Genetics of Menkes Kinky Hair Disease Clinical Presentation

The typical history of a patient with Menkes kinky hair disease (MKHD) includes healthy pregnancy and delivery. Birth frequently occurs several weeks in advance of the estimated date of confinement or due date.

• Classic Menkes kinky hair disease often escapes attention in the neonatal period because of its very subtle manifestations in neonates. However, several nonspecific physical and metabolic findings are commonly cited when birth histories of these babies are reviewed.

  • These findings include premature labor and delivery, large cephalohematomas in individuals born by abdominal delivery, hypothermia that necessitates warming lights or an isolette, hypoglycemia for which early feeding or support with intravenous (IV) glucose is instituted, and jaundice that requires several days of phototherapy.

  • Pectus excavatum and inguinal or umbilical herniae are found at birth in some patients with Menkes kinky hair disease.

  • Occasionally, unusual hair pigmentation may suggest the diagnosis in newborns. However, the appearance of the hair is often unremarkable. As in healthy babies, newborns with Menkes kinky hair disease may exhibit no hair or have normally pigmented hair. The pili torti Menkes kinky hair disease on microscopic examination of hair from older patients with Menkes kinky hair disease is not usually evident in the hair of newborns with Menkes kinky hair disease.

• Neurologically, newborns with Menkes kinky hair disease generally appear to be healthy. Excessive jitteriness was noted in one patient at age 1 week.

• Transient neonatal hypothermia and hypoglycemia are not uncommon; however, infants with Menkes kinky hair disease appear essentially healthy at birth and for the first 4-6 weeks of life.

• When the infant is aged approximately 2-2.5 months, the parents usually first suspect that something is wrong and voice their concerns to the healthcare provider.

• A steady downward spiral continues clinically, with development of progressive hypotonia, seizures, failure to thrive, and appearance of the characteristic coarse wiry hair by the time the individual with Menkes kinky hair disease is aged 4-5 months.

As noted previously, major clinical manifestations of persons with Menkes kinky hair disease include loss of early developmental milestones, truncal hypotonia, seizures, poor weight gain, abnormal hair, loose skin, pectus excavatum, and urinary bladder diverticula.

The severity widely varies in patients with Menkes kinky hair disease and certain disorders; although once considered distinct genetic entities, they most likely represent allelic variants.

• Patients with the classic phenotype may differ in certain respects (eg, presence of functional vision, level of infant personal-social development, severity of seizures), but they invariably demonstrate profound hypotonia and motor impairment. In contrast, patients with variants of the classic phenotype demonstrate less severe overall developmental outcomes.

  • One such child evaluated at the NIH was aged approximately 7 months when developmental delay was first noted.

    • When aged 9 months, the child required surgery for bladder outlet obstruction due to massive diverticula.

    • He was diagnosed with Menkes kinky hair disease when aged 22 months.

    • When aged 27 months, he was able to sit alone, crawl, play with toys, indicate his needs, and voice approximately 20 words with poor articulation. He was ataxic with head bobbing and tremor, and his brain MRI exhibited mild cerebellar hypoplasia. He had normal findings on EEG and no clinical seizures. His growth exhibited height between the 25th and 50th percentile for age, with weight and head circumference both slightly below the fifth percentile. His biochemical parameters (eg, plasma copper, plasma catecholamines, copper accumulation in cultured fibroblasts) did not differ significantly from values in infants with classic Menkes kinky hair disease.

    • Apart from his asymmetric growth retardation and history of bladder diverticula, this patient resembles a reported example of mild Menkes kinky hair disease.

  • Another atypical patient has been reported in whom motor development was relatively better, ataxia less severe, and connective tissue problems more prominent than the initially reported mild patient.

  • Because the neurologic impairment is less profound, other patients with conditions such as these seem unlikely to be suspected of having a condition related to Menkes kinky hair disease. Of note, both of the patients reported above had pili torti, which aided in establishing their diagnosis.

• Another probable Menkes variant is type IX Ehlers-Danlos syndrome, otherwise known as X-linked cutis laxa or occipital horn syndrome (OHS), in reference to the pathognomonic wedge-shaped calcification that forms within the trapezius and sternocleidomastoid muscles at their attachment to the occipital bone in affected individuals. See the image below.

  Adolescent patient with typical occipital horn syndrome. Note elbow dislocations and genu valgum. Radiographs exhibited bilateral occipital exostoses of the skull and club-shaped distal clavicles.

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  • This protuberance can be palpated in some patients and is demonstrable radiographically on Towne view or on appropriate sagittal CT scanning or MRI cuts. Radiologic abnormalities of the clavicles and long bones have also been noted.

  • Clinical findings include lax skin and joints, bladder diverticula, inguinal herniae, vascular tortuosity, and normal or slightly subnormal intelligence.

  • Biochemically, plasma copper and ceruloplasmin levels are in the low reference range, copper egress in cultured fibroblasts is impaired to the same degree as in classic Menkes kinky hair disease, and activity of fibroblast lysyl oxidase (LO) is markedly reduced.

  • Additionally, some patients have signs of autonomic dysfunction (eg, syncope, episodic diarrhea) suggestive of dopamine beta-hydroxylase (DBH) deficiency. Neurologic findings are otherwise essentially normal.

• Other clinical variants have been reported that involve features of both the classic Menkes and the occipital horn phenotypes.

  • One kindred study included 4 males aged 15 months to 35 years who had low or low-normal plasma copper; abnormal plasma catecholamines; and a syndrome of mental retardation, childhood-onset seizures (aged 3-8 y), neuromuscular weakness, joint abnormalities, and bladder diverticula.

  • Occipital exostoses were detected in the 15-year-old patient during radiologic studies performed after a sudden cerebral hemorrhage.

  • The 35-year-old man developed seizures when aged 3-4 years and required vesicostomy when aged 9 years for bladder outflow problems caused by diverticula. He learned to walk with the aid of crutches. His elbows became dislocated. He had the estimated intellectual capacity of an individual aged 8 years, and his speech was extremely inarticulate. His size was that of a small adult.

  • The members of this family who were affected with Menkes kinky hair disease were found to have splice site mutations in the 3' region of the Menkes/OHS gene that impaired but did not eliminate proper RNA splicing; a similar type of splicing mutation was found in an unrelated patient with typical OHS.

  • In 1994, Kaler et al quantitated the amount of proper splicing in cultured cells associated with these mutations at approximately 20-30% of normal. ^[12]

Mutations in the Menkes/OHS gene underlie both classic and milder phenotypes. The severity of mutations and amount of possible residual copper ATPase activity appear to be relevant to the variable clinical outcomes. ^[13] Secondary deficiencies of copper-dependent enzymes are believed to cause certain of the clinical manifestations, as reviewed earlier (see Pathophysiology).

• Menkes/OHS gene: Location of the Menkes/OHS gene on the X chromosome was indicated by pedigree analysis from the time of its earliest descriptions.

  • Progressively refined localization to Xq13 was enabled by linkage studies and characterization of X chromosomal rearrangements in 2 unrelated patients.

  • Genomic DNA from the chromosomal breakpoint region was used to identify portions of the Menkes locus by screening complementary DNA (cDNA) libraries or by exon trapping.

  • The candidate gene thus identified was found to be expressed abnormally in more than 70% of patients with Menkes kinky hair disease who were studied, and gene deletions were detectable by Southern blotting in more than 15% of these patients.

  • The Menkes/OHS gene is expressed in all human tissues tested, with the liver being a prominent exception. The messenger RNA (mRNA) transcript is approximately 8.5 kilobases (kb), with a long 3' noncoding portion and a coding sequence of 4.5 kb.

  • The predicted gene product is a 1500–amino acid molecule that is similar to numerous ion-motive ATPase molecules, including a pair of copper-transporting ATPases (copA and copB) in the bacterium Enterococcus hirae, an intracellular copper transporter (CCC2) in Saccharomyces cerevisiae, and the Wilson disease gene product.

• Menkes/OHS copper ATPase: A wide array of ion-motive ATPases has been characterized, and they are divided into 3 classes (ie, P, V, F).

  • The P-type ATPases, of which the Menkes gene product is an example, are so named because they form a covalently phosphorylated intermediate from transfer of the gamma-phosphate of ATP to a specific aspartate residue at the catalytic site of the protein.

  • V-type ATPases are those associated with vacuolar organelles (eg, lysosomes, endosomes, storage granules, Golgi vesicles).

  • The F-type ATPases are found in most bacteria and are associated with mitochondria in higher organisms.

  • The overall sequence similarity among prokaryotic and eukaryotic cation-transporting ATPases suggests that these proteins have been modified throughout evolution in response to the need for import and export of various cations across different cellular membranes.

  • P-type ATPases function in the regulation of intracellular ion concentrations. Those imbedded in plasma membranes function to extrude their respective ions from the cell. Other P-type ATPases are localized to the membranes of intracellular organelles (eg, sarcoplasmic reticulum, endoplasmic reticulum) and act to sequester ions within their lumens. Interaction with ATP at a specific site is believed to induce multiple conformational adjustments that reorient the molecule with respect to the membrane, creating channels for cation translocation from high-affinity binding sites on one membrane side to low-affinity binding domains oriented toward the other side. Alternative splicing that generates multiple isoforms with potential functional differences has been demonstrated for certain ATPases of this class.

• Menkes gene product: In the Menkes gene product, the N-terminal portion has a distinctive recurring amino acid pattern, cysteine-X-serine-cysteine, which is comparable to putative metal binding sites in the bacterial ATPases involved in transport of copper, cadmium, and mercury, respectively. Hydrophobicity analysis indicates 6-8 transmembrane domains, and the predicted protein demonstrates all the other functional domains expected of P-type ATPases. The expression of the Menkes gene in nearly all human tissues, the severe consequences of impaired function (ie, MKHD) and the high degree of evolutionary conservation all indicate the fundamental importance of the copper transport process that this gene encodes.

  • With the discovery of the Menkes gene, investigating the basic cell biological defect in greater depth became possible. Elegant studies in several laboratories (ie, Camakaris, Francis, Gitlin, Glover, Mercer) localized the Menkes/OHS gene product to the trans-Golgi apparatus, where it is involved in the delivery of copper to copper-dependent enzymes processed in the secretory pathway of cells. In addition, the Menkes/OHS ATPase appears to relocate in response to increased copper exposure, moving to the plasma membrane of cells where it presumably functions as a pump directly mediating copper exodus from cells.

  • This model of the gene product's locations (trans-Golgi and plasma membrane) is consistent with the copper retention phenotype of cultured Menkes cells and with induction of metallothionein (MT) in these cells at much lower experimental copper exposures than in healthy cells.

    • MT, a cysteine-rich heavy metal–binding protein, may represent a secondary line of defense against the toxicity of high intracellular copper that is required sooner than usual in Menkes cells because of a defect in the primary regulatory mechanism (ie, removal of copper).

    • The efficiency with which MT performs its detoxifying role and the extremely avid binding of MT to copper restricts the availability of the metal to cytosolic copper enzymes (eg, Cu/Zn SOD), as well as those synthesized or located in cellular compartments. Under normal conditions, MT presumably helps maintain low intracellular levels of free copper while permitting activation of the copper enzymes. However, in Menkes kinky hair disease, chronic MT induction due to failure of normal copper transport disrupts this balance.

    • Failure to extrude copper from intestinal mucosal cells into the blood explains the accumulation of copper in these cells and the consequent reduced delivery of copper to the liver in individuals with Menkes kinky hair disease. Similarly, failed copper export by placental cells accounts for the placental copper accumulation observed in pregnancies affected by Menkes kinky hair disease and the low hepatic copper levels in fetuses with Menkes kinky hair disease. Failure of copper export by vascular endothelial and glial cells comprising the blood-brain barrier explains low copper levels and reduced copper enzyme activities in the brain. Because the Menkes gene is weakly expressed in the liver, this organ must possess alternative mechanisms for copper excretion (ie, Wilson copper ATPase) ^[14] and thus does not manifest the copper accumulation, MT induction, or copper enzyme deficiencies found in other tissues.

• Mutational analysis of the Menkes/OHS gene: Identification of the Menkes/OHS gene provided the opportunity for molecular analysis of patients with Menkes kinky hair disease. Before the genomic organization of the gene was characterized completely, the reverse transcription–polymerase chain reaction method (RT-PCR) offered a particularly useful approach to mutation analysis.

  • RT-PCR involves isolation of intact RNA from cultured fibroblasts or lymphoblasts and reverse transcription of the 8.5-kb Menkes message using oligodeoxythymidine as a primer. This reaction generates a full-length cDNA that is suitable as a template for PCR using gene-specific primers. The entire cDNA from patients with Menkes kinky hair disease thus may be obtained for direct sequence analysis.

  • Menkes/OHS gene mutation analysis by this method, and more recently from genomic DNA directly, has been conducted in several laboratories (ie, Horn, Kaler, Mercer, Ogawa), contributing important information about the molecular correlates of certain clinical and biochemical phenotypes and about functional aspects of this copper-ATPase. Molecular diagnosis also provides practical benefit to Menkes families with regard to detection of fetuses with Menkes kinky hair disease and females who are carriers.