Clinical characteristics.

Individuals with NGLY1-related congenital disorder of deglycosylation (NGLY1-CDDG) typically display a clinical tetrad of developmental delay / intellectual disability in the mild to profound range, hypo- or alacrima, elevated liver transaminases that may spontaneously resolve in childhood, and a complex hyperkinetic movement disorder that can include choreiform, athetoid, dystonic, myoclonic, action tremor, and dysmetric movements. About half of affected individuals will develop clinical seizures. Other findings may include obstructive and/or central sleep apnea, oral motor defects that affect feeding ability, auditory neuropathy, constipation, scoliosis, and peripheral neuropathy.

Diagnosis/testing.

The diagnosis of NGLY1-CDDG is established in a proband by the identification of biallelic pathogenic variants in NGLY1 on molecular genetic testing. Typical serum screening tests for congenital disorders of glycosylation (i.e., analysis of serum transferrin glycoforms, N and O glycan profiling) will NOT reliably detect NGLY1-CDDG.

Management.

Treatment of manifestations: Lubricating eye drops and/or bland ointments for hypolacrima; feeding therapy and/or supplemental tube feeding for those with oromotor deficits and feeding difficulties; adequate access to water and a cool environment (including a cooling vest for those who live in hot climates) for hypohydrosis; vitamin D supplementation for those with vitamin D deficiency; evaluation by a developmental pediatrician and supportive therapies for developmental and cognitive issues; standard treatment for hearing loss, sleep apnea, constipation, scoliosis, and seizure disorder; consideration of referral to a hematologist for abnormal hematologic studies; consideration of referral to a gastroenterologist for elevated liver transaminases.

Surveillance: Annual follow up by a pediatrician/internist, rehabilitation medicine specialist, ophthalmologist, neurologist, and nutritionist is recommended. Periodic evaluation by a developmental pediatrician, gastroenterologist/hepatologist, and audiologist should be considered.

Agents/circumstances to avoid: Hot environment in those with hypohydrosis.

Genetic counseling.

NGLY1-CDDG is inherited in an autosomal recessive manner. At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Carrier testing for at-risk relatives, prenatal testing for pregnancies at increased risk, and preimplantation genetic testing are possible if the pathogenic variants in the family are known.

Suggestive Findings

NGLY1-related congenital disorder of deglycosylation (NGLY1-CDDG) should be suspected in individuals with the following clinical features and supportive laboratory findings.

Clinical features include:

• Developmental delay / intellectual disability, most often in the severe to profound range
• Hyperkinetic movement disorder
• Hypo- or alacrima

Supportive laboratory findings include elevated ALT and AST during early childhood that spontaneously normalize.

Note: Typical serum screening tests for congenital disorders of glycosylation (i.e., analysis of serum transferrin glycoforms, N and O glycan profiling) will NOT reliably detect NGLY1-CDDG (see Clinical Description, Biochemical).

Establishing the Diagnosis

The diagnosis of NGLY1-CDDG is established in a proband by the identification of biallelic pathogenic variants in NGLY1 on molecular genetic testing (see Table 1).

Clinical Description

NGLY1-related congenital disorder of deglycosylation (NGLY1-CDDG) is a multisystemic neurodevelopmental disorder in which individuals most commonly exhibit a tetrad of developmental delay / intellectual disability, hyperkinetic movement disorder, hypolacrima, and elevated transaminases during early childhood [Need et al 2012, Enns et al 2014, Heeley & Shinawi 2015, Caglayan et al 2015, Lam et al 2017]. Diagnosis has been achieved at ages ranging from three months to 20 years, mostly through broad molecular testing, such as exome analysis. While most individuals with NGLY1-CDDG survive into early adulthood, with a relatively stable clinical course [Enns et al 2014, Lam et al 2017], death during infancy from unclear causes has been reported. In addition, an affected child died following an infection complicated by uncontrolled seizure activity [Enns et al 2014], and an affected adolescent died from respiratory failure during a respiratory infection [Caglayan et al 2015]. Since so few cases have been identified, understanding of the clinical phenotypic spectrum and natural history continues to evolve.

Growth. In approximately half of individuals with NGLY1-CDDG birth weight is below the tenth centile, while the majority of birth lengths and birth head circumferences are appropriate for gestational age [Lam et al 2017]. Despite a robust appetite, individuals with NGLY1-CDDG develop failure to thrive with weight affected more than length/height. Acquired microcephaly has also been noted in some [Lam et al 2017].

Development. Developmental delay and/or intellectual disability is seen universally in individuals with NGLY1-CDDG. Severity of delay is broad and ranges from individuals having an IQ below average (70s) to individuals with profound intellectual disability. The majority of individuals are nonverbal or can only use single words or phrase speech. Despite lack of verbal communication, they use and benefit from alternate forms of augmentative communication tools, such as switch boards or electronic tablet-based tools. Affected individuals have a consistent developmental profile on the Vineland Adaptive Behavior Scales, Second Edition, in which individuals have relatively strong socialization skills, followed by communication skills, followed by weaknesses in motor skills with fine motor worse than gross motor skills, reflected in low daily living skills [Lam et al 2017].

Neurologic. Approximately half of affected individuals develop clinical seizures. While most develop myoclonic seizures, documented seizure types also include infantile spasms and atonic, tonic, absence, and gelastic seizures. Age of onset ranges from two months to ten years. In some individuals seizures have been intractable, while in others seizures have been controlled with levetiracetam or valproic acid [Lam et al 2017]. Compared to individuals with seizures of different etiologies, those with NGLY1-CDDG have not been more severely affected by any specific antiepileptic medication.

In addition, individuals with NGLY1-CDDG universally exhibit a complex hyperkinetic movement disorder that can include choreiform, athetoid, dystonic, myoclonic, action tremor, and dysmetric movements [Lam et al 2017].

Further findings may include the following:

• CSF laboratory results typically demonstrate:
  • Low total protein (from 8 affected individuals, mean protein level was 11 mg/dL, standard error of the mean [SEM] 1) and albumin (from 9 affected individuals, mean 9 mg/dL, SEM 1);
  • Low CSF/serum albumin ratios (from 9 affected individuals, mean ratio was 3, SEM 1);
  • Low CSF 5-hydroxyindolacetic acid, homovanillic acid, and tetrahydrobiopterin levels, especially in older individuals [Enns et al 2014, Lam et al 2017].
• Brain MRI can show:
  • Delayed myelination during early childhood (ages 0-5), but not in older individuals;
  • Progressive cerebral and occasional cerebellar atrophy, which correlates with worsening function. In 10/11 affected individuals imaged cerebral volume loss was found; in 4/11 cerebellar volume loss was also seen [Lam et al 2017].
• Brain MRS can be significant for:
  • Lower N-acetylaspartylglutamate and N-acetylaspartate levels compared to normal;
  • Higher choline and myo-inositol levels, becoming more prominent with increasing age, worsening function, and lower brain volume [Lam et al 2017].
• Nerve conduction studies most often demonstrate an axonal sensorimotor polyneuropathy with additional demyelinative features that are length dependent and appear progressive. Neuropathy has been documented in all nerves tested including the median, ulnar, radial, peroneal, tibial, and sural nerves. Individual testing typically reveals more severe neuropathy in the lower (compared to upper) extremities, with lower amplitudes and slower conduction.
• Needle electromyogram may show neurogenic findings with varying degrees of acute and chronic changes.
• QSWEAT testing can show absent sweat response, more frequently in the lower extremities than in the forearm, suggesting a length-dependent neuropathy [Lam et al 2017].

Ophthalmologic. Most affected individuals, with the exception of the youngest reported person, have evidence of hypo- or alacrima. Corneal findings include neovascularization, pannus formation, and scarring secondary to hypolacrima. Lagophthalmus, ptosis, exotropia and/or esotropia, optic nerve pallor or atrophy, retinal pigmentary changes including pigmentary granularity and pigmentary retinopathy, and cone dystrophy have been observed in individuals with NGLY1-CDDG [Lam et al 2017].

Audiologic. Tympanometry and behavioral hearing thresholds were normal in the individuals who could tolerate and cooperate with these exams. There is a consistent profile on auditory brain stem evoked response showing dyssynchronous and/or absent transmission through the auditory brain stem and/or eighth nerve in most individuals that appears to worsen with age [Lam et al 2017].

Cardiac. Echocardiogram is normal, and electrocardiogram shows heart rates in the low 100s with a minority of affected individuals with a QTcB >440 ms, but a normal QTcF.

Note: The QTcB is the standard clinical correction of the QT interval using Bazett's formula, calculated as QT interval divided by square root of the RR interval. The QtcF is the alternative correction based on Fridericia's formula, which is defined as the QT interval divided by the cube root of the RR interval. The QTcB is believed to overestimate the QT prolongation at higher heart rates, and the QTcF may underestimate the QT prolongation at slower heart rates [FDA 2005].

Sleep. Approximately half of tested individuals with NGLY1-CDDG have also been documented to have mild-to-profound obstructive and/or central sleep apnea [Lam et al 2017].

Feeding. Oral motor defects, including premature spillage, pharyngeal swallow response delays, poor oral bolus formation, weakness of the lips and tongue, dystonic movements of the tongue, and persistent oral reflexes of suckling and suck/swallow are seen in the majority of affected individuals. However, these findings typically do not prohibit oral feeding in the majority of individuals. Enteral feeds have been helpful with nutritional management, although this decision is made on a case-by-case basis [Lam et al 2017].

Gastrointestinal. The majority of affected individuals have some degree of constipation [Enns et al 2014].

Transaminases (AST and ALT):

• Are typically elevated and range from just slightly above the upper limit of normal to >1,000 U/L in the first two years of life;
• Usually normalize by age four years without any specific intervention.
  Note: In the few liver biopsies performed, findings have been normal or consistent with microvesicular steatosis, ductular proliferation, focal microvacuolation, and micronodular cirrhosis with bands of fibrosis with regenerative nodules.

Total cholesterol, low-density lipoprotein cholesterol, and triglyceride levels are low in about one third of tested individuals, but particle numbers and sizes of HDL, LDL, and VLDL are normal [Lam et al 2017].

Abdominal ultrasound findings can include splenomegaly, steatosis, coarse or inhomogeneous liver texture, and hepatomegaly.

Fibroscan scores show evidence of liver fibrosis in a few affected individuals [Lam et al 2017].

Hematologic. Coagulation studies in some individuals can be significant for low protein C, factor II, factor IX, factor XI, and fibrinogen levels. However, significant bleeding or clotting episodes have not yet been reported. Complete blood count is generally unremarkable [Lam et al 2017].

Immunologic. Affected individuals typically are reported to have fewer infections than their peers, with the exception of a few individuals with recurrent, more severe, respiratory infections. Antibody titers indicate that individuals with NGLY1-CDDG appear to respond typically to vaccinations, with the exception of rubella and rubeola vaccinations for which titers exhibited out-of-range elevations or were negative (for rubeola) in a majority of tested individuals [Lam et al 2017].

Musculoskeletal findings include delayed bone age despite a normal endocrine evaluation, low bone density in several individuals with a history of recurrent fractures, joint hypermobility, coxa valga, scoliosis, dislocations or subluxations of the hip or shoulder joints, and sclerosis of the phalanges or tarsal bones [Lam et al 2017]. These findings were present even in affected individuals who were ambulatory.

Biochemical findings include the following:

• Carbohydrate-deficient transferrin analysis in blood may show small elevations in mono- and a-oligosaccharides and tri-sialo-oligosaccharides, but not to the levels typically seen in PMM2-CDG.
• O-glycan profiling is normal.
• Urine quantitative mucopolysaccharides can be elevated, but with a normal pattern.
• Free and total carnitine, uric acid, white blood cell CoQ₁₀, plasma amino acids, and urine organic acids are essentially normal.
• Lactate was normal in the majority of affected individuals, but can be mild to moderately elevated (~5 mmol/L) especially in younger affected individuals.
• Lactate to pyruvate ratio is typically normal.
• Urine amino acids can show generalized aminoaciduria, especially in older individuals [Lam et al 2017].
• On liver biopsy, abnormal cristae and mitochondrial proliferation was noted in one individual, while depleted cristae and mitochondrial DNA depletion was seen in another individual [Kong et al 2018].
• On quadriceps muscle biopsy mitochondrial proliferation and mitochondrial DNA proliferation was noted in one affected individual [Kong et al 2018].

Genotype-Phenotype Correlations

The most common pathogenic variant is c.1201A>T (p.Arg401Ter), accounting for approximately one third of pathogenic alleles. Affected individuals harboring at least one copy of this pathogenic variant tend to have a more severe clinical course with higher scores on the Nijmegen Pediatric CDG Severity scale [Lam et al 2017].

A sib pair with the cryptic pathogenic c.930C>T splice site variant (predicted as a silent p.Gly310=) and a p.Gln208Ter nonsense variant exhibited relatively mild impairment in all domains [Lam et al 2017].

Nomenclature

NGLY1-CDDG was previously referred to as congenital disorder of glycosylation type Iv (CDG-Iv).

NGLY1-CDDG is the first primary defect of N-linked deglycosylation shown to cause human disease. Following the established nomenclature for congenital disorders of glycosylation, where disorders are formally named with the involved gene (not italicized) followed by -CDG (e.g., PMM2-CDG) [Jaeken et al 2009], the authors propose that this disorder and future disorders of N-linked deglycosylation follow a similar format, except using CDDG instead of CDG.

Prevalence

A total of 18 individuals from 14 families have been described in the literature [Need et al 2012, Enns et al 2014, Caglayan et al 2015, Heeley & Shinawi 2015, Bosch et al 2016, Lam et al 2017]. However, according to a database maintained by NGLY1.org, biallelic pathogenic variants in NGLY1 coupled with suggestive clinical phenotype have been identified in 46 individuals worldwide. Most of the reported affected individuals have been of northern European background, but this is likely due to ascertainment bias rather than a true increased prevalence in that population. Although not yet reported in the literature, individuals with African and non-white Hispanic background have been confirmed to have NGLY1-CDDG [Lam & Wolfe, personal observation].

Evaluations Following Initial Diagnosis

To establish the extent of disease and needs in an individual diagnosed with NGLY1-related congenital disorder of deglycosylation (NGLY1-CDDG), the evaluations summarized in Table 3 (if not performed as part of the initial evaluation that led to diagnosis) are recommended.