插入序列導致X染色體脆折症篩檢的CGG重複次數之判定困難

Insertion Sequence Interferes CGG Repeats Determination in the Fragile X Syndrome Screening Test

Repeat-primed PCR是X染色體脆折症(Fragile X syndrome; FXS)目前常用的檢測法，本案例在FXS致病區域(5’-UTR region of the FMR1 gene)內發生38bp的插入序列，造成repeat-primed PCR高估了13次CGG重覆，出現不連續、不典型的龍脊狀連續波鋒。透過定序確認案例的CGG重覆應為67次而非repeat-primed PCR判定之80次，兩者均屬FXS準突變型，但親代遺傳給子代發生完全突變型之風險不同（分別為5.3％與57.8％），可能影響後續遺傳諮詢建議。為避免不正確的CGG重覆影響臨床判斷，建議以repeat-primed PCR篩檢FXS時，若出現不連續、不典型的龍脊狀連續波鋒，應再以DNA定序法進行確認。

Repeat-primed PCR is the most common method for Fragile X syndrome (FXS) detection. A 38-bp insertion into 5’-UTR region of the FMR1 gene was noted to overestimate 13 repeats for the number of CGG repeats and present a discontinuous, atypical dragon’s back peaks in repeat-primed PCR. The correct number of CGG repeats of this case was confirmed to be 67 by direct sequencing, instead of 80 by repeat-primed PCR. Although both number of CGG repeats were belonging to pre-mutation, their rates of full-mutation transition on fetus were greatly different, 5.3% and 57.8% for 67 and 80 CGG repeats, respectively. Higher full-mutation transition rate weights more in the following genetic counseling. To prevent excessive or even miss interpretation caused by incorrectly determining CGG repeats, DNA sequencing should be used to confirm CGG repeats while the discontinuous or interrupted atypical dragon’s back peaks revealed in repeat-primed PCR.