- 學位論文
Silver-Russell syndrome與SRS-like病人間的分子遺傳研究
Molecular genetic studies in Silver-Russell syndrome and SRS-like patients
摘要
Silver-Russell syndrome (SRS)於1953年與1954年分別由Silver與Russell兩位醫師提出，人口發生率約三千分之一，男女均等。SRS是種多病徵的疾病，患者主要特徵為子宮內或產後的生長遲緩，產重過輕，骨齡不足，倒三角形的臉型，單側肢體發育不全，第五指（趾）屈指（趾）等等，其他較次要的症狀則為尿道下裂，咖啡牛奶斑，第二及第三趾的併趾以及過度流汗的現象；大部分的病例是偶發性的，只有少數家族性病患有被報導過。目前已知與SRS症狀或類似SRS症狀之異常染色體有第7、8、11、15、17、18對染色體，但其中只有第7對以及第17對染色體異常的患者在臨床診斷上一致性地符合SRS的病徵。 約7~10%的SRS病例在第七對染色體上帶有母系的單親雙倍體（maternal uniparental disomy, mUPD），他們的臨床表徵比較輕微且較為對稱。SRS患者中的mUPD7暗示了在第七對染色體內可能包含一個或多個與SRS相關的基因。目前已知在第七對染色體上有兩個imprinting區塊，分別位於7p11.2-p13與7q31-qter；在7p11.2-p13，與負調控生長相關的基因有IGFBP1，IGFBP3和GRB10，前兩者已被證實不具有imprinting的現象，而GRB10則被報導出在與直線生長最具關係的growth plate cartilage中並沒有imprinting的現象，所以也被質疑是否與SRS有直接關係。在7q31-qter區塊中，已知可能與生長相關之印記基因為父系表現的PEG1/MEST，學者指出可透過分析PEG1/MEST 促進子區域甲基化的情形來判斷病人是否屬於mUPD7，所以我們利用MSP(methylation-specific PCR)的技術偵測其promoter region甲基化的情形來初步篩選推知病患是否屬於mUPD7；然而目前在26個體型瘦小的病例中我們並沒有尋獲mUPD7的患者；此外在另一個候選區塊7q21上也沒有發現到異常的deletion，所以初步排除了我們的病例屬於在第七對染色體上有異常的族群之可能性。 在人類染色體11p15.5上有另一群包含與胎兒生長有相當程度關係之印記基因的區塊，這個印記區塊包含了父系表現的印記基因，例如KCNQ1OT1與IGF2，以及母系表現的印記基因例如CDKN1C及H19。這個染色體區塊過往被認為與胎兒過度生長的疾病Beckwith-Wiedemann syndrome (BWS)相關，近期有部分學者指出在一些SRS的患者中有發現ICR1甲基化異常的現象，在一篇報告裡異常的現象甚至高達31%；所以我們針對ICR1甲基化的情形在收集到的病人群裡作檢驗，然而結果卻沒有發現異常的個案。 藉由研究這些與SRS相關的基因，我們可以進一步瞭解這些與胎兒生長調控相關的基因以及印記機制；然而在我們收集到的病人裡，並沒有發現在這些被報導過與SRS相關的染色體候選區塊有異常的現象，這可能是由於我們收集到的病人檢體數量不夠大，也有可能是由於台灣族群的SRS患者致病機轉與國外不同。為了釐清這點，未來更大型的研究是必須的，除了患者檢體數目的增加外，對於候選基因與特定病徵的關連性也都必須更深入地探討。
關鍵字
分子遺傳並列摘要
Silver-Russell syndrome (Russell-Silver syndrome, SRS) is a heterogeneous syndrome which was reported by Silver & Russell in 1953 and 1954 respectively. The incidence is about 1 in 3,000, and equal to male and female. SRS is a multifactorial disease, most patients were characterized by intrauterine and postnatal growth retardation, low birth weight, delayed bone maturation, triangular face, hemihypotrophy and clinodactyly of the little fingers. Other less frequent anomalies are hypospadia, café-au-lait spots, syndactyly between the second and third toes and excessive sweating. Most cases are sporadic, only few familial cases have been described. The disease is clinically and genetically heterogenous, and various modes of inheritance and abnormalities involving chromosome 7, 8, 11, 15, 17 and 18 have been associated with SRS and SRS-like cases. However, only chromosomes 7 and 17 have been consistently implicated in patients with a strict clinical diagnosis of SRS. There are about 7~10% of maternal uniparental disomy 7 (mUPD7) in the SRS population, and these cases have symmetrical and mild phenotypes. Maternal UPD7 in SRS patients suggests that there might be one or more imprinted genes associated with SRS locate in chromosome 7. There are two known imprinting clusters in chromosome 7, locate at 7p11.2-p13 and 7q31-qter respectively. In 7p11.2-p13, this region was proposed to encompass the GRB10, IGFBP1 and IGFBP3 genes which were later considered as unlikely to be involved in SRS. In 7q31, PEG1/MEST is a known paternally expressed imprinting gene which was believed to have correlation with fetal growth. It was proposed that we can tell whether a patient is mUPD7 or not by using methylation-specific PCR (MSP) to detect the methylation status in the promoter region of PEG1/MEST. Besides the imprinting region, we also detected the 7q21 region, which was also thought might be a candidate region for SRS. So far in our 4 cases with SRS, 6 cases with SRS-like and 16cases with small for gestational age (SGA), we found neither abnormalities at PEG1/MEST promoter methylation status nor deletion at 7q21. Human chromosome 11p15.5 contains a cluster of imprinted genes that are crucial in the control of fetal growth. This cluster includes paternally expressed (maternally imprinted) genes (such as IGF2 and KCNQ1OT1) and maternally expressed (paternally imprinted) genes (such as CDKN1C and H19). Recently there are reports about demethylation at ICR1, which is in the telomeric imprinting domain in 11p15, were found in SRS and SRS-like patients and this region were later considered as another candidate region for SRS or SRS-like symptoms. It was also reported that around 31% of SRS patients were found with ICR1 demethylation, therefore we detected the methylation status in this region. However, our patients were found all normal with the methylation status of ICR1. By studying these candidate regions we can further understand more about genes and imprinting mechanisms in fetal growth. However, we excluded these regions of abnormalities as a common cause in our patients. These results might due to our small size of study population or by the ethnic origin of our patients. To answer this question and to determine the putative association between these candidate regions and SRS or SRS-like clinical symptoms, further distinct cohorts of growth retarded patients should be investigated.