Homocystinuria with methylmalonic acidemia is a rare metabolic disorder. There are distinct types of the disease, with the most common one being homocystinuria with methylmalonic acidemia type cblC (HMMAC). HMMAC is inherited in an autosomal recessive manner and is caused by mutations in the MMACHC gene. Only recently, recessive X-linked inheritance has been reported for an HMMAC-like disease that couldn't be related to MMACHC mutations. It has been proposed to refer to this disorder as homocystinuria with methylmalonic acidemia type cblX (HMMAX).
Most HMMAX patients appear normal at birth but small gestational age has been reported . According to current knowledge, HMMAX manifests within the first few months of life; symptom onset beyond infancy is not uncommon in case of HMMAC but has yet to be described in HMMAX patients . Affected infants are presented to the pediatrician with failure to thrive, severe developmental delays, and neurological symptoms. The spectrum of neurological deficits is broad and ranges from cognitive impairment to choreoathetosis and other movement disorders and anomalies of muscle tone. HMMAX patients commonly experience intractable seizures. Affected individuals may be microcephalic . Psychiatric conditions and behavioral disorders, visual impairment, and peripheral neuropathy have repeatedly been observed in HMMAC patients but no such findings have been made in those suffering from HMMAX    . Similarly, extraneural manifestations like cardiac malformation, cardiovascular accidents, renal involvement resulting in thrombotic microangiopathy and hemolytic uremic syndrome, gastrointestinal and dermatological symptoms, all of which have been associated with HMMAC, may eventually occur in HMMAX patients  .
Neurological symptoms are due to diffuse encephalopathy, delayed myelination, and cerebral atrophy, which are conditions that may be verified by means of diagnostic imaging. Hypsarrythmia is a common electroencephalographic finding . These findings are non-specific, though. Analyses of blood and urine samples have to be carried out and provide essential information as to the underlying metabolic disorder :
Still, these results don't allow for a reliable diagnosis of HMMAX since similar findings may be obtained in patients suffering from much more common disorders like vitamin B12 deficiency and folic acid deficiency, and in those with other hereditary diseases . While serum levels of vitamin B12 and folic acid can be assessed easily, the confirmation and differential diagnosis of HMMAX require genetic studies. Because HMMAC is assumed to be the most common inherited error of cobalamin metabolism, it is likely and reasonable that patients are first tested for MMACHC mutations. No such mutations will be detected in case of HMMAX, though. In a second step, complementation studies may be carried out: They will suggest HMMAC despite negative genetic analyses. This is the typical starting point for diagnostic measures aiming at confirming a tentative diagnosis of HMMAX, which is most reliably achieved by identifying the underlying mutation in the HCFC1 gene . Alternatively, a genealogical analysis of familial aggregation may point at a recessive X-linked inheritance of the disease, which should immediately raise suspicion of HMMAX.
Enzyme activity measurements in fibroblasts or lymphocytes constitute an alternative approach to the diagnosis of combined disorders of cobalamin metabolism, but are more cumbersome than genetic studies and don't provide any information as to the specific mutation. Nevertheless, such assays have to be carried out if the aforementioned analyses don't yield conclusive results despite strong suspicion, or if they are not available. Both methionine synthase and methylmalonyl-CoA mutase activities are decreased in case of HMMAX.
Guidelines for the diagnosis and management of cobalamin-related disorders including HMMAC have recently been published . Since HMMAX-causing HCFC1 mutations result in reduced expression of the MMACHC gene and thus in functional HMMAC, it seems reasonable to follow these guidelines in case of HMMAX. In general, treatment aims at improving clinical features and normalizing hematological and metabolic values:
At least one boy affected by HMMAX succumbed to the disease and maternal male infant deaths have repeatedly been reported, suggesting that HMMAX is a potentially fatal disorder. Even though data regarding the long-term outcome of HMMAX patients is scarce, assumptions may be made based on data obtained from those affected by HMMAC: Cobalamin supplementation is likely to improve neurological parameters like cognitive performance and motor function if initiated early enough. Notwithstanding, central nervous system damage is largely irreversible and substantial improvements in cerebral atrophy and white matter changes are not to be expected. Therefore, patients who are diagnosed and treated early probably have a much better prognosis than those who are diagnosed after the manifestation of clinical symptoms. In an ideal scenario, affected individuals are diagnosed before birth or identified by means of newborn screening . Still, even adequate therapy may not entirely prevent the development of complications of homocystinuria with methylmalonic acidemia and pre-existing complications may not respond to treatment. Most patients will probably experience some degree of disability over the course of their life .
HMMAX is caused by mutations in the HCFC1 gene. This gene is located on the long arm of the X chromosome and encodes in a coregulator of the zinc-finger transcription factor THAP11 that affects the expression of the MMACHC gene. In detail, HMMAX-related HCFC1 mutations are associated with severe reductions in MMACHC mRNA and protein and thus give rise to functional HMMAC. It should also be considered that the THAP11-HCFC1 complex may bind to genes MTR and ABCD4, which are involved in the pathogenesis of homocystinuria without methylmalonic aciduria type cblG and homocystinuria with methylmalonic acidemia type cblJ, respectively. Even though MTR and ABCD4 mRNA and protein levels didn't seem to be affected in those HMMAX patients examined by Yu and colleagues, the underexpression of these genes may contribute to the known phenotype of combined methionine synthase and methylmalonyl-CoA mutase deficiency. The high severity of symptoms observed in HMMAX patients may indeed be explained by a global impairment of cobalamin metabolism comparable to combined HMMAC, homocystinuria without methylmalonic aciduria type cblG and homocystinuria with methylmalonic acidemia type cblJ . Interestingly, THAP11 mutations have recently been identified in a patient who presented with clinical and biochemical features overlapping those of HMMAX, but who tested negative for MMACHC and HCFC1 mutations .
Of note, mutations in the HCFC1 gene have also been related to X-linked mental retardation type 3 (MRX3). Contrary to HMMAX-related mutations, sequence anomalies identified in MRX3 patients may result in HCFC1 overexpression. Furthermore, additional genetic defects have been detected in some individuals with MRX3 . Further research is required to shed more light on the genetic and possibly non-genetic differences between patients suffering from HMMAX or X-linked mental retardation type 3.
Inborn errors of cobalamin metabolism are rare diseases, with HMMAC being the most common one. More than 250 cases have been reported to date and the incidence of HMMAC has been estimated to 1-5 in 200,000 life births   . It may be assumed that some of these patients, particularly males, may indeed have suffered from HMMAX. There's room for reasonable doubt in all cases that have not been confirmed by genetic studies, which are 14/204 and 3/118 in two studies published by a Canadian research group  . The paper published by Yu and colleagues suggests that mutations in the HCFC1 gene are not uncommon in male patients that have been diagnosed with HMMAC: Such mutations have been detected in 13/17 patients who tested negative for MMACHC mutations.
In general, both males and females may be affected by homocystinuria with methylmalonic acidemia. However, recessive X-linked inheritance largely favors males over females. While males who are hemizygous for pathogenic HCFC1 mutations will develop HMMAX, females will not be affected unless they carry two pathogenic alleles.
All HMMAX patients reported to date developed clinical disease within five months of life . By contrast, the onset of HMMAC-associated symptoms may occur at any time between the neonatal period and adulthood . Further research is required to clarify whether there is a late-onset form of HMMAX.
Similar to HMMAC, HMMAX is a combined disorder of cobalamin metabolism, i.e., the underlying mutation interferes with the synthesis of methylcobalamin and adenosylcobalamin, cofactors of methionine synthase and methylmalonyl-CoA mutase . Thus, there are two pathophysiological cascades implicated in neurodegeneration and extraneural disease progression:
Enhanced oxidative stress due to impaired glutathione metabolism has been proposed as an additional factor in HMMAC pathogenesis , so it may be assumed that this is also true for HMMAX. The underlying molecular mechanisms have not yet been clarified, though. It also remains unknown whether methionine synthase or methylmalonyl-CoA mutase fulfill non-enzymatic functions. If this was the case, the dysregulation of the respective processes would constitute another pathogenetic mechanism in HMMAX and related disorders.
The prenatal diagnosis of HMMAX is feasible. Mutations in the HCFC1 gene can be identified in nucleic acids isolated from chorionic villi or amniotic fluid samples. Targeted genetic analyses require strong suspicion based on the carrier state of both parents. Most reliable results are obtained if the specific HCFC1 mutations of a child's mother or father are known. If genetic studies cannot be realized or yield inconclusive results, biochemical assays and enzyme activity measurements may be carried out. In this context, increased concentrations of homocysteine and methylmalonic acid in cell-free amniotic fluid indicate the possibility of homocystinuria with methylmalonic acidemia but don't allow for the identification of its type. In particular, the distinction between HMMAC and HMMAX is not possible unless genetic data of the child or their parents are available.
Of note, prenatal therapy via maternal treatment with hydroxycobalamin has yielded promising results in a child affected by HMMAC and may be considered if HMMAX is diagnosed .
According to the conventional classification of homocystinuria with methylmalonic acidemia, there are four types of the disease, namely cblC, cblD, cblF, and cblJ . All of them are induced by functional deficiencies of methionine synthase and methylmalonyl-CoA mutase, and they resemble each other in their clinical and biochemical presentation. The distinction of individual types has long since been based on the results of complementation studies. Because such studies are tedious, they have largely been replaced by molecular genetic analyses. The underlying mutations have been identified for cblC, cblD, cblF, and cblJ and can easily been detected by sequencing genes MMACHC, MMACHD, LMBRD1, and ABCD4, respectively  . However, scientist still rely on the results of somatic cell complementation if genetic analyses don't yield conclusive results despite strong suspicion, or if they are not available. Complementation studies are also carried out to prove the causal relation between genotypes and phenotypes . Only recently, it has been pointed out that HMMAC may indeed be caused by mutations not affecting the DNA sequence of the MMACHC gene  . Because X-linked inheritance has been shown for this variant of HMMAC, it is now referred to as HMMAX.
The term "homocystinuria" refers an increased excretion of homocystine in the urine. This condition may be due to distinct metabolic disorders associated with elevated serum concentrations of homocysteine, an intermediate amino acid and precursor of methionine. In patients suffering from inherited conditions that interfere with the conversion of homocysteine to methionine, blood and tissue levels of homocysteine are increased while methionine concentrations are below reference ranges.
There are different types of homocystinuria and they differ with regards to hematological and biochemical anomalies. Besides hyperhomocysteinemia and hypomethioninemia, patients may have increased serum levels of methylmalonic acids, which is why "homocystinuria with methylmalonic acidemia" is differentiated from "homocystinuria without methylmalonic aciduria". These differences originate from distinct gene defects, but they aren't reflected in the clinical presentation. Most patients suffering from homocystinuria with methylmalonic acidemia present within the neonatal period or infancy: Their parents may claim feeding difficulties and failure to thrive, and affected children show neurological deficits ranging from cognitive impairment to movement disorders and anomalies of muscle tone. Seizures are also common.
Individuals affected by homocystinuria with methylmalonic acidemia type cblX carry mutations in a gene named HCFC1. Thus, genetic studies can be carried out to confirm the diagnosis. Genetic studies may even be realized before birth if a child's parents are known to be carriers of HCFC1 mutations. Boys will develop the disease if they inherit a pathogenic allele from either their mother or their father; girls will develop the disease only if they inherit pathogenic alleles from both their mother and their father. Therefore, males are much more likely to be affected by methylmalonic acidemia type cblX. Few cases have been described to date and data regarding long-term outcomes are not available, so recommendations regarding diagnosis and treatment are mainly based on experiences with patients suffering from other forms of homocystinuria with methylmalonic acidemia. In this context, it is likely that the patients' prognosis largely depends on the time of diagnosis and initiation of treatment. The earlier an adequate treatment is started, the better the prognosis of the individual patient. Treatment consists in intramuscular injections of hydroxycobalamin and oral supplementation of betaine and folic acid. Lifelong therapy is required in all cases and despite utmost compliance with therapeutic regimens, it may not be possible to prevent all complications. If left untreated, homocystinuria with methylmalonic acidemia type cblX follows a slowly progressive course and may lead to severe disability or death.