provoked by variousstimuli,depending on the subtype of the condition.1
The condition is so named because of the appearances of the electrocardiogram (ECG/EKG), on which there is prolongation of the QT interval. In some individuals the QT prolongation occurs only after the administration of certain medications

genetics and mutation :
LQTS can arise from mutation of one of several gene

These mutations tend to prolong the duration of the ventricular action potential (APD), thus lengthening the QT interval. LQTS can be inherited in an autosomal dominant or an autosomal recessive fashions

The autosomal recessive forms of LQTS tend to have a more severe phenotype, with some variants having associated syndactyly (LQT8) or congenital neural deafness (LQT1). A number of specific gene loci have been identified that are associated with LQTS. Genetic testing for LQTS is clinically available and may help to direct appropriate therapies . The most common causes of LQTS are mutations in the genes KCNQ1 (LQT1), KCNH2 (LQT2), and SCN5A (LQT3); the following is a list of all known genes associated with LQTS:

LQT1
LQT1 is the most common type of long QT syndrome, making up about 30 to 35 percent of all cases. The LQT1 gene is KCNQ1, which has been isolated to chromosome 11p15.5. KCNQ1 codes for the voltage-gated potassium channel KvLQT1 that is highly expressed in the heart. It is believed that the product of the KCNQ1 gene produces an alpha subunit that interacts with other proteins (in particular, the minK beta subunit) to create the IKs ion channel, which is responsible for the delayed potassium rectifier current of the cardiac action potential.
Mutations to the KCNQ1 gene can be inherited in an autosomal dominant or an autosomal recessive pattern in the same family. In the autosomal recessive mutation of this gene, homozygous mutations in KVLQT1 leads to severe prolongation of the QT interval (due to near-complete loss of the IKs ion channel), and is associated with increased risk of ventricular arrhythmias and congenital deafness. This variant of LQT1 is known as the Jervell and Lange-Nielsen syndrome.
Most individuals with LQT1 show paradoxical prolongation of the QT interval with infusion of epinephrine. This can also unmark latent carriers of the LQT1 gene.
Many missense mutations of the LQT1 gene have been identified. These are often associated with a high frequency of syncopes but less sudden death than LQT2.
LQT2
The LQT2 type is the second most common gene location that is affected in long QT syndrome, making up about 25 to 30 percent of all cases. This form of long QT syndrome most likely involves mutations of the human ether-a-go-go related gene (hERG) on chromosome 7. The hERG gene (also known as KCNH2) is part of the rapid component of the potassium rectifying current (IKr). (The IKr current is mainly responsible for the termination of the cardiac action potential, and therefore the length of the QT interval.) The normally functioning hERG gene allows protection against early after depolarizations (EADs).
Most drugs that cause long QT syndrome do so by blocking the IKr current via the hERG gene. These include erythromycin, terfenadine, and ketoconazole. The hERG channel is very sensitive to unintended drug binding due to two aromatic amino acids, the tyrosine at position 652 and the phenylalanine at position 656. These amino acid residues are poised so that a drug binding to them will block the channel from conducting current. Other potassium channels do not have these residues in these positions and are, therefore, not as prone to blockage.
Photo: ‎سندرم کیوتی/QT syndrome

سندرم کیو.تی (به انگلیسی: QT syndrome) یک اختلال مادرزادی است، که به صورت افزایش اینتروالQT در نوارقلب(ECG) خود را نشان می‌دهد و ممکن است به آریتمی‌های بطنی و متعاقباً به سنکوپ و ایست قلبی و مرگ ناگهانی ختم شود. شایعترین تظاهر کلینیکی در بیماران دچار LQT، حملهٔ ناگهانی یا ایست قلبی می‌باشد. شیوع بیماری در حد ۱۰۰۰۰:۱ نفر می‌باشد وقوع این بیماری در تمام نقاط دنیا تقریباً یکسان است. بیماران دچار QT معمولاً دارای حملات قلبی مانند سنکوپ، ایست قلبی و یا مرگ ناگهانی در زمان کودکی و اوایل بلوغ می‌باشند اما در دههٔ پنجم نیز ممکن است ظاهر شود. خطر مرگ و میر LQTs در پسران بیشتر از دختران در سنین کمتر از ۱۰ سال است سالیانه حدود ۴۰۰۰ مورد مرگ به علت QT در ایالات متحدهٔ آمریکا رخ می‌دهد که ۶ درصد موارد تا سن ۴ سالگی می‌باشد.
---------------------------------ٌ
ژنتیک:
LQTS می توانیددر اثر  یکی از چند جهش ژنی بوجود  آید.
این جهش تمایل به طولانی تر کردن طول مدت پتانسیل عمل بطن و در نتیجه طولانی شدن فاصله QT دارد. این جهش می تواند به صورت اتوزومال غالب یا  اتوزومال مغلوب به ارث برسد. فرم اتوزومال مغلوب LQTS ها تمایل دارند که یک فنوتیپ شدید تر همراه با ناشنوایی عصبی مادرزادی ایجاد کنند.
آزمایش ژنتیک بالینی برای LQTS ها  در دسترس است و ممکن است  به هدایت درمان های مناسب کمک کند.
علل شایع ترین LQTSها، جهش در ژن های KCNQ1 (LQT1)، KCNH2 (LQT2)، و SCN5A (LQT3) هستند!

در LQ نوع ۱ عامل محرک در ایست قلبی، ورزش و شنا کردن است و تماس ناگهانی آب سرد با صورت بیمار باعث تحریک رفلکس واگ می‌شود.

تشخیص:
LQT معمولاً بعد از اینکه شخصی یک حملهٔ قلبی مانند سنکوپ یا ایست قلبی داشته باشد تشخیص داده می‌شود و یا بعد از تشخیص LQTs در یک فرد فامیل یا مرگ ناگهانی مورد ارزیابی قرار می‌گیرد که بهترین و قطعی‌ترین روش بررسی ژنتیک می‌باشد.

علائم:
یکی از علائم این اختلال وجود یک نمودار طولانی تر شده از حد نرمال QT در نوار الکتروکاردیوگرام است. اما برخی از افرادی که در معرض خطرند این علامت را نشان نمی‌دهند و درنتیجه تشخیص داده نمی‌شوند. و ۱۰ درصد از آنها تا سن چهل سالگی دچار حمله قلبی می‌شوند.

Long QT syndrome
The long QT syndrome (LQTS) is a rare inherited heart condition in which delayed repolarization of the heart following a heartbeat increases the risk of episodes of torsades de pointes (TDP, a form of irregular heartbeat that originates from the ventricles). These episodes may lead to palpitations, fainting and sudden death due to ventricular fibrillation. Episodes may be provoked by  variousstimuli,depending on the subtype of the condition.1
The condition is so named because of the appearances of the electrocardiogram (ECG/EKG), on which there is prolongation of the QT interval. In some individuals the QT prolongation occurs only after the administration of certain medications

genetics and mutation : 
 LQTS can arise from mutation of one of several gene

These mutations tend to prolong the duration of the ventricular action potential (APD), thus lengthening the QT interval. LQTS can be inherited in an autosomal dominant or an autosomal recessive fashions

The autosomal recessive forms of LQTS tend to have a more severe phenotype, with some variants having associated syndactyly (LQT8) or congenital neural deafness (LQT1). A number of specific gene loci have been identified that are associated with LQTS. Genetic testing for LQTS is clinically available and may help to direct appropriate therapies . The most common causes of LQTS are mutations in the genes KCNQ1 (LQT1), KCNH2 (LQT2), and SCN5A (LQT3); the following is a list of all known genes associated with LQTS:

LQT1
LQT1 is the most common type of long QT syndrome, making up about 30 to 35 percent of all cases. The LQT1 gene is KCNQ1, which has been isolated to chromosome 11p15.5. KCNQ1 codes for the voltage-gated potassium channel KvLQT1 that is highly expressed in the heart. It is believed that the product of the KCNQ1 gene produces an alpha subunit that interacts with other proteins (in particular, the minK beta subunit) to create the IKs ion channel, which is responsible for the delayed potassium rectifier current of the cardiac action potential.
Mutations to the KCNQ1 gene can be inherited in an autosomal dominant or an autosomal recessive pattern in the same family. In the autosomal recessive mutation of this gene, homozygous mutations in KVLQT1 leads to severe prolongation of the QT interval (due to near-complete loss of the IKs ion channel), and is associated with increased risk of ventricular arrhythmias and congenital deafness. This variant of LQT1 is known as the Jervell and Lange-Nielsen syndrome.
Most individuals with LQT1 show paradoxical prolongation of the QT interval with infusion of epinephrine. This can also unmark latent carriers of the LQT1 gene.
Many missense mutations of the LQT1 gene have been identified. These are often associated with a high frequency of syncopes but less sudden death than LQT2.
LQT2
The LQT2 type is the second most common gene location that is affected in long QT syndrome, making up about 25 to 30 percent of all cases. This form of long QT syndrome most likely involves mutations of the human ether-a-go-go related gene (hERG) on chromosome 7. The hERG gene (also known as KCNH2) is part of the rapid component of the potassium rectifying current (IKr). (The IKr current is mainly responsible for the termination of the cardiac action potential, and therefore the length of the QT interval.) The normally functioning hERG gene allows protection against early after depolarizations (EADs).
Most drugs that cause long QT syndrome do so by blocking the IKr current via the hERG gene. These include erythromycin, terfenadine, and ketoconazole. The hERG channel is very sensitive to unintended drug binding due to two aromatic amino acids, the tyrosine at position 652 and the phenylalanine at position 656. These amino acid residues are poised so that a drug binding to them will block the channel from conducting current. Other potassium channels do not have these residues in these positions and are, therefore, not as prone to blockage.

http://en.wikipedia.org/wiki/Long_QT_syndrome‎


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