Abstract
We have previously reported that an X-linked recessive form of chronic idiopathic intestinal pseudo-obstruction (CIIPX) maps to Xq28. To select candidate genes for the disease, we analyzed the expression in murine fetal brain and intestine of 56 genes from the critical region. We selected and sequenced seven genes and found that one affected male from a large CIIPX-affected kindred bears a 2-bp deletion in exon 2 of the FLNA gene that is present at the heterozygous state in the carrier females of the family. The frameshift mutation is located between two close methionines at the filamin N terminus and is predicted to produce a protein truncated shortly after the first predicted methionine. Loss-of-function FLNA mutations have been associated with X-linked dominant nodular ventricular heterotopia (PVNH), a central nervous system (CNS) migration defect that presents with seizures in females and lethality in males. Notably, the affected male bearing the FLNA deletion had signs of CNS involvement and potentially has PVNH. To understand how the severe frameshift mutation we found can explain the CIIPX phenotype and its X-linked recessive inheritance, we transiently expressed both the wild- type and mutant filamin in cell culture and found that filamin translation can start from either of the two initial methionines in these conditions. Therefore, translation of a normal shorter filamin can occur in vitro from the second methionine downstream of the 2-bp insertion we found. We confirmed this, demonstrating that the filamin protein is present in the patient's lymphoblastoid cell line that shows abnormal cytoskeletal actin organization compared with normal lymphoblasts. We conclude that the filamin N terminal region between the initial two methionines is crucial for proper enteric neuron development.