McLeod phenotype associated with a XK missense mutation without hematologic, neuromuscular, or cerebral involvement

A case of McLeod syndrome caused by a nonsense variation c.942G>A in the XK gene: A case report

McLeod syndrome is a rare XK gene-related progressive, debilitating disease involving multiple systems. The blood group phenotypes in McLeod syndrome patients usually display the Kx antigen loss and a decrease in the Kell blood group system antigen expression. This paper describes a 41-year-old male Chinese patient with McLeod syndrome. He first attended a hospital in 2015 and developed progressively worsening symptoms 4 years ago. As the disease progressed, the patient exhibited memory loss, unresponsiveness, and chorea and displayed elevated creatine kinase levels. However, McLeod syndrome could not be diagnosed by these signs and laboratory results. The patient was readmitted to the hospital in 2020 and was suspected of having McLeod syndrome. Serological analysis of the Kell blood group system and genotyping for the XK blood group system were performed, revealing the weak expression of the K antigen and the negative K antigen. Sequencing of the coding region of the XK gene showed a hemizygous c.942G>A variation in the XK gene, which resulted in a premature stop codon at position 314 (p.Trp314Ter). Therefore, the patient was diagnosed with McLeod syndrome. In conclusion, this paper presents a case of McLeod syndrome caused by a nonsense variation c.942G>A in the XK gene. The analysis of the XK gene and blood group antigen is helpful for the diagnosis of McLeod syndrome and for distinguishing it from many other diseases.

VPS13 Forum Proceedings: XK, XK-Related and VPS13 Proteins in Membrane Lipid Dynamics

In 2020, the pandemic interrupted the series of biannual International Neuroacanthocytosis Meetings that brought together clinicians, scientists, and patient groups to share research into a small group of devastating genetic diseases that combine both acanthocytosis (deformed red blood cells) and neurodegeneration with movement disorders. This Meeting Report describes talks at the 5th VPS13 Forum in January 2022, one of a series of online meetings held to fill the gap. The meeting addressed the basic biology of two key proteins implicated in chorea-acanthocytosis (mutations in VPS13A) and McLeod syndrome (mutations in XK). In a remarkable confluence of ideas, the speakers described different aspects of a single functional unit that comprises of VPS13A and XK proteins working together. Conditions caused by VPS13 (A-D) gene family mutations and related genes, such as XK, previously footnote knowledge, seem to turn central for a novel disease paradigm: bulk lipid transfer disorders.

Disposition of Proteins and Lipids in Synaptic Membrane Compartments Is Altered in Q175/Q7 Huntington's Disease Mouse Striatum

Dysfunction at synapses is thought to be an early change contributing to cognitive, psychiatric and motor disturbances in Huntington's disease (HD). In neurons, mutant Huntingtin collects in aggregates and distributes to the same sites as wild-type Huntingtin including on membranes and in synapses. In this study, we investigated the biochemical integrity of synapses in HD mouse striatum. We performed subcellular fractionation of striatal tissue from 2 and 6-month old knock-in Q175/Q7 HD and Q7/Q7 mice. Compared to striata of Q7/Q7 mice, proteins including GLUT3, Na+/K+ ATPase, NMDAR 2b, PSD95, and VGLUT1 had altered distribution in Q175/Q7 HD striata of 6-month old mice but not 2-month old mice. These proteins are found on plasma membranes and pre- and postsynaptic membranes supporting hypotheses that functional changes at synapses contribute to cognitive and behavioral symptoms of HD. Lipidomic analysis of mouse fractions indicated that compared to those of wild-type, fractions 1 and 2 of 6 months Q175/Q7 HD had altered levels of two species of PIP2, a phospholipid involved in synaptic signaling, increased levels of cholesterol ester and decreased cardiolipin species. At 2 months, increased levels of species of acylcarnitine, phosphatidic acid and sphingomyelin were measured. EM analysis showed that the contents of fractions 1 and 2 of Q7/Q7 and Q175/Q7 HD striata had a mix of isolated synaptic vesicles, vesicle filled axon terminals singly or in clusters, and ER and endosome-like membranes. However, those of Q175/Q7 striata contained significantly fewer and larger clumps of particles compared to those of Q7/Q7. Human HD postmortem putamen showed differences from control putamen in subcellular distribution of two proteins (Calnexin and GLUT3). Our biochemical, lipidomic and EM analysis show that the presence of the HD mutation conferred age dependent disruption of localization of synaptic proteins and lipids important for synaptic function. Our data demonstrate concrete biochemical changes suggesting altered integrity of synaptic compartments in HD mice that may mirror changes in HD patients and presage cognitive and psychiatric changes that occur in premanifest HD.

The tertiary structure of the human Xkr8-Basigin complex that scrambles phospholipids at plasma membranes

Xkr8-Basigin is a plasma membrane phospholipid scramblase activated by kinases or caspases. We combined cryo-EM and X-ray crystallography to investigate its structure at an overall resolution of 3.8 Å. Its membrane-spanning region carrying 22 charged amino acids adopts a cuboid-like structure stabilized by salt bridges between hydrophilic residues in transmembrane helices. Phosphatidylcholine binding was observed in a hydrophobic cleft on the surface exposed to the outer leaflet of the plasma membrane. Six charged residues placed from top to bottom inside the molecule were essential for scrambling phospholipids in inward and outward directions, apparently providing a pathway for their translocation. A tryptophan residue was present between the head group of phosphatidylcholine and the extracellular end of the path. Its mutation to alanine made the Xkr8-Basigin complex constitutively active, indicating that it plays a vital role in regulating its scramblase activity. The structure of Xkr8-Basigin provides insights into the molecular mechanisms underlying phospholipid scrambling.

Novel Imaging Biomarkers for Huntington's Disease and Other Hereditary Choreas

PURPOSE OF THE REVIEW

Imaging biomarkers for neurodegenerative disorders are primarily developed with the goal to aid diagnosis, to monitor disease progression, and to assess the efficacy of disease-modifying therapies in support to clinical outcomes that may either show limited sensitivity or need extended time for their evaluation. This article will review the most recent concepts and findings in the field of neuroimaging applied to Huntington's disease and Huntington-like syndromes. Emphasis will be given to the discussion of potential pharmacodynamic biomarkers for clinical trials in Huntington's disease (HD) and of neuroimaging tools that can be used as diagnostic biomarkers in HD-like syndromes.

RECENT FINDINGS

Several magnetic resonance (MR) and positron emission tomography (PET) molecular imaging tools have been identified as potential pharmacodynamic biomarkers and others are in the pipeline after preclinical validation. MRI and 18F-fluorodeoxyglucose PET can be considered useful supportive diagnostic tools for the differentiation of other HD-like syndromes. New trials in HD have the primary goal to lower mutant huntingtin (mHTT) protein levels in the brain in order to reduce or alter the progression of the disease. MR and PET molecular imaging markers have been developed as tools to monitor disease progression and to evaluate treatment outcomes of disease-modifying trials in HD. These markers could be used alone or in combination for detecting structural and pharmacodynamic changes potentially associated with the lowering of mHTT.

Stepwise partitioning of Xp21: a profiling method for XK deletions causative of the McLeod syndrome

BACKGROUND

McLeod syndrome (MLS) is hematologically defined by the absence of the red blood cell (RBC) antigen Kx on the transmembrane RBC protein, XK, representing a highly specific diagnostic marker. Direct molecular assessment of XK therefore represents a desirable diagnostic tool. Whereas pathogenic point mutations may be simply identified, partial and complete deletions of XK on Xp21.1, eventually covering adjacent genes and causing multifaceted "continuous gene syndromes," are difficult to localize.

STUDY DESIGN AND METHODS

Three different McLeod patient samples were tested using 16 initial positional polymerase chain reaction (PCR) procedures distributed over an approximately 2.8-Mbp Xp-chromosomal region, ranging telomeric from MAGEB16 to OTC, centromeric of XK. The molecular breakpoint of one sample with an apparent large Xp deletion was iteratively narrowed down by stepwise positioning further PCR procedures and sequenced. Two mutant XK genes, one previously published and serving as a positive control, were also sequenced.

RESULTS

We confirmed the positive control as previously published and listed as XK*N.20 by the International Society of Blood Transfusion (ISBT). The other XK showed a novel four-nucleotide deletion in Exon 1, 195-198delCCGC (newly listed as XK*N.39 by the ISBT). The third sample had an approximately 151-kbp X-chromosomal deletion, reaching from Exon 2 of LANCL3, across XK to Exon 3 of CYBB (newly listed as XK*N.01.016 by the ISBT). Carrier status of the patients' sister was diagnosed using a diagnostic "gap-PCR."

CONCLUSIONS

The stepwise partitioning of Xp21.1 is pragmatic and cost-efficient in comparison to other diagnostic techniques such as "massive parallel sequencing" given the rarity of MLS. All males with suspected MLS should be considered for molecular XK profiling.

Untangling the Thorns: Advances in the Neuroacanthocytosis Syndromes

There have been significant advances in neuroacanthocytosis (NA) syndromes in the past 20 years, however, confusion still exists regarding the precise nature of these disorders and the correct nomenclature. This article seeks to clarify these issues and to summarise the recent literature in the field. The four key NA syndromes are described here-chorea-acanthocytosis, McLeod syndrome, Huntington's disease-like 2, and pantothenate kinase- associated neurodegeneration. In the first two, acanthocytosis is a frequent, although not invariable, finding; in the second two, it occurs in approximately 10% of patients. Degeneration affecting the basal ganglia is the key neuropathologic finding, thus the clinical presentations can be remarkably similar. The characteristic phenotype comprises a variety of movement disorders, including chorea, dystonia, and parkinsonism, and also psychiatric and cognitive symptoms attributable to basal ganglia dysfunction. The age of onset, inheritance patterns, and ethnic background differ in each condition, providing diagnostic clues. Other investigations, including routine blood testing and neuroimaging can be informative. Genetic diagnosis, if available, provides a definitive diagnosis, and is important for genetic counseling, and hopefully molecular therapies in the future. In this article I provide a historical perspective on each NA syndrome. The first 3 disorders, chorea-acanthocytosis, McLeod syndrome, Huntington's disease-like 2, are discussed in detail, with a comprehensive review of the literature to date for each, while pantothenate kinase-associated neurodegeneration is presented in summary, as this disorder has recently been reviewed in this journal. Therapy for all of these diseases is, at present, purely symptomatic.

[A KEL*02mod allele responsible for an apparent maternity exclusion]

The patient's rare KEL:1,-2 phenotype was highlighted in course of a routine preoperative erythrocyte typing. Unexpectedly, her two daughters presented a KEL:-1,2 phenotype what appeared first as an apparent maternity exclusion. Flow cytometry, genotyping and adsorption-elution analyses were then performed for those three patients. KEL genotyping showed that the patient's genotype was KEL*01/KEL*02 whereas that of her daughters was KEL*02/KEL*02. By using polyclonal anti-KEL2 reagent, weak amount of KEL2 was identified on the patient's erythrocytes, a result which was confirmed by both flow cytometry and adsorption-elution assays, suggesting that patient's phenotype was in fact KEL:1,2w. These results are in favour of a weak expressed KEL*02 allele (KEL*2mod) transmission coding for a KEL2 antigen detected in some technical conditions only. Those results allowed to explain the apparent maternity exclusion based on initial KEL phenotype. This study also seems to confirm the presence of a compensatory mechanism of the KELmod allele deficient expression in heterozygote patients. A KEL phenotype retrospective study of 80,000 subjects showed a local KEL:1,-2 frequency four times lower than that described in literature. Moreover, a significant number of those individuals would in reality be KEL:1,2w, what still would decrease the real frequency of the KEL:1,2 subjects.

Ablation of the Kell/Xk complex alters erythrocyte divalent cation homeostasis

XK is a putative transporter of unknown function that is ubiquitously expressed and linked through disulfide bonds to Kell protein, an endothelin-3 (ET-3)-converting enzyme. We generated three knockout (KO) mice that lacked either Xk, Kell or both proteins and characterized erythrocyte cation levels, transport and hematological parameters. Absence of Xk or Kell was accompanied by changes in erythrocyte K(+), Mg(2+), Na(+) and Ca(2+) transport that were associated with changes in mean cellular volume and corpuscular hemoglobin concentration mean. Baseline Ca(2+)-ATPase activity was undetected in erythrocytes from all three mouse types but was restored upon pre-incubation with ET-3. Consistent with these alterations in Ca(2+) handling, we observed increased Gardos channel activity in Kel and Xk KO mice. In addition Kel deletion was associated with increased Mg(2+) permeability while Xk deletion blocked Na/Mg exchanger activity. Our results provide evidence that cellular divalent cation regulation is functionally coupled to the Kell/XK system in erythrocytes and loss of this complex may contribute to acanthocytosis formation in McLeod syndrome.

McLeod syndrome: a neurohaematological disorder

The X-linked McLeod syndrome is defined by absent Kx red blood cell antigen and weak expression of Kell antigens, and this constellation may be accidentally detected in routine screening of apparently healthy blood donors. Most carriers of this McLeod blood group phenotype have acanthocytosis and elevated serum creatine kinase levels and are prone to develop a severe neurological disorder resembling Huntington's disease. Onset of neurological symptoms ranges between 25 and 60 years, and the penetrance of the disorder appears to be high. Additional symptoms of the McLeod neuroacanthocytosis syndrome that warrant therapeutic and diagnostic considerations include generalized seizures, neuromuscular symptoms leading to weakness and atrophy, and cardiopathy mainly manifesting with atrial fibrillation, malignant arrhythmias and dilated cardiomyopathy. Therefore, asymptomatic carriers of the McLeod blood group phenotype should have a careful genetic counseling, neurological examination and a cardiologic evaluation for the presence of a treatable cardiomyopathy.

McLeod phenotype without the McLeod syndrome

BACKGROUND

McLeod neuroacanthocytosis syndrome is a late-onset X-linked multisystem disorder affecting the peripheral and central nervous systems, red blood cells (RBCs), and internal organs. A variety of mutations have been found in the responsible gene (XK) including single nonsense and missense mutations, nucleotide mutations at or near the splice junctions of introns of XK, and different deletion mutations. To date no clear phenotype-genotype correlation is apparent. The clinical details of one case of McLeod phenotype without apparent neuromuscular abnormalities have been reported. Here the clinical details of two additional cases are presented, of which the genetic details have previously been published.

STUDY DESIGN AND METHODS

Two asymptomatic or minimally symptomatic cases at ages expected to manifest the McLeod syndrome (MLS) were evaluated. The first case had been authenticated as a genuine McLeod both by serology and by genotyping (R222G missense mutation) and the second case had a mutation in XK (IVS2+5G>A) and by serology exhibited very weak Kx antigen and no detectable Kell antigens, except extremely low k antigen by adsorption-elution technique. The patients were examined for hematologic, neurologic, and other clinical abnormalities.

RESULTS

Despite documented McLeod phenotype on RBCs, and identified mutations of XK, neurologic and other clinical findings were minimal at ages expected to manifest MLS.

CONCLUSIONS

The different XK mutations may have different effects upon the XK gene product and thus may account for the variable phenotype.

Phenotypic variability of a distinct deletion in McLeod syndrome

The X-linked McLeod neuroacanthocytosis syndrome strongly resembles Huntington's disease and has been reported in various countries world-wide. Herein, we report two Chilean brothers with predominant psychiatric features at disease onset including schizophrenia-like psychosis and obsessive compulsive disorder. Molecular genetic analysis revealed a small deletion in the XK gene (938-942delCTCTA), which has been already described in a North American patient of Anglo-Saxon descent and a Japanese family, presenting with seizures, muscle atrophy or chorea yet absence of psychiatric features. These findings argue against a founder effect and indicate a profound phenotypic variability associated with the 938-942delCTCTA deletion. Our report supports the inclusion of McLeod syndrome in the differential diagnosis of Huntington's disease as well as acute psychosis in male subjects.

Expression profiles of mouse Kell, XK, and XPLAC mRNA

Kell and XK are related because in red cells they exist as a disulfide-bonded complex. Kell is an endothelin-3-converting enzyme, and XK is predicted to be a transporter. Absence of XK, which is accompanied by reduced Kell on red cells, results in acanthocytosis and late-onset forms of central nervous system and neuromuscular abnormalities that characterize the McLeod syndrome. In this study, expression of mouse XK, XPLAC, a homolog of XK, and Kell were compared by in situ hybridization histochemistry (ISHH) and RT-PCR. ISHH showed that Kell and XK are coexpressed in erythroid tissues. ISHH detected XK, but not Kell, mRNA in testis, but RT-PCR indicated that both Kell and XK are coexpressed. XK, but not Kell, was significantly expressed in brain, spinal cord, small intestine, heart, stomach, bladder, and kidney. ISHH did not detect XK in skeletal muscle but RT-PCR did. In brain, XK was predominantly expressed in neuronal rather than in supportive cells. By contrast, XPLAC was predominantly expressed in the thymus. Coexpression of Kell and XK in erythroid tissues and the different expressions in non-erythroid tissues suggest that XK may have a complementary hematological function with Kell and a separate role in other tissues.

Developments in neuroacanthocytosis: Expanding the spectrum of choreatic syndromes

As with other neurodegenerative disorders, research into the group of diseases known under the umbrella term of "neuroacanthocytosis" has greatly benefited from the identification of causative genes. The distinct and unifying aspect of these disorders is the presence of thorny deformations of circulating erythrocytes. This may be due to abnormal properties of red cell membranes, which could lead to insights into mechanisms of neurodegeneration. Research approaches in this field, in addition to examining functions and protein interactions of the affected proteins with particular respect to neurons, have also drawn upon the expertise of hematologists and red cell membrane biologists. In this article, recent developments in the field are presented.

Cerebral metabolic alterations in McLeod syndrome

The X-linked McLeod neuroacanthocytosis syndrome is a multisystem disorder with central nervous system manifestations resembling Huntington's disease. We examined 5 McLeod patients and 5 asymptomatic heterozygous females with fast multiple spin-echo spectroscopic imaging. Three patients with pronounced psychiatric or cognitive manifestations had pathological N-acetyl aspartate/(creatine + choline) ratios in frontal, temporal, and insular areas, with an individual pattern. Two patients with a severe choreatic movement disorder had unilateral thalamic alterations. One patient with moderate movement disorder and personality disorder had bilateral occipital alterations. One female heterozygote had unilateral insular metabolic alterations, possibly indicating subclinical cerebral involvement. Although the prominent psychiatric and cognitive manifestations in McLeod patients suggest significant and widespread cortical abnormalities, previous neuroradiological and histopathological data had not revealed definite extrastriatal pathology. Our findings demonstrating metabolic abnormalities in different brain regions of McLeod patients might either reflect neuronal dysfunction due to impaired basal ganglia-thalamo-cortical circuits or subtle structural alterations in the particular cerebral areas.

Neuroacanthocytosis

PURPOSE OF REVIEW

The term neuroacanthocytosis describes a group of phenotypically and genetically heterogeneous disorders, and thus has long been a source of confusion and diagnostic imprecision. It is vital to distinguish between the lipoprotein deficiency disorders which affect gait, but do not cause movement disorders or neuropsychiatric problems, and the diseases described here, of which these are characteristic features. This review summarizes the current state of knowledge regarding this group of diseases in order to facilitate clinical recognition, accurate diagnosis and appropriate management.

RECENT FINDINGS

Advances in molecular medicine have enabled us to distinguish precisely among the disorders described under the label of neuroacanthocytosis, most notably between autosomal recessive chorea-acanthocytosis and the X-linked McLeod syndrome. This has facilitated appreciation of the range of phenotypes in each of the various conditions. Acanthocytosis is also found in a smaller percentage of cases with pantothenate kinase-associated neurodegeneration (PKAN) and Huntington's disease-like 2 (HDL2). An improved method of determination of acanthocytosis has been described, which if adopted as standard practice may facilitate detection of these conditions.

SUMMARY

Genetic testing has led to increased diagnostic accuracy of the neuroacanthocytosis syndromes, which is essential to extend recognition of these disorders, as well as to improve understanding of the disease process. Most importantly, given the absence of a cure, it is vital for appropriate genetic counselling. Treatments, as in other neurodegenerative conditions, are at present limited to symptomatic therapies.

Neuroacanthocytosis: new developments in a neglected group of dementing disorders

Neurological abnormalities associated with spiculated, "acanthocytic" red cells in blood have been summarized as neuroacanthocytosis. This is a heterogeneous group of conditions that can now be clearly subdivided on the basis of genetic discoveries. The core neuroacanthocytosis syndromes are autosomal recessive chorea-acanthocytosis (ChAc) and the X-linked McLeod syndrome (MLS). Huntington's disease-like 2 (HLD2) and pantothenate kinase associated neurodegeneration (PKAN) can now also be included. All of these share dyskinesias, cognitive deterioration and progressive neurodegeneration mainly of the basal ganglia, but they are sufficiently distinct to permit a specific working diagnosis on the basis of clinical, laboratory and imaging findings. In addition, the VPS13A (formerly called CHAC), XK, JPH3 and PANK2 genes, respectively, may be examined for mutations. Unfortunately, little is yet known about the normal and abnormal physiology of the protein products of these genes, but they appear to be involved in membrane function and intracellular protein sorting. Since no cures are yet available, development and study of disease models in experimental animals (mouse, C. elegans) is a priority for current research. From a clinical point of view, the common occurrence of cardiomyopathy in MLS, the transfusion hazards due to the McLeod Kell phenotype and the possibility of improving the violent trunk spasms and orofacial dyskinesias typical for ChAc (with subsequent lip or tongue mutilations and feeding dystonia) by deep brain surgery or stimulation should be considered in patient management.

A McLeod phenotype detected by random screening for K:-4 [Kp(b-)] blood donors in Brazil

BACKGROUND

The red blood cells of the McLeod phenotype have weak expression of Kell System antigens due to no expression of XK protein.

STUDY DESIGN AND METHODS

One blood donor reacted as K:-4 [Kp(b-)] during a screening assay. Subsequent serologic studies demonstrated weak expression of K:4 and all other high-incidence Kell system antigens tested; however, no expression of Kx antigen was observed.

RESULTS

One apparently healthy blood donor demonstrated low expression of K:2, K:4, K:5, K:7, K:14, K:22, and no Kx antigen in his red blood cells. His brother and mother showed the same weak expression, and his father showed normal expression of antigens tested. Flow cytometry studies confirmed the mother's status as a McLeod carrier female. Genotyping determined the presence of KEL2 and KEL4 alleles in mother and siblings. Southern blot with an exon-1 probe showed fragments shorter than predicted for the siblings and the mother, suggesting a deletion. Polymerase chain reaction with primers spanning exon 1 and flanking regions displayed a similar pattern. Deoxyribonucleic acid sequence allowed the precise characterization of a deletion of 392 bp, beginning at the 5' of the coding region up to nucleotide 201 of exon 1, which putatively abrogates the production of XK protein.

CONCLUSION

Two brothers with McLeod phenotype in a Brazilian blood-donor population were identified. The molecular basis for this phenotype is a 392-bp deletion spanning from 5' of the coding region to exon 1 of the XK gene, never described before.