Mutations disrupting selenocysteine formation cause progressive cerebello-cerebral atrophy.

The essential micronutrient selenium is found in proteins as selenocysteine (Sec), the only genetically encoded amino acid whose biosynthesis occurs on its cognate tRNA in humans. In the final step of selenocysteine formation, the essential enzyme SepSecS catalyzes the conversion of Sep-tRNA to Sec-tRNA. We demonstrate that SepSecS mutations cause autosomal-recessive progressive cerebellocerebral atrophy (PCCA) in Jews of Iraqi and Moroccan ancestry. Both founder mutations, common in these two populations, disrupt the sole route to the biosynthesis of the 21st amino acid, Sec, and thus to the generation of selenoproteins in humans.

A syndrome of congenital microcephaly, intellectual disability and dysmorphism with a homozygous mutation in FRMD4A.

A consanguineous Bedouin Israeli kindred presented with a novel autosomal recessive intellectual disability syndrome of congenital microcephaly, low anterior hairline, bitemporal narrowing, low-set protruding ears, strabismus and tented thick eyebrows with sparse hair in their medial segment. Brain imaging demonstrated various degrees of agenesis of corpus callosum and hypoplasia of the vermis and cerebellum. Genome-wide linkage analysis followed by fine mapping defined a 7.67 Mb disease-associated locus (LOD score 4.99 at θ=0 for marker D10S1653). Sequencing of the 48 genes within the locus identified a single non-synonymous homozygous duplication frameshift mutation of 13 nucleotides (c.2134_2146dup13) within the coding region of FRMD4A, that was common to all affected individuals and not found in 180 non-related Bedouin controls. Three of 50 remotely related healthy controls of the same tribe were heterozygous for the mutation. FRMD4A, member of the FERM superfamily, is involved in cell structure, transport and signaling. It regulates cell polarity by playing an important role in the activation of ARF6, mediating the interaction between Par3 and the ARF6 guanine nucleotide exchange factor. ARF6 is known to modulate cell polarity in neurons, and regulates dendritic branching in hippocampal neurons and neurite outgrowth. The FRMD4 domain that is essential for determining cell polarity through interaction with Par3 is truncated by the c.2134_2146dup13 mutation. FRMD4A polymorphisms were recently suggested to be a risk factor for Alzheimer's disease. We now show a homozygous frameshift mutation of the same gene in a severe neurologic syndrome with unique dysmorphism.