FAM46A mutations are responsible for autosomal recessive osteogenesis imperfecta.

BACKGROUND: Stüve-Wiedemann syndrome (SWS) is characterised by bowing of the lower limbs, respiratory distress and hyperthermia that are often responsible for early death. Survivors develop progressive scoliosis and spontaneous fractures. We previously identified LIFR mutations in most SWS cases, but absence of LIFR pathogenic changes in five patients led us to perform exome sequencing and to identify homozygosity for a FAM46A mutation in one case [p.Ser205Tyrfs*13]. The follow-up of this case supported a final diagnosis of osteogenesis imperfecta (OI), based on vertebral collapses and blue sclerae. METHODS AND RESULTS: This prompted us to screen FAM46A in 25 OI patients with no known mutations.We identified a homozygous deleterious variant in FAM46A in two affected sibs with typical OI [p.His127Arg]. Another homozygous variant, [p.Asp231Gly], also classed as deleterious, was detected in a patient with type III OI of consanguineous parents using homozygosity mapping and exome sequencing.FAM46A is a member of the superfamily of nucleotidyltransferase fold proteins but its exact function is presently unknown. Nevertheless, there are lines of evidence pointing to a relevant role of FAM46A in bone development. By RT-PCR analysis, we detected specific expression of FAM46A in human osteoblasts andinterestingly, a nonsense mutation in Fam46a has been recently identified in an ENU-derived (N-ethyl-N-nitrosourea) mouse model characterised by decreased body length, limb, rib, pelvis, and skull deformities and reduced cortical thickness in long bones. CONCLUSION: We conclude that FAM46A mutations are responsible for a severe form of OI with congenital bowing of the lower limbs and suggest screening this gene in unexplained OI forms.

Maternal uniparental disomy of chromosome 2 in a patient with a DGUOK mutation associated with hepatocerebral mitochondrial DNA depletion syndrome.

We report maternal uniparental disomy of chromosome 2 (matUPD2) in a 9-month-old girl presenting with hepatocerebral mitochondrial DNA depletion syndrome. This patient was homozygous for the c.352C>T (p.Arg118Cys) mutation in DGUOK gene. The proband's mother was heterozygous for the mutation was absent in DNA of the father. For proband, the absence of paternal contribution at the DGUOK locus prompted us to exclude intragenic DGUOK deletion of the paternal allele with Multiplex ligation-dependent probe amplification (MLPA) analysis. We also excluded non-paternity by studying various markers at different loci. Then we performed an analysis of copy number variations and absence of heterozygosity (AOH) on the proband DNA using high resolution oligonucleotides microarray. Several large regions of AOH with no copy number change were detected on chromosome 2 and one of these AOH regions encompassed DGUOK gene. These results were confirmed with haplotype analysis using polymorphic markers. Informative SNPs and microsatellites markers spanning the whole chromosome 2 showed a matUPD2 with heterodisomy and isodisomy regions, the absence of paternal allele and presence of two maternal alleles, with only one maternal allele on the region of DGUOK locus in 2p13.1. This is the first demonstration of matUPD2 with segmental isodisomy at 2p13.1 locus in hepatocerebral mitochondrial DNA depletion syndrome. The identification of UPD2 will impact genetic counseling for the proband's parents. Because the recurrence risk for UPD2 is very low, the risk for disease in further offspring for this couple is negligible.

New SUCLG1 patients expanding the phenotypic spectrum of this rare cause of mild methylmalonic aciduria.

Deficiencies in two subunits of the succinyl-coenzyme A synthetase (SCS) have been involved in patients with encephalomyopathy and mild methylmalonic aciduria (MMA). In this study, we described three new SUCLG1 patients and performed a meta-analysis of the literature. Our report enlarges the phenotypic spectrum of SUCLG1 mutations and confirms that a characteristic metabolic profile (presence of MMA and C4-DC carnitine in urines) and basal ganglia MRI lesions are the hallmarks of SCS defects. As mitochondrial DNA depletion in muscle is not a constant finding in SUCLG1 patients, this may suggest that diagnosis should not be based on it, but also that alternative physiopathological mechanisms may be considered to explain the combined respiratory chain deficiency observed in SCS patients.