YWY Chu, WL Wong, BHY Chung

Answer

The propositus' most prominent feature is macroglossia, presented since birth. His birth weight was in normal range. There were no other obvious features that suggest any syndromic diagnosis at birth. It was considered that the patient had isolated macroglossia at that time. Macroglossia is a clinical feature of several disorders, including neurofibromatosis type I, mucopolysaccharidosis type I and II and Pompe disease. In the recent published diagnostic approach of macroglossia,² the diagnostic algorithm for macroglossia is proposed (Figure 2). The algorithm suggests that patients with isolated macroglossia should have at least initial evaluation with abdominal ultrasound and molecular testing for Beckwidth-Wiedemann syndrome (BWS) before a final diagnosis is given.

Figure 2 Proposed diagnostic algorithm for macroglossia. (Modified from Prada CE, Zarate YA, Hopkin RJ. Genetic causes of macroglossia: diagnostic approach. Pediatrics 2012;129:e431-7.)2

The propositus in this case had normal abdominal ultrasound at 3 months of age. During the physical examination at 3 months of age, he was noted to have ear pit/creases on his left ear, which is one of the features of BWS. His body weight and body height were above 97th percentile (macrosomia). In general, a diagnosis of BWS is made if at least three major diagnostic findings, or two major and one minor³ (Table 1) were present, which the propositus was found to have when he was three months old. A clinical diagnosis of BWS was consistent with this three months old propositus. Molecular testing for BWS, which consisted of methylation specific multiplex ligation-dependent probe amplification (MS-MLPA) on chromosome 11 and bisulfite-converted DNA pyrosequencing, was arranged for him. The result found that he had loss of methylation in the KvDMR region, which confirmed the diagnosis of BWS.

BWS is a clinically heterogeneous overgrowth syndrome.³ There are different molecular mechanisms that cause BWS, as shown in Figure 3. BWS is caused by the deregulation of imprinted genes in the chromosomal region 11p15. Human chromosome 11p15 contains two distinct domains that are two clusters of imprinted genes and are critical regulators of early human growth, each expressed under the control of an imprinting centre (IC), also called imprinting control region (ICR) or differentially methylated region (DMR).⁴ Imprinted domain 1 includes the paternally expressed IGF2 gene, the maternally expressed H19 gene, and a DMR upstream of H19, known as H19 DMR or IC1. H19 encodes a non-translated transcript that may function as a tumour suppressor gene. IGF2 encodes a potent fetal growth factor and its overexpression is thought to be important in the pathogenesis of BWS. The H19 DMR is normally methylated on the paternal but not the maternal allele, maintaining imprinted expression in imprinted domain 1. Imprinted domain 2 contains the maternally expressed genes, KCNQ1 and CDKN1C, as well as the paternally expressed but non-translated KCNQ1OT1 gene. CDKN1C negatively regulates cell proliferation. Intron 10 of the KCNQ1 gene contains another DMR called KvDMR1 or IC2, which controls the regulation of domain 2. A number of different mechanisms can lead to BWS via epigenetic and/or genetic alterations in the chromosome 11p15 imprinted domains. These alterations include: Loss of methylation at the IC2 (50%), paternal uniparental disomy 11 (20%), gain of methylation at the IC1 (5%), mutation in the CDKN1C gene (5% in sporadic cases) and duplication, inversion, or translocation involving chr11p15 (each is <1%).³

Figure 3 Genetic and epigenetic aetiologies underlying Beckwith-Wiedemann syndrome.

MS-MLPA is the most robust method clinically available for detecting the majority of epigenetic and genetic aetiologies associated with BWS. It can detect changes in both CpG methylation and copy number of more than 30 chromosomal sequences in a single experiment.^5,6 Another molecular analysis in BWS used is bisulfite-converted DNA pyrosequencing.^7-10 This technique quantitatively evaluates the methylation profiles of IC1 and IC2 and can be used to confirm the clinical diagnosis of BWS. In the case of our patient, MS-MLPA detected no copy number change of 11p15 region, while it found an aberrant loss of methylation at the IC2 by MS-MLPA (Figure 4D). Moreover, methylation analysis for IC1 and IC2 by pyrosequencing showed loss of methylation at the IC2 in the patient (Figure 5).

Figure 4 The ratio charts of the multiplex ligation-dependent probe amplification (MS-MLPA) results for patients with Beckwith-Wiedemann syndrome (BWS) (C to H) and a normal control (A and B). Panels A, C, E and G show the copy number variation pattern of each individual. The copy numbers are normal in the patients with BWS and the normal control. Probe ratios below 0.7 (bottom line) or above 1.3 (top line) are usually regarded as indicative of a heterozygous deletion (copy number change from two to one allele) or duplication (copy number change from two to three alleles), respectively. Panels B, D, F and H show the methylation pattern of each individual. Panel D shows the patient with loss of methylation (LOM) at IC2 since the signal of the four probes for IC2 increase to approach zero (box), while panel B shows the normal methylation in the normal control. Panel F shows the patient with gain of methylation (GOM) at IC1 since the signal of the four probes for IC1 decrease to approach one (box). Panel H shows the patient with both partial GOM at IC1 and partial LOM at IC2 (the probe ratios for both IC1 and IC2 change aberrantly and are still in the range of the two reference lines) (boxes). This reflects mosaic paternal uniparental disomy of 11p15 (patUPD 11p15).

Figure 5 (A) Pyrograms showing loss of methylation at IC2 in a patient with Beckwith-Wiedemann syndrome and (B) normal methylation status at IC2 in a normal control.

In BWS caused by other molecular mechanisms, MS-MLPA would show different results. Figure 4F showed the gain of methylation of IC1, where IGF2 gene expressions on both paternal and maternal chromosomes were activated and caused overgrowth. Figure 4H showed the mosaic paternal uniparental disomy (patUPD), in which partial gain of DNA methylation was detected in IC1 and partial loss of methylation was detected in IC2. Overgrowth occurred when two copies of IGF2 growth promoting genes were activated, while the growth suppressing genes H19 and CDKN1C were inactivated.

BWS caused by patUPD found in approximately 20% of patients, the second most common molecular cause of BWS. The vast majority of patients with patUPD have mosaicism. Usually, this group of patients has the feature of hemihyperplasia, where there is an asymmetrical overgrowth of one or more region of the body. Mosaicism usually occurs post-zygotically. Therefore, the patUPD on 11p15 region can only be found in some somatic tissues. Molecular testing on lymphocytes may be insufficient, as it may not show the mosaic patUPD finding. Sample from hemihyperplasia region, such as skin fibroblast, should be taken for molecular testing for mosaic patUPD confirmation.

The management for BWS would depend on the severity of individuals' BWS features. In particular for this patient, since BWS is associated with increased risk of embryonal tumour development, surveillance is required and we have recommended abdominal ultrasound examination every 3-6 months until eight years of age and serum alpha-fetoprotein (AFP) concentration measurement in the first four years of life for hepatoblastoma early detection.¹¹

The risk of recurrence varies between different molecular subtypes of BWS. Table 2 shows the aetiology and risk of recurrence to parents of a child with BWS. PatUPD occurs when there is post-zygotic somatic recombination. The recurrence risk to parents of affected patient and offspring of affected patient would be low. Hypomethylation on IC2 and hypermethylation on IC1 are arise due to the imprinting error occurs during the germ cell formation. During the germ cells formation, imprints are erased and re-established depended on the sex of transmitting parents. Therefore, the imprinting error in these two subgroups is usually sporadic. The recurrence risk to parents of affected patient and offspring of affected patient is low. In chromosomal rearrangements involving 11p15 region, translocation and inversion typically show maternal inheritance, while duplication typically shows paternal inheritance. These chromosomal rearrangements could be de novo or inherited from parental chromosomal alternations. Regardless of the presence of family history of BWS, it should consider testing on both parents for any chromosomal rearrangements. If one parent has the same chromosomal rearrangement as the patient, parent-of-origin effect must be considered. CDKN1C mutation occurs either in sporadic manner or inherited from parents. When the family history is absent for BWS, it needs to consider testing on both parents. The recurrence risk when one parent is identified to have CDKN1C mutation may be as high as 50%, depends on the parent-of-origin of CDKN1C mutation. This is same as the recurrence risk of offspring of affected patient. The preferential of maternal transmission of CDKN1C mutation is found. For the paternal transmission, the recurrence risk would be lower than 50%. The exact figure is unknown due to lack of empirical data of paternal transmitted cases.

Acknowledgements

We would like to thank Small Project Funding for their support in the project of imprinting disorder (project code: 201109176084). We would also like to thank the patient and the family for their contribution.

References

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