Redefining infantile-onset multisystem phenotypes of coenzyme Q10-deficiency in the next-generation sequencing era

Primary coenzyme Q10 (CoQ10) deficiency encompasses a subset of mitochondrial diseases caused by mutations affecting proteins involved in the CoQ10 biosynthetic pathway. One of the most frequent clinical syndromes associated with primary CoQ10 deficiency is the severe infantile multisystemic form, which, until recently, was underdiagnosed. In the last few years, the availability of genetic screening through whole exome sequencing and whole genome sequencing has enabled molecular diagnosis in a growing number of patients with this syndrome and has revealed new disease phenotypes and molecular defects in CoQ10 biosynthetic pathway genes. Early genetic screening can rapidly and non-invasively diagnose primary CoQ10 deficiencies. Early diagnosis is particularly important in cases of CoQ10 deficient steroid-resistant nephrotic syndrome, which frequently improves with treatment. In contrast, the infantile multisystemic forms of CoQ10 deficiency, particularly when manifesting with encephalopathy, present therapeutic challenges, due to poor responses to CoQ10 supplementation. Administration of CoQ10 biosynthetic intermediate compounds is a promising alternative to CoQ10; however, further pre-clinical studies are needed to establish their safety and efficacy, as well as to elucidate the mechanism of actions of the intermediates. Here, we review the molecular defects causes of the multisystemic infantile phenotype of primary CoQ10 deficiency, genotype-phenotype correlations, and recent therapeutic advances.


INTRODUCTION
Coenzyme Q 10 (ubiquinone; CoQ 10 , EC 206-147-9) is a lipid molecule widely but variably distributed among cellular organelles and tissues. Intracellular CoQ 10 concentration is highest in the lysosomes and Golgi vesicles, followed by microsomes and mitochondria [1,2] . This essential molecule is required for multiple cellular functions and aspects of metabolism, including ATP synthesis via the mitochondrial respiratory chain; antioxidant defenses; regulation of the mitochondrial permeability transition pore; activation of uncoupling proteins; and metabolism of sulfides, proline, arginine, glycine, fatty acids, and pyrimidines [1,3,4] . CoQ 10 contains a long polyisoprenyl tail of ten isoprene units, which positions the molecule in the mid-plane of membrane bilayer, as well as a fully substituted benzoquinone ring that undergoes reversible reduction and oxidation [3,5] . The various functions of CoQ 10 depend on the capacity of the benzoate ring to assume three different redox states: (1) oxidized (ubiquinone); (2) semioxidized (semiubiquinone); and (3) reduced (ubiquinol) [1][2][3][4] . Although the main ubiquinone antioxidant function is protection against lipid and protein peroxidation, ubiquinol also regenerates other powerful antioxidants, such as α-tocopherol and ascorbate, via electron donation, and recycles them back to their active reduced forms, thereby enhancing activities of other antioxidant defenses [1][2][3][4]6] .
Among the non-mitochondrial enzymatic systems involved in the continuous regeneration of ubiquinol is selenoprotein thioredoxin reductase (TrxR1), an essential antioxidant enzyme known to reduce many compounds, as well as thioredoxin [6] . TrxR1-mediated reduction of CoQ 10 is dependent on its selenocysteine, which may account for the relationship between levels of ubiquinone and selenium [7,8] .
Similar to most other mitochondrial disorders, primary CoQ 10 deficiency is clinically heterogenous, presenting at different ages of onset, with variable, multiple organs involvement [9,10] . In the past, diagnosis of this condition relied only on biochemical assays [10,11] . Specifically, low levels of CoQ 10 in muscle, often, but not always, associated with deficiency of CoQ 10 -dependent respiratory chain enzymes (complexes I + III and II + III) activities [10] ; however, identification of pathogenic gene variants, wider use of nextgeneration sequencing, and recognition of characteristic phenotypes have greatly facilitated diagnosis of this condition. For example, the two most frequent and earliest phenotypes associated with CoQ 10 deficiency, steroid-resistant nephrotic syndrome (SRNS) and cerebellar ataxia, have been linked to specific molecular defects in CoQ 10 biosynthetic enzymes, and specific COQ genes have been added to targeted diagnostic panels [e.g., COQ8A, previously known as ADCK3, is included in ataxia gene panels because pathogenic variants in this gene cause autosomal recessive cerebellar ataxia 2 (ARCA2)] [12,13] .
In contrast, until very recently, diagnoses of the lethal, infantile or childhood-onset multisystemic forms were reached at late stage of disease or even postmortem, through linkage or homozygous analysis in the family, in conjunction with biochemical diagnosis, and thus fewer patients were reported, compared to the other two phenotypes. However, in the last few years, implementation of next generation sequencing (NGS)-based diagnostics such as whole exome sequencing (WES) and whole genome sequencing (WGS) has caused a dramatic shift in the diagnosis, from a biochemical approach towards a molecular one, of this phenotype too. The unbiased genetic screening approach enables early diagnosis in infants and children with complex multisystemic syndromes, unveiling novel phenotypes, and molecular defects [14] ; however, it is important to note that some gene variants of uncertain significance have been reported, without the functional studies necessary to prove pathogenicity.
Another patient, with compound heterozygous for two novel variants (p.Arg221Leufs* and p.Ser370Arg) in PDSS1 was reported in 2012. The infant presented developmental delay, nephrotic syndrome, and failure to thrive, and died at 16 months of age due to renal failure. Brain MRI showed leukoencephalopathy and brainstem lesions [16] .
In 2000, Rötig et al. [17] described three siblings with similar symptoms, albeit varying degrees of severity, which included: severe SRNS, neurological impairment (ataxia, dystonia, and amyotrophy), retinitis pigmentosa, sensorineural deafness, and cardiomyopathy. Trans-prenyltransferase deficiency was identified, which was subsequently demonstrated to be due to a homozygous PDSS2 variant. patient was hypotonic at birth with rapid evolution of the encephalopathy. At 3 months of age, low dose CoQ 10 supplementation (50 mg) was initiated, and he developed intractable seizures, progressing to refractory focal status epilepticus, and death at 8 months.
Quinzii and Loos reported another infant, with PDSS2 pathogenic variants, who presented at age 2 months with severe global developmental delay and failure to thrive. Later evaluations showed bilateral optic atrophy, severe hypotonia, lactic acidosis, renal glomerular dysfunction, Leigh syndrome, and hypertrophy of the left ventricle. At 8 months oral therapy with L-carnitine (50 mg/kg/day), CoQ 10 (10 mg/kg/day), riboflavin (100 mg/day), and thiamine (50 mg/day) was started without clinical response. The proband developed generalized status epilepticus; his neurological status deteriorated and he died at 19 months. Postmortem sequencing identified two novel heterozygous missense mutations: c.590 C>A, p. Ala197Glu and c.932 T>C, p. Phe311Ser [19] .
More recently, two novel mutations in PDSS2 were reported in a 7-month-old infant with nephrotic syndrome, along with encephalomyopathy, hypertrophic cardiomyopathy, deafness, retinitis pigmentosa, and elevated serum lactate level. Clinical exome sequencing revealed a heterozygous missense variant c.485A>G (p.His162Arg) and a heterozygous 2923-bp deletion (c.1042_1148-2816del), which causes a 107-base-long deletion of exon 8. The patient died at 8 months of age, despite CoQ 10 supplementation (20 mg/kg/day) [20] . Pathogenic variants in PDSS2 were also reported in two patients with isolated SRNS [21] .
The combination of neurological symptoms with SRNS are the hallmark of the neonatal multisystemic presentation of mutations in COQ2. SRNS may be the first and predominating feature within the first year of life, followed by later onset of other manifestations such as refractory seizures, hypotonia, psychomotor delay, nystagmus, and optic atrophy [15,25,26] . Nevertheless, a few exceptional cases have lacked renal involvement [26,27] .
In 2006, Quinzii et al. [24] reported the first genetic cause of primary CoQ 10 deficiency, a homozygous c.890 A>G (p.Tyr297Cys) variant in COQ2, in a 33-month-old boy [23] . The clinical picture was dominated by nephro-encephalopathy with SRNS (proteinuria 4.3 g/ day), psychomotor regression, optic atrophy, tremor, and acute-onset status epilepticus with focal electroencephalogram abnormalities predominantly in the left occipital region. Brain magnetic resonance imaging showed cerebellar atrophy, mild diffuse cerebral atrophy, and stroke-like lesions in the left cingulate cortex and subcortical area. His sister presented only with SRNS at 12 months but was treated before she developed significant neurological manifestations [23,24] .
In 2007, Diomedi-Camassei et al. [28] described two other patients with early-onset glomerular lesions. The first patient presented with SRNS at the age of 18 months due to collapsing glomerulopathy, with no extrarenal symptoms. He had compound heterozygous COQ2 variants c.590G>A (p.Arg197His) and c.683A>G (p.Asn228Ser). The second patient presented at 5 days of life with oliguria, with severe extracapillary proliferation on renal biopsy. He rapidly developed end-stage renal disease and died at the age of 6 months after a course complicated by progressive epileptic encephalopathy. He harbored a homozygous c.437G>A (p.Ser146Asn) variant. In 2018, Eroglu et al. [29] reported four patients from two different families with SRNS and three with insulin dependent neonatal diabetes were described. Despite initial response to CoQ 10 supplementation in three, all patients developed neurological features, including intractable seizures that did not improve with oral CoQ 10 treatment.
In contrast to the original cases with COQ2 defects and encephalonephropathies, Desbats et al. [25] described a neonatal case with severe lactic acidosis, proteinuria, dicarboxylic aciduria, hepatic insufficiency, hypokinetic, and dilated left ventricle on echocardiography, although without clinical signs of cardiomyopathy, who died within the first 24 h of life. Scalais et al. [27] described a patient without renal involvement, who presented at 3 weeks of age with myoclonic epilepsy and hypertrophic cardiomyopathy. Serial brain MRIs performed at 4 months showed bilateral and symmetrical increased signal intensities within the posterior putamen and temporal areas and in the rolandic and parasagittal cerebral regions as well as cerebral atrophy and increased CSF lactate. Jakobs et al. [26] described dizygotic twins from consanguineous Turkish parents born prematurely who died at the ages of five and 6 months, respectively, after fluctuating disease courses with apneas, seizures, feeding problems, and generalized edema. Again, in these patients, there was no evidence of renal involvement. The patients carried a novel homozygous mutation in COQ2 (c.905C>T, p.Ala302Val).
The initial evidence of COQ4 dysfunction as cause of encephalomyopathy was the report of Salviati et al. [32] , who, in 2012, reported a 3.9-Mb deletion of chromosome 9q34.13 encompassing COQ4 in a 3-year-old boy with mental retardation, encephalomyopathy, and dysmorphic features who responded to CoQ 10 supplementation (30 mg/kg per day of ubiquinone).
In 2015, the first five patients with point mutations in COQ4 were described. Four of them had prenatal or perinatal onset with early fatal outcome. Two unrelated individuals presented with severe hypotonia, bradycardia, respiratory insufficiency, and heart failure. Two sisters showed antenatal cerebellar hypoplasia, neonatal respiratory-distress syndrome, and epileptic encephalopathy. Only one patient had a gradually progressive condition characterized by spastic ataxic gait and seizures. Except for the solitary patient with the progressive condition, CoQ 10 supplementation was not administered due to fatal early onset. All these individuals carried homozygous or compound-heterozygous variants, clearly indicating that the disease is inherited as autosomal-recessive trait, indicating that haploinsufficiency might not be pathogenic because the parents, heterozygous for the nonsense variant, were unaffected [33] .
Chung et al. [34] described five recessive missense mutations in COQ4 segregating with disease in four families. All patients presented with a severe multisystemic neonatal form including nervous system manifestations such as hypotonia, encephalopathy with EEG abnormalities, neonatal seizures, and cerebellar atrophy. Other manifestations included lactic acidosis, cardiomyopathy, and secondary breathing difficulties. Cerebellar hypoplasia was a common finding and nephropathy was not present. Only two patients received CoQ 10 supplementation, without response.
Sondheimer et al. [35] identified novel mutations in COQ4 in an infant presenting with early onset biventricular hypertrophic cardiomyopathy, hypotonia, hearing loss, seizures, and lactic acidosis associated with severe muscle CoQ 10 deficiency.
Ling et al. [36] showed three unrelated Chinese families presenting with the COQ4 c.370G>A (p.G124S) variant, manifesting as either encephalopathy with intractable seizures and developmental delay or cardiomyopathy with left ventricle hypertrophy. In the first case of this series, CoQ 10 supplementation (600 mg/day) was started at six years, which resulted in improvement in the patient's alertness only. In the second patient, CoQ 10 supplementation was started at 250 mg per day; then, it was increased to 400 mg per day, 3 months after symptom onset with some improvement in the control of seizures and patient's alertness. The patient had only one further episode of epilepsy at the age of three. The third patient was not treated. The same homozygous c.370G>A (p.G124S) COQ4 variant was reported in another Chinese patient, who presented in the second month of life with Leigh syndrome, respiratory distress, lactic acidosis, dystonia, seizures, and failure to thrive, without renal involvement [37] .
A recent paper reported 11 additional southern Chinese patients, the largest cohort of COQ4 deficient patients to date. Five had classical neonatal-onset encephalo-cardiomyopathy, while the other six had infantile-onset characterized by different constellations of symptoms such as hypotonia, cortical visual impairment, severe developmental delay, and seizures. Although dystonia was observed in two out of the six patients with infantile-onset presentation, none displayed basal ganglia lesions. The patients carried the variant c.370G>A, (p.Gly124Ser), previously reported by Ling et al. [36] and Lu et al. [37] , suggesting a founder effect in the southern Chinese population. Among the 10 patients who received CoQ 10 supplement and with continuous follow-up, only 3 showed stabilization of the cardiopathy or seizure control; all were homozygous for c.370G>A, p. (Gly124Ser). Some improvement was observed in one patient with the heterozygous missense variants c.370G>A and c.371G>T. Five patients harbored the splicing mutation c.402+1G>A, inducing a severe early onset phenotype that was not responsive to CoQ 10 supplementation [38] .
A recent report expanded the spectrum phenotype of COQ4 mutations to include childhoodonset spinocerebellar ataxia with stroke-like episodes, associated with a homozygous variant in the COQ4 gene c.230C>T (p.Thr77Ile), reported in two siblings. After the diagnosis at ages 11 and 13 years, CoQ 10 supplementation (1000 mg/day) was initiated for both siblings. Although motor outcomes were stable for the first year of treatment, one of the patients developed a second stroke-like episode at age 14 [39] .
Finally, a homozygous mutation c.164G>T, p.Gly55Val in COQ4 was reported in two siblings with a combination of slowly progressive ataxia, spasticity, and seizures, constituting an autosomal recessive cerebellar ataxia (ARCA) syndrome. The more severely affected patient received high-dose CoQ 10 (2000 mg/day) and showed clinically significant improvement; he was originally wheelchair-bound, unable to walk with support or standing unaided. With treatment, he became able to ambulate with a walker and stand without support. After this response, the other patient was also treated, with some improvement as well [40] .

COQ5 (MIM616359)
COQ5 catalyzes the only C-methylation in the biosynthesis of CoQ 10 [ Figure 1] [41] . Mutations in COQ5 have been reported in only three sisters of non-consanguineous Iraqi-Jewish descent. They had varying degrees of cerebellar ataxia, encephalopathy, generalized tonic-clonic seizures, and cognitive disability, with childhood onset and slow progression [ Table 1 and Figure 2]. Neither WES nor WGS was able to identify a potential pathogenic variant, whereas a SNP array study, performed on the parents and all siblings, identified a tandem duplication affecting the last four exons of the gene, confirmed by Sanger analysis [42] .

COQ7 (MIM616733)
COQ7 is required for one of the three hydroxylations of CoQ benzoquinone ring [ Figure 1] [43] . In 2015, Freyer et al. [44] described a 9-year-old boy with COQ7 pathogenic variants with complex clinical multiple organ involvement. The child had a history of neonatal lung hypoplasia, joint contractures, early infantile hypertension, and left ventricular cardiac hypertrophy, likely secondary to his prenatal kidney dysplasia with renal dysfunction resulting in oligohydramniosis. Although renal dysfunction normalized during the first year of life, he progressively developed mental retardation, axono-demyelinating neuropathy, hypotonia, and hearing loss. The homozygous c.422T>A (p.Val141Glu) variant in COQ7 was identified through WES. Additional functional studies in the patient fibroblasts confirmed the pathogenicity of the variant.
A second report described a patient carrying the combination of a novel homozygous mutation (p.Leu111Pro) in COQ7, with the mitochondrial DNA m.1555A>G mutation, commonly associated with deafness. The phenotype was characterized by a mild form of spastic paraparesia and cognitive impairment as well as hearing loss. No functional studies were performed to define the cause of the deafness. The authors hypothesized that the combination of CoQ 10 deficiency and the m.1555A>G mutation leads to synergistic inhibition of mitochondrial function, causing irreversible damages and/or cell death and finally the clinical manifestation of hearing loss [45] .
In 2019, Kwong et al. [46] reported a patient with a severe phenotype characterized by encephalomyonephrocardiopathy, persistent lactic acidosis, and basal ganglia lesions, who died at 12 months. The patient had intrauterine growth restriction, cardiomegaly, and tricuspid regurgitation since antenatal period. WES identified two compound heterozygous variants in the COQ7 gene: a deletion insertion resulting in frameshift c.599_600delinsTAATGCATC, p.(Lys200Ilefs*56) and a missense substitution c.319C>T, p. (Arg107Trp). The proband started CoQ 10 supplementation at 2 months of life; the initial dose is unknown, but it was increased to 20 mg/kg/day at 12 months of life. Nevertheless, the patient cardiorespiratory manifestations deteriorated and the patient died of sepsis. Skin fibroblast studies supported pathogenicity by revealing decreased combined complex II + III activity and reduction in CoQ 10 level.

COQ9 (MIM614654)
COQ9 is required for the stability and function of COQ7 [ Figure 1] [47,48] . Mutations in COQ9 have been reported in few patients, presenting with the similar lethal neonatal phenotypes characterized by encephalomyopathy and kidney involvement, including tubulopathy [ Table 1 and Figure 2].
Danhauser et al. [51] described another infant carrying a homozygous splice-site variant c.521+1del, p.(Ser127_Arg202del) in COQ9, manifesting with neonatal encephalopathy with hypotonia, poor breathing, and severe lactic acidosis with symmetrical hyperechoic signal alterations in the basal ganglia, suggestive of neonatal Leigh-like syndrome. The patient subsequently developed seizures and recurrent episodes of apnea and bradycardia and died at 18 days of life.
In 2018, Smith et al. [52] reported four siblings, who presented prenatally with an unknown and an ultimately lethal condition characterized by intrauterine growth retardation, oligohydramnios, variable dilated cardiomyopathy, anemia, abnormal appearing kidneys, and autopsy brain findings suggestive of Leigh disease. The patients had the variants c.521+2T>C and c.711+3G>C in COQ9, which cause in-frame deletions (p.Ser127_Arg202del and p. Ala203_Asp237del).
In 2019, a novel frameshift c.384delG (Gly129Valfs*17) homozygous mutation was reported in a 9-month-old girl, born from consanguineous parents of Pakistani origin, presenting with growth retardation, microcephaly, and seizures. She was born at 38 weeks gestation, weighed 2000 g, after an uncomplicated pregnancy, and was hospitalized for 3 days due to respiratory distress. At age 4 months, she had sustained clonic seizures. Physical examination showed microcephaly, truncal hypotonia, and dysmorphic features. Abdominal ultrasonography revealed cystic kidneys. Non-compaction of the left ventricle was detected in echocardiography. Cranial MRI showed hypoplasia of the cerebellar vermis and brain stem, corpus callosum agenesis, and cortical atrophy. CoQ 10 supplementation (5 mg/kg/day) was started when she was 10 months old. Despite increasing the dose to 50 mg/kg/day after the molecular diagnosis, no neurological improvement was observed [53] .
In general, clinical features alone are insufficient to definitively diagnose CoQ 10 deficiency or to distinguish between primary and secondary CoQ 10 deficiencies, or even from other mitochondrial conditions. Therefore, evaluation of patients with suspected CoQ 10 deficiency relies on genetic or biochemical studies. If the clinical picture and/or family history raise the possibility of a metabolic/genetic condition, WES, including sequencing of mitochondrial DNA, if available, should be considered the first step. However, only 35% of Mendelian diseases are solved by WES [54] because the majority of undiagnosed cases are subject to limitations in variant-calling and prioritization, as well as inability to detect intronic and regulatory pathogenic variants. WGS enables complete coverage of the genome; however, interpretation is often hindered by difficulty in prioritization of the vast numbers of variants detected and our incomplete understanding of the non-coding sequences. Consequently, the diagnostic yield with WGS is only modestly increased to just over 40% [55][56][57] . In parallel with NGS, laboratory analyses should include routine tests such as blood lactate and urine organic acids, although normal values do not exclude CoQ 10 deficiency.
If genetic analysis shows pathogenic homozygous or compound heterozygous variants in any of the previously reported genes involved in CoQ 10 synthesis with a compatible clinical picture, definitive diagnosis of primary CoQ 10 can be established without further analyses. In presence of variants of uncertain significance, functional and/or complementary studies are needed. Blood mononuclear cells represent a readily accessible sample, which is often suitable as an alternative to muscle for the measurement of CoQ 10 , by high performance liquid chromatography or mass spectrometry [11,58] . In contrast, plasma levels of CoQ 10 are influenced by the amount of plasma lipoproteins (carriers of CoQ 10 in circulation), dietary intake, or supplementation, therefore cannot be used for diagnostic purpose. In addition, COQ 10 levels can be measured in other tissues, such as lymphoblastoid cell lines or primary fibroblasts, although normal values in these tissues do not exclude the diagnosis of CoQ 10 deficiency, as some patients with genetically confirmed CoQ 10 biosynthetic defects have had normal CoQ 10 levels in fibroblasts. As mentioned above, reduced activity of complexes I + III and II + III (and I + III) is highly suggestive of CoQ 10 deficiency [10] .

Current treatments
Humans-Varying doses of CoQ 10 have been used for the treatment of primary CoQ 10 deficiencies, ranging from 5 to 50 mg/kg/day for both adults and children [10,17] . We cannot compare the effects of different dosages because formulations and durations of treatment also varied [10] . We recommend high doses of CoQ 10 supplementation (> 30 mg/kg), because inadequate dosage and duration of intake have often constrained uptake of exogenous CoQ 10 [59][60][61] , with few mild reported side effects [10] .
Early intervention with CoQ 10 supplementation in high doses has been shown to improve renal function [62] . However, in neonatal cases with neurological involvement, response of CoQ 10 supplementation is poor, probably due to the irreversible brain damage at the time of the diagnosis, as well as the poor bioavailability of CoQ 10 , which does not cross the bloodbrain barrier [29,46,53] . New solubilized and stabilized formulations that are able to preserve CoQ 10 in its reduced form (CoQH 2 or ubiquinol) have been developed and increase bioavailability after oral dosing compared to standard ubiquinone [63] . Experience in patients with primary CoQ 10 deficiency is limited and there are no clear indications about the doseequivalence of ubiquinone and ubiquinol. Short-tail Q 10 analogs, such as idebenone (IDB), are more bioavailable than CoQ 10 but are not effective in patients with primary CoQ 10 deficiency [64] .
In vitro and in vivo studies-In vitro studies in human fibroblasts show that short-tail Q 10 analogs, such as CoQ 2 and IDB, are not effective in primary CoQ 10 deficiency because they do not correct the respiratory chain defects [65] .
Studies in Pdss2 mutant mice, a mouse model of CoQ-deficient NS, show that CoQ 10 supplementation prevents renal failure through rescue of sulfides metabolism and oxidative stress. In contrast, IDB treatment was ineffective and comparable to placebo [66,67] . In a mouse model of CoQ 10 deficiency and encephalomyopathy due to Coq9 dysfunction, the water-soluble formulation of ubiquinol was shown to be more effective than ubiquinone in rescuing brain abnormalities [68] .

Investigational treatments
Administration of metabolic intermediates able to "bypass" the enzymatic block and to enable endogenous synthesis of CoQ 10 has been attempted in experimental in vitro and in vivo models of primary CoQ deficiency, as an alternative to CoQ 10 supplementation [69] , whose therapeutic effects are hampered by its poor bioavailability. In vitro studies-Treatment with 2,4-dihydroxybenzoic acid (DHB, β-resorcylic acid, β-RA) was shown to be effective in human fibroblasts carrying COQ7 pathogenic variants [44,45] and in COQ2-deficient cell lines, increasing the levels of CoQ 10 as well as increasing the viability of mutant cells growth in galactose medium [70] .
Luna-Sánchez et al. [71] also investigated the effect of DHB in mouse embryonic fibroblasts from two different mouse models of COQ9 dysfunction (Coq9 R239X/R239X and Coq9 Q95X/Q95X ) showing similar results to those obtained in COQ2 and COQ7 mutant cells, with different response to treatment based on the severity of the biochemical defect and the residual levels of COQ7.
Treatment with vanillic acid (VA) recovered CoQ 10 biosynthesis, ATP production, and reduced levels of reactive oxygen species in a human cell line lacking functional COQ6 [72] , a FAD-dependent monooxygenase responsible for the addition of the hydroxyl group in position C5 of the quinone ring [73] . Mutations in COQ6 cause SRNS associated with sensorineural deafness and a variable degree of encephalopathy [74] .
In vivo studies-The first studies to show in vivo efficacy of hydroxylated CoQ precursor compounds 3,4-dihydroxybenzoic acid, DHB, and VA to rescue endogenous CoQ biosynthesis were performed in yeast models of COQ6 and COQ7 deficiencies [75,76] .
DHB was found to rescue not only the clinical phenotype but also morphological and histopathological signs of encephalopathy in the Coq9 R239X mouse. The therapeutic effect of DHB was not attributed to the increase of CoQ 10 levels, but rather to the reduction of DMQ 10 , an intermediated metabolite that may be toxic for mitochondrial function when accumulated in the organelle. Thus, the authors proposed that DHB should be preferentially considered for the treatment of human CoQ 10 deficiency with accumulation of DMQ 10 , as mutations in COQ4, COQ7, and COQ9 [79] .
Although all these experimental data suggest that biosynthesis intermediates might be a promising alternative, further studies are needed to assess therapeutic response, safety, and bioavailability and to understand their mechanism of action before their translation to the clinical practice.

CONCLUSION
Multisystemic forms of primary CoQ 10 deficiency are usually devastating conditions manifesting in prenatal, neonatal, or infantile period of life. Clinical symptoms include variable combinations of encephalomyocardionephropathy syndromes. Although the diagnosis of these primary CoQ 10 deficiency syndromes is usually not straightforward, renal involvement, particularly SRNS, can be a clinical clue. In the severe multisystemic forms, WES is often the first step in the diagnostic workup. Nevertheless, detection of novel genetic variants of uncertain significance should be followed by biochemical assays and/or functional studies in patient cells to prove pathogenicity. Eventually, comprehensive characterization of the clinical spectrum of these syndromes and associated molecular defects will establish pathogenicity of variants identified by WES and obviate further studies that are available only in specialized research laboratories.
Although in the suspect of primary coenzyme Q 10 deficiency high doses of coenzyme Q 10 supplementation are recommended, early-onset neurological features are often not responsive to supplementation. CoQ 10 biosynthetic analogs might be suitable alternatives to CoQ 10 supplementation, but additional analyses are required before these compounds can be translated to the clinical setting.  Phenotypes associated with CoQ 10 biosynthesis defects