Maurizio De Fusco

Haploinsufficiency of ATP1A2 encoding the Na+/K+ pump alpha2 subunit associated with familial hemiplegic migraine type 2.

Headache attacks and autonomic dysfunctions characterize migraine, a very common, disabling disorder with a prevalence of 12% in the general population of Western countries. About 20% of individuals affected with migraine experience aura, a visual or sensory-motor neurological dysfunction that usually precedes or accompanies the headache. Although the mode of transmission is controversial, population-based and twin studies have implicated genetic factors, especially in migraine with aura. Familial hemiplegic migraine is a hereditary form of migraine characterized by aura and some hemiparesis. Here we show that mutations in the gene ATP1A2 that encodes the α2 subunit of the Na+/K+ pump are associated with familial hemiplegic migraine type 2 (FHM2) linked to chromosome 1q23 (OMIM 602481). Functional data indicate that the putative pathogenetic mechanism is triggered by a loss of function of a single allele of ATP1A2. This is the first report associating mutations of Na+K+ pump subunits to genetic diseases.

Spastic Paraplegia and OXPHOS Impairment Caused by Mutations in Paraplegin, a Nuclear-Encoded Mitochondrial Metalloprotease

Hereditary spastic paraplegia (HSP) is characterized by progressive weakness and spasticity of the lower limbs due to degeneration of corticospinal axons. We found that patients from a chromosome 16q24.3-linked HSP family are homozygous for a 9.5 kb deletion involving a gene encoding a novel protein, named Paraplegin. Two additional Paraplegin mutations, both resulting in a frameshift, were found in a complicated and in a pure form of HSP. Paraplegin is highly homologous to the yeast mitochondrial ATPases, AFG3, RCA1, and YME1, which have both proteolytic and chaperon-like activities at the inner mitochondrial membrane. Immunofluorescence analysis and import experiments showed that Paraplegin localizes to mitochondria. Analysis of muscle biopsies from two patients carrying Paraplegin mutations showed typical signs of mitochondrial OXPHOS defects, thus suggesting a mechanism for neurodegeneration in HSP-type disorders.

Increased Sensitivity of the Neuronal Nicotinic Receptor α2 Subunit Causes Familial Epilepsy with Nocturnal Wandering and Ictal Fear

Sleep has traditionally been recognized as a precipitating factor for some forms of epilepsy, although differential diagnosis between some seizure types and parasomnias may be difficult. Autosomal dominant frontal lobe epilepsy is characterized by nocturnal seizures with hyperkinetic automatisms and poorly organized stereotyped movements and has been associated with mutations of the alpha 4 and beta 2 subunits of the neuronal nicotinic acetylcholine receptor. We performed a clinical and molecular genetic study of a large pedigree segregating sleep-related epilepsy in which seizures are associated with fear sensation, tongue movements, and nocturnal wandering, closely resembling nightmares and sleep walking. We identified a new genetic locus for familial sleep-related focal epilepsy on chromosome 8p12.3-8q12.3. By sequencing the positional candidate neuronal cholinergic receptor alpha 2 subunit gene (CHRNA2), we detected a heterozygous missense mutation, I279N, in the first transmembrane domain that is crucial for receptor function. Whole-cell recordings of transiently transfected HEK293 cells expressing either the mutant or the wild-type receptor showed that the new CHRNA2 mutation markedly increases the receptor sensitivity to acetylcholine, therefore indicating that the nicotinic alpha 2 subunit alteration is the underlying cause. CHRNA2 is the third neuronal cholinergic receptor gene to be associated with familial sleep-related epilepsies. Compared with the CHRNA4 and CHRNB2 mutations reported elsewhere, CHRNA2 mutations cause a more complex and finalized ictal behavior.

Autosomal recessive Rolandic epilepsy with paroxysmal exercise-induced dystonia and writer's cramp : Delineation of the syndrome and gene mapping to chromosome 16p12-11.2

We describe a pedigree in which 3 members in the same generation are affected by Rolandic epilepsy (RE), paroxysmal exercise‐induced dystonia (PED), and writers cramp (WC). Both the seizures and paroxysmal dystonia had a strong age‐related expression that peaked during childhood, whereas the WC, also appearing in childhood, has been stable since diagnosis. Genome‐wide linkage analysis performed under the assumption of recessive inheritance identified a common homozygous haplotype in a critical region spanning 6 cM between markers D16S3133 and D16S3131 on chromosome 16, cosegregating with the affected phenotype and producing a multipoint LOD score value of 3.68. Although its features are unique, this syndrome presents striking analogies with the autosomal dominant infantile convulsions and paroxysmal coreoathetosis (ICCA) syndrome, linked to a 10 cM region between D16S401 and D16S517, which entirely includes the 6 cM of the RE–PED–WC critical region. The same gene may be responsible for both RE–PED–WC and ICCA, with specific mutations explaining each of these Mendelian disorders. This report shows that idiopathic focal disorders such as epilepsy and dystonia, can be caused by the same genetic abnormality, may have a transient expression, and may be inherited as an autosomal recessive trait. Ann Neurol 1999;45:344–352

A recessive variant of the Romano-Ward long-QT syndrome?

BACKGROUND The congenital long-QT syndrome (LQTS) is a genetically heterogeneous disease characterized by prolonged ventricular repolarization and life-threatening arrhythmias. Mutations of the KVLQT1 gene, a cardiac potassium channel, generate two allelic diseases: the Romano-Ward syndrome, inherited as a dominant trait, and the Jervell and Lange-Nielsen syndrome, inherited as an autosomal recessive trait. METHODS AND RESULTS A consanguineous family with the clinical phenotype of LQTS was screened for mutations in the KVLQT1 gene. Complementary RNAs for injection into Xenopus oocytes were prepared, and currents were recorded with the double microelectrode technique. A homozygous missense mutation, leading to an alanine-to-threonine substitution at the beginning of the pore domain of the KVLQT1 channel, was found in the proband, a 9-year-old boy with normal hearing, a prolonged QT interval, and syncopal episodes during physical exercise. The parents of the proband were heterozygous for the mutation and had a normal QT interval. The functional evaluation of the mutant channel activity showed reduction in total current, a hyperpolarizing shift in activation, and a faster activation rate consistent with a mild mutation likely to require homozygosity to manifest the phenotype. CONCLUSIONS These findings provide the first evidence for a recessive form of the Romano-Ward long-QT syndrome and indicate that homozygous mutations on KVLQT1 do not invariably produce the Jervell and Lange-Nielsen syndrome. The implications of this observation prompt a reconsideration of the penetrance of different mutations responsible for LQTS and suggest that mild mutations in LQTS genes may be present among the general population and may predispose to drug-induced ventricular arrhythmias.

Increased Susceptibility to Cortical Spreading Depression in the Mouse Model of Familial Hemiplegic Migraine Type 2

Familial hemiplegic migraine type 2 (FHM2) is an autosomal dominant form of migraine with aura that is caused by mutations of the α2-subunit of the Na,K-ATPase, an isoform almost exclusively expressed in astrocytes in the adult brain. We generated the first FHM2 knock-in mouse model carrying the human W887R mutation in the Atp1a2 orthologous gene. Homozygous Atp1a2R887/R887 mutants died just after birth, while heterozygous Atp1a2+/R887 mice showed no apparent clinical phenotype. The mutant α2 Na,K-ATPase protein was barely detectable in the brain of homozygous mutants and strongly reduced in the brain of heterozygous mutants, likely as a consequence of endoplasmic reticulum retention and subsequent proteasomal degradation, as we demonstrate in transfected cells. In vivo analysis of cortical spreading depression (CSD), the phenomenon underlying migraine aura, revealed a decreased induction threshold and an increased velocity of propagation in the heterozygous FHM2 mouse. Since several lines of evidence involve a specific role of the glial α2 Na,K pump in active reuptake of glutamate from the synaptic cleft, we hypothesize that CSD facilitation in the FHM2 mouse model is sustained by inefficient glutamate clearance by astrocytes and consequent increased cortical excitatory neurotransmission. The demonstration that FHM2 and FHM1 mutations share the ability to facilitate induction and propagation of CSD in mouse models further support the role of CSD as a key migraine trigger.

A New Locus for Autosomal Recessive Hereditary Spastic Paraplegia Maps to Chromosome 16q24.3

Hereditary spastic paraplegia is a genetically and phenotypically heterogeneous disorder. Both pure and complicated forms have been described, with autosomal dominant, autosomal recessive, and X-linked inheritance. Various loci (SPG1-SPG6) associated with this disorder have been mapped. Here, we report linkage analysis of a large consanguineous family affected with autosomal recessive spastic paraplegia with age at onset of 25-42 years. Linkage analysis of this family excluded all previously described spastic paraplegia loci. A genomewide linkage analysis showed evidence of linkage to chromosome 16q24.3, with markers D16S413 (maximum LOD score 3.37 at recombination fraction [theta] of .00) and D16S303 (maximum LOD score 3.74 at straight theta=.00). Multipoint analysis localized the disease gene in the most telomeric region, with a LOD score of 4.2. These data indicate the presence of a new locus linked to pure recessive spastic paraplegia, on chromosome 16q24.3, within a candidate region of 6 cM.

Identification of a New Locus for Medullary Cystic Disease, on Chromosome 16p12

Autosomal dominant medullary cystic disease (ADMCKD) is an interstitial nephropathy that has morphologic and clinical features similar to autosomal recessive nephronophthisis. The typical renal dysfunction associated with ADMCKD results mainly from a defect in urinary concentration ability, although results of urinalysis are normal. Recently, a locus on chromosome 1 was associated with ADMCKD, in DNA from two large Cypriot families, and genetic heterogeneity was inferred. We describe the genomewide linkage mapping of a new locus for medullary cystic disease, ADMCKD2, on chromosome 16p12 in a four-generation Italian pedigree. The family with ADMCKD2 fulfills the typical diagnostic criteria of ADMCKD, complicated by hyperuricemia and gouty arthritis. Marker D16S3036 shows a maximum two-point LOD score of 3.68, and the defined critical region spans 10.5 cM, between D16S500 and SCNN1B1-2. Candidate genes included in the critical region are discussed.

Familial hemiplegic migraine type 2 is linked to 0.9Mb region on chromosome 1q23.

Familial hemiplegic migraine (FHM) is a rare autosomal dominant disorder characterized by episodes of transient hemiparesis followed by headache. Two chromosomal loci are associated to FHM: FHM1 on chromosome 19 and FHM2 on chromosome 1q21‐23. Mutations of the α‐1A subunit of the voltage gated calcium channel (CACNA1A) are responsible for FHM1. FHM2 critical region spans 28cM, hence hampering the identification of the responsible gene. Here, we report the FHM2 locus refining by linkage analysis on two large Italian families affected by pure FHM. The new critical region covers a small area of 0.9Mb in 1q23 and renders feasible a positional candidate approach. By mutation analysis, we excluded the calsequestrin and two potassium channel genes mapping within the narrowed FHM2 locus. Ann Neurol 2003

A New Benign Adult Familial Myoclonic Epilepsy (BAFME) Pedigree Suggesting Linkage to Chromosome 2p11.1‐q12.2

Summary:  Benign adult familial myoclonic epilepsy (BAFME) is an autosomal dominant condition characterized by cortical tremor and generalized seizures, mapped on chromosome 8q24 by Japanese authors. Recently the same phenotype also was reported in European families, with linkage on chromosome 2. We present a new family with suggestion of linkage to chromosome 2p11.1‐2q12.2 (lod score value, 1.55). This observation would confirm that BAFME is a worldwide, genetically heterogeneous condition, probably with Japanese families linked to 8q24 and European families to 2p11.1‐q12.2.