Borjeson-Forssman-Lehmann syndrome and multiple pituitary hormone deficiency

Transgenic mice with an R342X mutation in Phf6 display clinical features of Börjeson-Forssman-Lehmann Syndrome

The PHF6 mutation c.1024C > T; p.R342X, is a recurrent cause of Börjeson-Forssman-Lehmann Syndrome (BFLS), a neurodevelopmental disorder characterized by moderate-severe intellectual disability, truncal obesity, gynecomastia, hypogonadism, long tapering fingers and large ears (MIM#301900). Here, we generated transgenic mice with the identical substitution (R342X mice) using CRISPR technology. We show that the p.R342X mutation causes a reduction in PHF6 protein levels, in both human and mice, from nonsense-mediated decay and nonsense-associated alternative splicing, respectively. Magnetic resonance imaging studies indicated that R342X mice had a reduced brain volume on a mixed genetic background but developed hydrocephaly and a high incidence of postnatal death on a C57BL/6 background. Cortical development proceeded normally, while hippocampus and hypothalamus relative brain volumes were altered. A hypoplastic anterior pituitary was also observed that likely contributes to the small size of the R342X mice. Behavior testing demonstrated deficits in associative learning, spatial memory and an anxiolytic phenotype. Taken together, the R342X mice represent a good preclinical model of BFLS that will allow further dissection of PHF6 function and disease pathogenesis.

Downregulation of the GHRH/GH/IGF1 axis in a mouse model of Börjeson-Forssman-Lehman syndrome

Börjeson-Forssman-Lehmann syndrome (BFLS) is an intellectual disability and endocrine disorder caused by plant homeodomain finger 6 (PHF6) mutations. Individuals with BFLS present with short stature. We report a mouse model of BFLS, in which deletion of Phf6 causes a proportional reduction in body size compared with control mice. Growth hormone (GH) levels were reduced in the absence of PHF6. Phf6 ^{- /Y} animals displayed a reduction in the expression of the genes encoding GH-releasing hormone (GHRH) in the brain, GH in the pituitary gland and insulin-like growth factor 1 (IGF1) in the liver. Phf6 deletion specifically in the nervous system caused a proportional growth defect, indicating a neuroendocrine contribution to the phenotype. Loss of suppressor of cytokine signaling 2 (SOCS2), a negative regulator of growth hormone signaling partially rescued body size, supporting a reversible deficiency in GH signaling. These results demonstrate that PHF6 regulates the GHRH/GH/IGF1 axis.

A Novel Nonsense Mutation of PHF6 in a Female with Extended Phenotypes of Borjeson-Forssman-Lehmann Syndrome

Borjeson-Forssman-Lehmann syndrome (BFLS) is a rare X-linked disease caused by PHF6 mutations. Classic BFLS has been associated with intellectual disability (ID), developmental delay (DD), obesity, epilepsy, typical facial features and anomalies of fingers and toes. Endocrinological phenotypes and outcome of treatment in this condition remain to be delineated. Here we report a patient who exhibited complete growth hormone deficiency who responded to hormonal treatment but with adverse effects. Horseshoe kidney was present in this patient, which is also atypical in BFLS. A heterozygous nonsense mutation c.673C>T (p.R225X) of PHF6 gene was identified in the patient, inherited from her unaffected mother. Both the patient and her mother showed highly skewed X-inactivation. We reviewed the phenotypes of all reported BFLS cases, and summarized their endocrine presentations. This first report of an Asian patient with BFLS further delineated the genetic and phenotypic spectrum of the syndrome. The adverse effect experienced by the patient suggests caution in the use of growth hormone treatment in this condition.

PHF6 Deletions May Cause Borjeson-Forssman-Lehmann Syndrome in Females

In a 16-year-old girl with intellectual disability, irregular teeth, slight body asymmetry, and striated skin pigmentation, highly skewed X-inactivation increased the likelihood of an X-linked cause of her condition. Among these, prominent supraorbital ridges and hearing loss suggested a filaminopathy, but no filamin A mutation was found. The correct diagnosis, Borjeson-Forssman-Lehmann syndrome (BFLS, MIM#301900), was first made when a copy number array identified a de novo 15-kb deletion of the terminal 3 exons of the PHF6 gene. In retrospect, her phenotype resembled that of males with BFLS. Such deletions of PHF6 have not been reported previously. This might be because PHF6 mutations are rarely looked for in females since classical BFLS so far has been thought to be a male-specific syndrome, and large PHF6 deletions might be incompatible with male fetal survival. If this is the case, sporadic BFLS could be more frequent in females than in males.

Protein and gene expression analysis of Phf6, the gene mutated in the Börjeson-Forssman-Lehmann Syndrome of intellectual disability and obesity

The Plant homeodomain finger gene 6 (PHF6) was identified as the gene mutated in patients suffering from the Börjeson-Forssman-Lehmann Syndrome (BFLS), an X-linked mental retardation disorder. BFLS mental disability is evident from an early age, suggesting a developmental brain defect. The PHF6 protein contains four nuclear localisation signals and two imperfect plant homeodomain (PHD) fingers similar to the third, imperfect PHD fingers in members of the trithorax family of transcriptional regulators. The PHF6 gene is highly conserved in vertebrate species. Despite the devastating effects of mutation of the PHF6 gene, nothing is known about the cellular function of PHF6. In order to lay the base for functional studies, we identify here the cell types that express the murine Phf6 gene and protein during prenatal and postnatal development. The Phf6 gene and protein are expressed widely. However, expression levels vary from strong to barely detectable. Strongest Phf6 gene expression and nuclear localisation of Phf6 protein were observed in the developing central nervous system, the anterior pituitary gland, the primordia of facial structures and the limb buds. Expression levels of both mRNA and protein decline over the course of development. In the adult brain moderate Phf6 expression is maintained in projection neurons, such as mitral cells in the olfactory bulb, cerebrocortical pyramidal cells and cerebellar Purkinje cells. Phf6 gene expression and nuclear localisation of Phf6 protein correlate with clinical symptoms in BFLS patients, namely mental disability, pan-anterior pituitary hormonal deficiency and facial as well digit abnormalities.