Zhong N, Kajanoja E, Smits B, Pietrofesa J, Curley D, Wang D, Ju W, Nolin S, Dobkin C, Ryynänen M, Brown W T
Department of Human Genetics, New York State Institute for Basic Research in Developmental Disabilities, Staten Island 10314, USA.
Am J Med Genet. 1996 Jul 12;64(1):226-33. doi: 10.1002/(SICI)1096-8628(19960712)64:1<226::AID-AJMG41>3.0.CO;2-M.
The apparent associations between fragile X mutations and nearby microsatellites may reflect both founder effects and microsatellite instability. To gain further insight into their relative contributions, we typed a sample of 56 unrelated control and 37 fragile X chromosomes from an eastern Finnish population for FMR1 CGG repeat lengths, AGG interspersion patterns, DXS548, FRAXAC1, FRAXE and a new polymorphic locus, Alu-L. In the controls, the most common FMR1 allele was 30 repeats with a range of 20 to 47 and a calculated heterozygosity of 88%. A strong founder effect was observed for locus DXS548 with 95% of fragile X chromosomes having the 21 CA repeat (196 bp) allele compared to 17% of controls, while none of the fragile X but 69% of controls had the 20 repeat allele. Although the FRAXAC1 locus is much closer than DXS548 to FMR1 (7 kb vs. 150 kb), there was no significant difference between fragile X and control FRAXAC1 allele distributions. The FRAXE repeat, located 600 kb distal to FMR1, was found to show strong linkage disequilibrium as well. A newly defined polymorphism, Alu-L, located at approximately 40 kb distal to the FMR1 repeat, showed very low polymorphism in the Finnish samples. Analysis of the combined loci DXS548-FRAXAC1-FRAXE showed three founder haplotypes. Haplotype 21-19-16 was found on 27 (75%) of fragile X chromosomes but on none of controls. Three (8.4%) fragile X chromosomes had haplotypes 21-19-15, 21-19-20, and 21-19-25 differing from the common fragile X haplotype only in FRAXE. These could have arisen by recombination or from mutations of FRAXE. A second haplotype 21-18-17 was found in four (11.1%) fragile X chromosomes but only one (1.9%) control. This may represent a more recent founder mutation. A third haplotype 25-21-15, seen in two fragile X chromosomes (5.6%) and one (1.9%) control, was even less common and thus may represent an even more recent mutation or admixture of immigrant types. Analysis of the AGG interspersions within the FMR1 CGG repeat showed that 7/8 premutation chromosomes lacked an AGG whereas all controls had at least one AGG. This supports the hypothesis that the mutation of AGG to CGG leads to repeat instability and mutational expansion.
脆性X突变与附近微卫星之间明显的关联可能既反映了奠基者效应，也反映了微卫星不稳定性。为了进一步深入了解它们的相对作用，我们对来自芬兰东部人群的56个无关对照样本和37条脆性X染色体进行了FMR1 CGG重复长度、AGG散布模式、DXS548、FRAXAC1、FRAXE以及一个新的多态性位点Alu-L的分型。在对照样本中，最常见的FMR1等位基因是30次重复，范围为20至47次重复，计算得出的杂合度为88%。观察到DXS548位点有很强的奠基者效应，95%的脆性X染色体具有21次CA重复（196 bp）等位基因，而对照样本中只有17%；脆性X染色体中没有一个具有20次重复等位基因，而对照样本中有69%具有该等位基因。尽管FRAXAC1位点比DXS548位点距离FMR1近得多（分别为7 kb和150 kb），但脆性X染色体和对照样本的FRAXAC1等位基因分布没有显著差异。位于FMR1下游600 kb处的FRAXE重复序列也显示出很强的连锁不平衡。一个新定义的多态性位点Alu-L，位于FMR1重复序列下游约40 kb处，在芬兰样本中显示出非常低的多态性。对DXS548 - FRAXAC1 - FRAXE联合位点的分析显示出三种奠基者单倍型。在27条（75%）脆性X染色体上发现了单倍型21 - 19 - 16，而对照样本中没有。三条（8.4%）脆性X染色体具有单倍型21 - 19 - 15、21 - 19 - 20和21 - 19 - 25，它们与常见的脆性X单倍型仅在FRAXE上不同。这些可能是通过重组或FRAXE的突变产生的。在四条（11.1%）脆性X染色体上发现了第二种单倍型21 - 18 - 17，但对照样本中只有一条（1.9%）。这可能代表一个更近的奠基者突变。第三种单倍型25 - 21 - 15，在两条脆性X染色体（5.6%）和一条对照样本（1.9%）中出现，更为罕见，因此可能代表一个更新的突变或移民类型的混合。对FMR1 CGG重复序列内AGG散布的分析表明，8条前突变染色体中有7条缺乏AGG，而所有对照样本至少有一个AGG。这支持了AGG突变为CGG会导致重复序列不稳定和突变扩展的假说。
