"Ears of the Lynx" MRI Sign Is Associated with SPG11 and SPG15 Hereditary Spastic Paraplegia.

BACKGROUND AND PURPOSE: The "ears of the lynx" MR imaging sign has been described in case reports of hereditary spastic paraplegia with a thin corpus callosum, mostly associated with mutations in the spatacsin vesicle trafficking associated gene, causing Spastic Paraplegia type 11 (SPG11). This sign corresponds to long T1 and T2 values in the forceps minor of the corpus callosum, which appears hyperintense on FLAIR and hypointense on T1-weighted images. Our purpose was to determine the sensitivity and specificity of the ears of the lynx MR imaging sign for genetic cases compared with common potential mimics. MATERIALS AND METHODS: Four independent raters, blinded to the diagnosis, determined whether the ears of the lynx sign was present in each of a set of 204 single anonymized FLAIR and T1-weighted MR images from 34 patients with causal mutations associated with SPG11 or Spastic Paraplegia type 15 (SPG15). 34 healthy controls, and 34 patients with multiple sclerosis. RESULTS: The interrater reliability for FLAIR images was substantial (Cohen κ, 0.66-0.77). For these images, the sensitivity of the ears of the lynx sign across raters ranged from 78.8 to 97.0 and the specificity ranged from 90.9 to 100. The accuracy of the sign, measured by area under the receiver operating characteristic curve, ranged from very good (87.1) to excellent (93.9). CONCLUSIONS: The ears of the lynx sign on FLAIR MR imaging is highly specific for the most common genetic subtypes of hereditary spastic paraplegia with a thin corpus callosum. When this sign is present, there is a high likelihood of a genetic mutation, particularly associated with SPG11 or SPG15, even in the absence of a family history.

Mutations in the motor and stalk domains of KIF5A in spastic paraplegia type 10 and in axonal Charcot-Marie-Tooth type 2.

Spastic paraplegia type 10 (SPG10) is an autosomal dominant form of hereditary spastic paraplegia (HSP) due to mutations in KIF5A, a gene encoding the neuronal kinesin heavy chain implicated in anterograde axonal transport. KIF5A mutations were found in both pure and complicated forms of the disease; a single KIF5A mutation was also detected in a CMT2 patient belonging to an SPG10 mutant family. To confirm the involvement of the KIF5A gene in both CMT2 and SPG10 phenotypes and to define the frequency of KIF5A mutations in an Italian HSP patient population, we performed a genetic screening of this gene in a series of 139 HSP and 36 CMT2 affected subjects. We identified five missense changes, four in five HSP patients and one in a CMT2 subject. All mutations, including the one segregating in the CMT2 patient, are localized in the kinesin motor domain except for one, falling within the stalk domain and predicted to generate protein structure destabilization. The results obtained indicate a KIF5A mutation frequency of 8.8% in the Italian HSP population and identify a region of the kinesin protein, the stalk domain, as a novel target for mutation. In addition, the mutation found in the CMT2 patient strengthens the hypothesis that CMT2 and SPG10 are the extreme phenotypes resulting from mutations in the same gene.

Clinical phenotype variability in patients with hereditary spastic paraplegia type 5 associated with CYP7B1 mutations.

Spastic paraplegia type 5 (SPG5) is caused by mutations in CYP7B1, a gene encoding the cytochrome P-450 oxysterol 7-α-hydroxylase, CYP7B1, an enzyme implicated in the cholesterol metabolism. Mutations in CYP7B1 were found in both pure and complicated forms of the disease with a mutation frequency of 7.7% in pure recessive cases. The mutation frequency in complex forms, approximately 6.6%, is more controversial and needs to be refined. We studied in more detail the SPG5-related spectrum of complex phenotypes by screening CYPB1 for mutations in a large cohort of 105 Italian hereditary spastic paraplegias (HSPs) index patients including 50 patients with a complicated HSP (cHSP) phenotype overlapping the SPG11- and the SPG15-related forms except for the lack of thin corpus callosum and 55 pure patients. Five CYP7B1 mutations, three of which are novel, were identified in four patients, two with a complex form of the disease and two with a pure phenotype. The CYP7B1 mutation frequencies obtained in both complicated and pure familial cases are comparable to the known ones. These results obtained extend the range of SPG5-related phenotypes and reveal variability in clinical presentation, disease course and functional profile in the SPG5-related patients while providing with some clues for molecular diagnosis in cHSP.

The genetic features of 24 patients affected by familial and sporadic hemiplegic migraine.

Familial hemiplegic migraine (FHM) is the only migraine subtype for which a monogenic mode of inheritance, autosomal dominant has been clearly established. It is genetically heterogeneous and at least three different genes exist (CACNA1A, ATP1A2, and SCN1A), the so-called FHM1, FHM2, and FHM3 genes, respectively. Sporadic hemiplegic migraine (SHM) is a disorder, in which some patients may have their pathophysiology identical to FHM, but others, possibly the majority, may have different pathophysiology, probably related to the mechanisms of typical migraine with aura. In our study, we have screened the DNA of 24 patients affected by FHM and SHM. Only in three patients, 2 sporadic and 1 familial cases, we have described genetic mutations, all of them in the ATP1A2 gene. In our opinion, these results demonstrate a more frequent involvement of the ATP1A2 gene not only in the sporadic form, but probably also in the Italian FHM patients without permanent cerebellar signs. Moreover, the absence of CACNA1A, ATP1A2 and SCN1A mutations in the other 12 familial cases suggests the involvement of still unknown genes.

The GST domain of GDAP1 is a frequent target of mutations in the dominant form of axonal Charcot Marie Tooth type 2K.

BACKGROUND: Mutations in GDAP1 associate with demyelinating (CMT4A) and axonal (CMT2K) forms of CMT. While CMT4A shows recessive inheritance, CMT2K can present with either recessive (AR-CMT2K) or dominant segregation pattern (AD-CMT2K), the latter being characterised by milder phenotypes and later onset. The majority of the GDAP1 mutations are associated with CMT4A and AR-CMT2K, with only four heterozygous mutations identified in AD-CMT2K. METHODS: We screened GDAP1 gene in a series of 43 index patients, 39 with CMT2 and 4 with intermediate CMT, with sporadic and familial occurrence of the disease. RESULTS: Three novel mutations were identified in three families with dominant segregation of the disease: two missense changes, p.Arg226Ser and p.Ser34Cys, affecting the GST domain of the GDAP1 protein and a novel deletion (c.23delAG) leading to early truncation of the protein upstream the GST domain. Wide variability in clinical presentation is shared by all three families mostly in terms of age at onset and disease severity. A rare variant p.Gly269Arg, located within the GST domain, apparently acts as phenotype modulator in the family carrying the deletion. CONCLUSION: The results obtained reveal a GDAP1 mutation frequency of 27% in the dominant families analysed, a figure still unreported for this gene, thus suggesting that GDAP1 involvement in dominant CMT2 might be higher than expected.

Novel splice-site mutations and a large intragenic deletion in PLA2G6 associated with a severe and rapidly progressive form of infantile neuroaxonal dystrophy.

Infantile neuroaxonal dystrophy, INAD, is a severe progressive psychomotor disorder with infantile onset and characterized by the presence of axonal spheroids throughout the central and peripheral nervous systems. A subset of INAD patients shows also brain iron accumulation which represents instead the distinctive feature of the idiopathic neurodegeneration with brain iron accumulation, NBIA. These diseases share the same causative gene, PLA2G6, encoding iPLA2-VIA, a calcium-independent phospholipase. Mutations that lead to a complete absence of protein are associated with a severe INAD profile, while compound heterozygous mutations with possibly a residual protein activity are instead associated with the less severe NBIA phenotype. Here we describe two INAD patients both with an unusually rapid disease progression and a peculiar neuroradiological presentation in one of them. Compound heterozygosity for a large intragenic deletion and a nonsense mutation was found in one of them while the other is carrying two novel splice-site mutations. Breakpoint-sequence analysis suggests a non-allelic-homologous-recombination (NAHR) event, probably underlying the rearrangement. These findings, while supporting the genotype-phenotype correlation already observed in INAD patients, provide the first sequence characterization of a genomic rearrangement in PLA2G6 gene, thus orienting the search for missing mutant alleles in PLA2G6 related diseases.

Nitric oxide inhibition of Drp1-mediated mitochondrial fission is critical for myogenic differentiation.

During myogenic differentiation the short mitochondria of myoblasts change into the extensively elongated network observed in myotubes. The functional relevance and the molecular mechanisms driving the formation of this mitochondrial network are unknown. We now show that mitochondrial elongation is required for myogenesis to occur and that this event depends on the cellular generation of nitric oxide (NO). Inhibition of NO synthesis in myogenic precursor cells leads to inhibition of mitochondrial elongation and of myogenic differentiation. This is due to the enhanced activity, translocation and docking of the pro-fission GTPase dynamin-related protein-1 (Drp1) to mitochondria, leading also to a latent mitochondrial dysfunction that increased sensitivity to apoptotic stimuli. These effects of NO inhibition were not observed in myogenic precursor cells containing a dominant-negative form of Drp1. Both NO-dependent repression of Drp1 action and maintenance of mitochondrial integrity and function were mediated through the soluble guanylate cyclase. These data uncover a novel level of regulation of differentiation linking mitochondrial morphology and function to myogenic differentiation.

Point mutations and a large intragenic deletion in SPG11 in complicated spastic paraplegia without thin corpus callosum.

BACKGROUND: Hereditary spastic paraplegia (HSP) with thin corpus callosum (HSP-TCC) is a frequent subtype of complicated HSP clinically characterised by slowly progressive spastic paraparesis with cognitive impairment and thin corpus callosum (TCC). SPG11, the gene associated with the major locus involved, encodes spatacsin, a protein of unknown function. METHODS: Different types of mutations were identified in patients with the complex form of HSP (cHSP) including TCC. We screened a series of 45 index patients with different types of cHSP with (n = 10) and without (n = 35) TCC. RESULTS: Ten mutations, of which five are novel, were detected in seven patients. Of importance, three out of seven mutated patients present with cHSP without TCC. Among the novel mutations identified, we characterised a large intragenic rearrangement deleting 2.6 kb of the SPG11 gene. The rearrangement is due to non-allelic homologous recombination between Alu sequences flanking the breakpoints. CONCLUSIONS: These findings expand the mutation spectrum of SPG11 and suggest that SPG11 mutations may occur more frequently in familial than sporadic forms of cHSP without TCC. This helps to define further clinical and molecular criteria for a correct diagnosis of the SPG11 related form of cHSP. In addition, the intragenic deletion detected here, and the mechanism involved, both provide clues to address the issue of SPG11 missing mutant alleles previously reported.

Mutated mitofusin 2 presents with intrafamilial variability and brain mitochondrial dysfunction.

BACKGROUND: The axonal forms of Charcot-Marie-Tooth (CMT2) disease are a clinically and genetically heterogeneous group of disorders. Mitofusin 2 gene (MFN2) mutations are the most common cause of CMT2. Complex phenotypes have been described in association with MFN2 gene mutations, including CMT2 with pyramidal features (hereditary motor and sensory neuropathy [HSMN V]) and CMT2 with optic atrophy (HMSN VI). OBJECTIVE: To report on the clinical, neurophysiologic, and neuropathologic features of an Italian family with a novel MFN2 gene mutation and investigate brain functional parameters using magnetic resonance spectroscopy (MRS). METHODS: Three family members, a father and his two sons, were affected by peripheral neuropathy, cognitive impairment, and poor nocturnal vision (also optic neuropathy in one case). A member of this family also showed spastic paraparesis. The MFN2 gene sequence was analyzed. A sural nerve biopsy as well as brain (1)H-MRS and (31)P-MRS were evaluated in two patients. RESULTS: Affected family members carried a novel MFN2 missense mutation, namely R104W, located within the critical GTPase domain of the protein which affects a highly conserved amino acid position. Sural nerve biopsies showed a normal mitochondrial network, particularly at the nodes of Ranvier, upon electron microscopy examination. A significant defect of high energy phosphates (HEPs) in the visual cortex was observed at rest by (31)P-MRS in the adult proband, while his son showed a defective recovery of HEPs after stimulation of the visual cortex. CONCLUSION: Cognitive impairment may be another feature of the MFN2-related phenotype. The widespread peripheral and CNS involvement, as well as the neurosensorial defects, underline the similarities among MFN2-related and primary mitochondrial disorders.

Amino acid changes in the amino terminus of the Na,K-adenosine triphosphatase alpha-2 subunit associated to familial and sporadic hemiplegic migraine.

Familial hemiplegic migraine (FHM) is a rare subtype of migraine with aura inherited with an autosomal dominant pattern. Here, we report the genetic analysis of four families and one sporadic case with hemiplegic migraine (HM) in whom we searched for mutations in the three genes associated with the disease CACNA1A, ATP1A2 and SCN1A. Two novel amino acid changes p.Arg65Trp and p.Tyr9Asn, in the Na,K-adenosine triphosphatase (ATPase) alpha-2 subunit encoded by the ATP1A2 gene, were found in one FHM family and in the sporadic case, respectively. These mutations are peculiar for their location in the extreme N-terminus, an uncommon mutation target in this protein. Low frequency of migraine attacks in all our mutant patients with low complexity of the associated aura symptoms in the sporadic case is also observed. Besides the two novel mutations, the data here reported confirm the involvement of ATP1A2 gene in the sporadic form of HM, while the negative results on the other families tested for all genes known in HM strengthen the hypothesis of the existence of at least another locus involved in FHM.

A novel mutation in KCNQ2 associated with BFNC, drug resistant epilepsy, and mental retardation.

BACKGROUND: Benign familial neonatal convulsion (BFNC) is a rare autosomal dominant disorder caused by mutations in two genes, KCNQ2 and KCNQ3, encoding for potassium channel subunits underlying the M-current. This current limits neuronal hyperexcitability by causing spike-frequency adaptation. METHODS: The authors describe a BFNC family with four affected members: two of them exhibit BFNC only while the other two, in addition to BFNC, present either with a severe epileptic encephalopathy or with focal seizures and mental retardation. RESULTS: All affected members of this family carry a novel missense mutation in the KCNQ2 gene (K526N), disrupting the tri-dimensional conformation of a C-terminal region of the channel subunit involved in accessory protein binding. When heterologously expressed in CHO cells, potassium channels containing mutant subunits in homomeric or heteromeric configuration with wild-type KCNQ2 and KCNQ3 subunits exhibit an altered voltage-dependence of activation, without changes in intracellular trafficking and plasma membrane expression. CONCLUSION: The KCNQ2 K526N mutation may affect M-channel function by disrupting the complex biochemical signaling involving KCNQ2 C-terminus. Genetic rather than acquired factors may be involved in the pathophysiology of the phenotypic variability of the neurologic symptoms associated with BFNC in the described family.

Overexpression of wild-type and mutant mucolipin proteins in mammalian cells: effects on the late endocytic compartment organization.

Mucolipin-1 is a 65-kDa membrane protein encoded by the MCOLN1 gene, which is mutated in patients with mucolipidosis type IV (MLIV), a rare neurodegenerative lysosomal storage disorder. We studied the subcellular localization of wild-type and three different mutant forms (T232P, F408del and F465L) of mucolipin by expressing Myc-tagged proteins in HeLa cells. The overexpressed wild-type mucolipin colocalizes to late endocytic structures and induces an aberrant distribution of these compartments. F408del and F465L MLIV mutant proteins show a distribution similar to the wild-type protein, whereas T232P is retained in the endoplasmic reticulum. Among the mutants, only F408del induces a redistribution of the late endocytic compartment. These findings suggest that the overexpression of the mucolipin cation channel influences the dynamic equilibrium of late endocytic compartments.

Molecular cloning and characterization of NEU4, the fourth member of the human sialidase gene family.

Several mammalian sialidases have been cloned so far and here we describe the identification and expression of a new member of the human sialidase gene family. The NEU4 gene, identified by searching sequence databases for entries showing homologies to the human cytosolic sialidase NEU2, maps in 2q37 and encodes a 484-residue protein. The polypeptide contains all the typical sialidase amino acid motifs and, apart from an amino acid stretch that appears unique among mammalian sialidases, shows a high degree of homology for NEU2 and the plasma membrane-associated (NEU3) sialidases. RNA dot-blot analysis showed a low but wide expression pattern, with the highest level in liver. Transient transfection in COS7 cells allowed the detection of a sialidase activity toward the artificial substrate 4MU-NeuAc in the acidic range of pH. Immunofluorescence staining and Western blot analysis demonstrated the association of NEU4 with the inner cell membranes.

Identification and characterisation of human xCT that co-expresses, with 4F2 heavy chain, the amino acid transport activity system xc-.

We have identified a new human complementary deoxyribonucleic acid (cDNA), for the xc- amino acid transporter (HGMW-approved name SLC7A11; also known as human xCT), that, when co-expressed with the heavy chain of surface antigen 4F2 (4F2hc, also termed CD98), but not with rBAT, (related to the bo,+ amino acid transporter), induces system xc- transport activity in Xenopus oocytes. Human xCT is the seventh human member of the family of amino acid transporters that are subunits of 4F2hc or rBAT and, inview of its amino acid sequence identity (89%) with mouse xCT, is most probably the human orthologue thereof. The amino acid transport activity induced by the co-expression of human 4F2hc and xCT in Xenopus oocytes was sodium independent and specific for L-cystine, L-glutamate and L-aspartate. This activity also functioned in an exchange mode (e.g. cystine/glutamate) with a substrate stoichiometry of 1:1. Expression of human xCT alone in oocytes did not induce amino acid transport activity and the expressed xCT protein was localised intracellularly. When human xCT was co-expressed with 4F2hc, the former localised to the oocyte plasma membrane. Tissue-expression studies showed that human SLC7A11 mRNA is expressed mainly in the brain, but also in pancreas and in cultured cell lines. The transport characteristics of human xCT and the distribution of its tissue expression strongly suggest that it corresponds to the human amino acid transporter system xc-.