Here we report two familial interstitial deletions of Xp11.22 in the same X chromosome of the same patient presenting with hydrocephalus, severe growth and psychomotor retardation, renal proximal tubulopathy with low-molecular-weight proteinuria, hypercalciuria, hyperaminoaciduria, hypophosphatemia and hyperuricemia. Maternal transmission of the deletions was also confirmed in a second fetus who was aborted due to the causality of the deletions.
So far, 11 cases of Xp11.22 deletions in 4 families have been described [7-10]. The patients’ deletions involving this region presented a unique deletion often associated with autism spectrum disorders, variable degrees of ID, craniofacial dysmorphism, long hands and fingers, normal growth parameters and significant language delay [7]. The genomic positions of these previously described deletions did not overlap with ours. Two other cases described in DECIPHER database concerned deletions that were much smaller than our patient’s deletions. These patients presented ID and short seizures. Thus, to our knowledge, this is the first case of two deletions in the same chromosomal region Xp11.22 associated with severe psychomotor retardation and renal tubulopathy.
Among the genes partially or totally included in the deletions, SHROOM4 and CLCN5 are candidate genes whose total loss of expression could explain clinical manifestations. Severe ID is one of the major sign that presented our patient. In 2006, Hagens et al., investigated the breakpoints of two balanced X;autosome translocations in two unrelated female patients with mild/moderate syndromic ID and found that the Xp11.2 breakpoints disrupt SHROOM4 in both cases [15]. They also identified a missense exchange in this gene, segregating with the Stocco dos Santos XLMR syndrome in a large four-generation pedigree but absent in more than 1000 control X-chromosomes. Among other phenotypic characteristics, the affected males in this family presented with severe ID, delayed or no speech, seizures and hyperactivity.
Yoder and Hildebrand described the characterization of mouse SHROOM4 and observed that the protein localizes to a distinct population of F-actin and appears to regulate the formation of this cytoskeletal compartment [16]. Myosin II is also localized to this compartment and appears to be required for maintaining its proper organization. Thus, SHROOM4 encodes a protein involved in cytoskeletal architecture and may function to regulate cellular and cytoskeletal architecture by modulating the spatial distribution of myosin II [16]. SHROOM4 is also expressed in human adult and fetal brain structures and is needed to establish or maintain a particular cytoskeletal structure that is necessary for the function or survival of a specific neuronal population of cells [16]. These data suggest that SHROOM4 is important in cognitive function and/or development. SHROOM4 is included in the distal Xp11.22 deletion of our patient. The total loss of function could be responsible for his phenotype. Screening of many more XLMR patients is also warranted to further confirm the implication of this gene in XLMR. This could include cellular properties such as morphology, vesicular trafficking or polarity, all of which are regulated by the actin cytoskeleton. Based on these observations, it will be important to determine what other developmental processes might be regulated by SHROOM4 and if there are genetic interactions between SHROOM4 and other regulators of cellular morphology.
Others genes could be potentially candidate for the ID of the patient. Indeed, western blot analysis of MAGED1 detected expression in embryos, especially in cerebral cortex and cerebellum. MAGED1 plays important roles in the central nervous system in both developmental and adult stages [17].
CLCN5 (chloride channel 5) is located on chromosome Xp11.22 and encodes CIC-5, member of the CIC family (chloride ion channels and ion transporters). CIC-5 is a Cl-/H+ exchanger that is primarily localized to endosomal membranes and may function to facilitate reabsorption by the renal proximal tubule [18,19]. Mutations in CLCN5 have been found in Dent disease, an X-linked inherited renal condition characterized by proteinuria, hypercalciuria and hyperphosphaturia, kidney stones, and in some cases, kidney failure [11-13].
To date, mutations in the gene are described and alternatively spliced transcript variants encoding different isoforms have been found. The total number of reported CLCN5 mutations is 148, and these are scattered throughout the coding region, with no evidence for major mutational hot spots [20]. The majority of the mutations are predicted to result in truncated or absent ClC-5 protein, which would lead to complete loss of Cl-/H+ exchanger function. There is genetic heterogeneity for Dent disease, with approximately 60% of patients having CLCN5 mutations (Dent disease 1), ~15% harboring OCRL1 mutations (Dent disease 2) and the remaining 25% of patients having neither CLCN5 nor OCRL1 mutations but possibly defects in other genes [21]. The possibility that these other genes may encode some of the proteins (e.g. ClC-4 and cofilin) that interact with ClC-5 [22] has been investigated but no mutations in CLCN4 or COFILIN were identified yet [20].
Besides the gene content of the two interstitial deletions and the most plausible candidates, one may not exclude position effects on neighboring genes which are not included in the deletions.
To our knowledge, this patient is the first description of a double Xp11.22 deletion causing a CLCN5 disruption and associated with a such characteristic renal proximal tubulopathy. The diagnosis of Dent disease is usefully based on the presence of all three of the following criteria: low-molecular-weight proteinuria, hypercalciuria and at least one of the following: nephrocalcinosis, kidney stones, hematuria, hypophosphatemia or renal insufficiency. Generally, molecular genetic testing and the presence of a CLCN5 mutation confirm the diagnosis. For our patient, the Xp11.22 deletion including CLCN5 and, consequently, the loss of CIC-5 exchanger allowed us to confirm the diagnosis of Dent disease. The genotype-phenotype correlation was discussed with the family during genetic counseling. Prenatal diagnosis was proposed for the second pregnancy and the couple decided, according to the French law, to terminate the pregnancy with an affected male.
All findings strongly implicate the inherited Xp11.22 deletions as causative of the described somatic and developmental phenotypes in the affected male patient. The Xp11.22 deletions with loss of CLCN5 gene allowed us to confirm the diagnostic of Dent disease for our patient. Our results also strengthen the potential role of SHROOM4 in syndromic XLMR.
